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UF/IFAS Citrus Research and Education Center

UF/IFAS Citrus Research and Education Center

Flower Bud Induction

NOTICE FOR CITRUS EXTENSION AGENTS & SPECIALISTS AND GROWER NEWSLETTERS
The following information has been developed as part of the Decision Information System for Citrus

Overview of flower bud induction in Florida

Dr. Tripti Vashisth, Horticulturist
Citrus Research & Education Center, Lake Alfred, FL

See below for previous flower bud advisories.


 

2022-23

  • 12/2/2022

    Flowering Bud Induction Advisory #2

    Season Forecast: This is going to be a La Niña winter, which means Florida will experience temperatures warmer than normal.  Rainfall pattern, though, is not very clear but it is expected to be below average for at least until the end of the year. Basically, it is going to warmer and drier for the next few weeks (at least)! The precipitation pattern for February-March is still unclear!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a prolonged bloom. 

    Current Condition (as of December 2, 2022): Currently, a majority of the citrus producing regions have low to moderate flower bud induction. So far Ummatila has accumulated the highest amount of induction hours, 550 IH and the rest of the areas surrounding Lake Alfred, and Immokalee are between 300-350 IH.  The next 10 days will be intermediate for cool temperature accumulation with about 100 to 170 IH in most of the citrus producing area. More IH in the next couple of weeks will be good and will push the trees toward moderate flower bud induction period, possibly leading to first wave of flowering around mid-late February (warm-weather depending). The Indian River region has accumulated least amount IH so far and is still in low flower bud induction period. It is expected that Indian River region will be low to moderate flower bud induction period in next 3-5 days.

    This year flowering can be unpredictable not only because of the upcoming weather conditions but also due to the effects of Hurricanes Ian and Nicole. Our trees lost a significant amount of leaves and fruit and endured high speed wind and rain, thus, increasing the stress level on trees. Generally, fruit presence on the tree interrupts flower bud induction, therefore, with fruit loss one can expect to see early flowering. However, the significant leaf loss that has happened in last month also plays a major role in upcoming flowering. Due to significant leaf loss most of the trees are flushing, therefore utilizing the carbohydrate resources thus reduction in carbohydrate level may reduce overall flowering. Under these circumstances and the current weather condition, I do not expect to see a significant early flowering to happen though I expect to see 2-3 cohorts of flowering. The situation will become clearer in the next couple of weeks as we head into cooler temperatures.   

    Gibberellic Acid Application Consideration Update: Gibberellic acid sprays can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA (to suppress flowering) should be applied before warm temperatures (that is before differentiation begins). Previous research on HLB-affected trees has shown that when GA applied monthly in fall, early flowering was suppressed. Therefore, if you have a weak crop load and are forecasted to have warm spells, GA application can be considered to suppress off season flowering.

    GA induced suppression of flowering benefits HLB-affected trees by preserving carbohydrates that can be later used by growing fruit and tree. Therefore, growers who are considering GA application to suppress flowering should carefully choose time to spray. GA application should be targeted at moderate-high flower bud induction period to get most effect. A late GA application might not be useful either. As a general rule of thumb in most of the citrus producing regions of Florida moderate to high flower bud induction should be by 2-3rd week in December.

    *As per current weather conditions and predictions (as of December 2, 2022), growers who are interested in suppressing early flowering and reducing the intensity of flowering, should consider a GA spray in next 1-2 weeks for central and southwest Florida (when the trees are predicted to be in moderate-high flower bud induction). It is advised to regularly check the flower bud induction model to get a better idea of existing conditions for your region.*

    DO NOT spray GA after first of January to manage flowering.

    GA can keep the fruit green, therefore can be a concern for fresh market fruit.

    General Information: Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. After moderate induction, a warm spell when it coincides with rainfall can initiate differentiation, which after sufficient days of warm temperatures will lead to bloom. Trees will be very vulnerable to growth stimulation by a warm period after they accumulate 300-400 hours of cool temps if soil moisture is adequate. 

    Keep track of induction hours in your area and watch for projected warm periods from the weather services. Normal healthy trees could have their induction boosted by applying some drought stress. Unfortunately, with vulnerable root systems associated with HLB you shouldn’t risk heavier preharvest fruit drop of the current crop by using water stress to prevent unwanted early vegetative growth and enhance induction of flowers. Based on weather predictions, if you are concerned about early flowering in your region, a gibberellic acid (GA) application can prevent some early flowering.

    Flowering-related management considerations for HLB-affected trees:

    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth. You should not risk the current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm,
    • Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are very less likely to benefit because of 2 reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flower; interestingly a good branch of severe HLB trees has same flowering potential as mild HLB trees. So, additional flowering promoting fertilizer is not needed; (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop, therefore, pushing tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.

     

  • 11/21/2022

    Flowering Bud Induction Advisory #1

    Season Forecast: This is going to be a La Niña winter, which means Florida will experience temperatures warmer than normal.  Rainfall pattern, though, is not very clear but it is expected to be below average for at least until the end of the year. Basically, it is going to warmer and drier for the next few weeks (at least)! The precipitation pattern for February-March is still unclear!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a prolonged bloom. 

    Current Condition: Currently, a majority of the citrus producing regions have low flower bud induction. So far Ummatila has accumulated about 400 IH and the rest of the areas surrounding Lake Alfred, Immokalee, and Indian River are still a 300 IH which means low bud induction. It is still early in the induction. The next 10 days will be intermediate for cool temperature accumulation with about 100 to 170 IH in most of the citrus producing area. More IH in the next couple of weeks will be good and will push the trees toward low-moderate flower bud induction period, which is good and needed.

    This year flowering can be unpredictable not only because of the upcoming weather conditions but also due to the effects of Hurricanes Ian and Nicole. Our trees lost a significant amount of leaves and fruit and endured high speed wind and rain, thus, increasing the stress level on trees. Generally, fruit presence on the tree interrupts flower bud induction, therefore, with fruit loss one can expect to see early flowering. However, the significant leaf loss that has happened in last month also plays a major role in upcoming flowering. Due to significant leaf loss most of the trees are flushing, therefore utilizing the carbohydrate resources thus reduction in carbohydrate level may reduce overall flowering. Under these circumstances and the current weather condition, I do not expect to see a significant early flowering to happen though I expect to see 2-3 cohorts of flowering. The situation will become clearer in the next couple of weeks as we head into cooler temperatures.   

    General Information: Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. After moderate induction, a warm spell when it coincides with rainfall can initiate differentiation, which after sufficient days of warm temperatures will lead to bloom. Trees will be very vulnerable to growth stimulation by a warm period after they accumulate 300-400 hours of cool temps if soil moisture is adequate. 

    Keep track of induction hours in your area and watch for projected warm periods from the weather services. Normal healthy trees could have their induction boosted by applying some drought stress. Unfortunately, with vulnerable root systems associated with HLB you shouldn’t risk heavier preharvest fruit drop of the current crop by using water stress to prevent unwanted early vegetative growth and enhance induction of flowers. Based on weather predictions, if you are concerned about early flowering in your region, a gibberellic acid (GA) application can prevent some early flowering.

    Gibberellic Acid Application Consideration: Gibberellic acid sprays can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA (to suppress flowering) should be applied before warm temperatures (that is before differentiation begins). Previous research on HLB-affected trees has shown that when GA applied monthly in fall, early flowering was suppressed. Therefore, if you have a weak crop load and are forecasted to have warm spells, GA application can be considered to suppress off season flowering.

    GA induced suppression of flowering benefits HLB-affected trees by preserving carbohydrates that can be later used by growing fruit and tree. Therefore, growers who are considering GA application to suppress flowering should carefully choose time to spray. GA application should be targeted at moderate-high flower bud induction period to get most effect. A late GA application might not be useful either. As a general rule of thumb in most of the citrus producing regions of Florida moderate to high flower bud induction should be by 2-3rd week in December. For example, this year so far we are have had low bud induction. Even within the next 10-14 days we are expected to be marginally in a moderate flower bud induction period, therefore, under such situation growers should wait for a week or so to apply GA (in order to suppress flowering). Nevertheless, multiple GA sprays in Fall improves the chance of influencing flowering.

    DO NOT spray GA after first of January to manage flowering.

    GA can keep the fruit green, therefore can be a concern for fresh market fruit.

    Flowering-related management considerations for HLB-affected trees:

    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth. You should not risk the current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm,
    • Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are very less likely to benefit because of 2 reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flower; interestingly a good branch of severe HLB trees has same flowering potential as mild HLB trees. So, additional flowering promoting fertilizer is not needed; (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop, therefore, pushing tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.

 

2021-22

  • 2/17/2022

    Flowering Bud Induction Advisory #5

    Season Forecast: This is a La Niña winter, second in a row, which means Florida is experiencing temperatures warmer than normal and rainfall lower than normal. We are in for a warm and dry winter weather!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a very prolonged bloom. On the positive side if dry weather prevails, not much differentiation will happen and dry conditions during the bloom period there could potentially lower the risk for postbloom fruit drop (PFD).

    Current Condition: Unfortunately, this season we have seen significant fluctuation in temperatures, where November received some cold weather, December was warmer and then January experienced cold weather again. By December 1st, 2021, most of the citrus producing regions of Florida had experienced moderate flower bud induction. Thus a prolonged flowering window with light flowering in January to mid-March, about 3 waves of flowering, was expected. In the past 2-3 weeks, we have received significant cold weather, including freezing temperatures in many areas. The freezing temperatures resulted in freeze injury on any newly emerged flower and leaves. Briefly, for the Ridge, it can be said that most of the flowers from first wave of flowering have succumbed to freeze damage. A silver lining to freezing temperatures is that it has slowed down the flowering progression and eliminated early flowers.

    Currently, every citrus producing region has experienced sufficient induction hours to be experiencing another wave of flowering soon. However, the cold weather has slowed it down. It is expected to see next wave of flowering from end of February-first week of March, depending upon location. Based on the model prediction and weather pattern, it is expected that flowering will occur until 4th week of March. With such prolonged flowering window multiple cohorts of fruit set will be seen.

    If you have not started yet, it is time to start fertilizer application. It is ideal to have the fertilizer on ground before the flowering begins in next 10-15 days. As the weather warms up in next week or so, it is time schedule more frequent irrigation. It is expected to stay dry for next two weeks therefore, a good irrigation program during fruit set and early fruit growth period is critical. Recent research shows that water deficit during early fruit growth period can suppress fruit growth and contribute to increased fruit drop later in the season

    Flowering related management considerations for HLB-affected trees:

    • Gibberellic acid sprays (20 g a.i. per acre) can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins). DO NOT spray GA after first of January to manage flowering, a spray too close to flowering period can negatively influence flowering. GA can keep the fruit green, therefore can be a concern for fresh market fruit.
    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppresses vegetative growth. You should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm.
    • Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are much less likely to benefit because of 2 reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flower. Interestingly a good branch on severely HLB-affected trees has same flowering potential as mildly HLB-affected trees. So additional flowering promoting fertilizer is not needed. (2) High rates of twig dieback and low fruitlet retention is the main concern with severely HLB-affected trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop therefore, pushing tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.
    • If low fruit set has been a reoccurring issue in some groves, a GA spray to boost fruit set can be made. Such sprays should be applied after the peak flowering for the season has been observed.

    Flowering and PFD Advisory by Dr. Megan Dewdney

    In the current weather forecasts, temperatures are expected to be warm enough for a postbloom fruit drop (PFD) infection period while flowers are open. The optimum temperature range is 72-79°F for infection. Luckily, there is little to no rain predicted. For the fungus that causes PFD to infect, there needs to be sufficient wetting, usually more than 16 hours, and the required wetness period can be longer if the temperatures are not within the optimal range. These long wetting periods generally only occur with rainfall or long periods of fog. It is hard to see too far into the future for the March wave of bloom, but if this season is like last year’s La Niña, the spring will be dry with little likelihood of PFD. If we get forecasts for warm rain during March when there are many flowers, I recommend looking for signs of recent infection like buttons to see how much inoculum may be present and follow the application recommendations of the Citrus Advisory System (http://agroclimate.org/tools/cas/).

  • 1/12/2022

    Flower Bud Induction Advisory #4

    Season Forecast: This is a La Niña winter, second in a row, which means Florida is experiencing temperatures warmer than normal and rainfall lower than normal. We are in for warm and dry winter weather!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flowering waves and a very prolonged bloom. On the positive side, if dry weather prevails, not much differentiation will happen and dry conditions during the bloom period could potentially lower the risk for postbloom fruit drop (PFD).

    Current Condition: We continue to experience warm weather, as expected in a La Niña winter. Currently, all citrus producing regions have induction hours (IH) ranging from 1100 to 840 IH, north to south. This puts us in a high induction period, it is predicted that we will have at least 2 waves of bloom, pretty much everywhere. Unfortunately, with the current warm weather we are already seeing small number of early off-season blooms in many blocks, especially the ones that have been harvested. The first major wave of flowering is likely to happen in the first week of February in central Florida and by February 15th in southwest. The second wave of flowering is expected with a 2-3 week gap from first wave. Currently, for Indian River region, the model predicts one wave of flowering around February 20th but it is very likely that instead of one strong flower wave there might be a prolonged flowering period spanning from mid-February to mid-March. Altogether, it is very likely that we will see prolonged and sporadic flowering this season especially in the groves that have been picked or have lighter than usual crop or stressed trees. It is time to start thinking about fertilizer application. With warm weather, trees are starting to put out vegetative and flower buds. It will be ideal to have your spring fertilizer on ground before first wave of flower is obvious.

    Rainfall is going to play a significant role! Dry and cool weather will be ideal. If we get some cold weather (which is unlikely in La Niña) it will really help with flowering. Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. After moderate induction (we are in high induction right now), a warm spell coinciding with rainfall can initiate differentiation, which after sufficient days of warm temperatures will lead to bloom. Trees will be very vulnerable to growth stimulation by a warm period after they accumulate 300-400 hours of cool temps if soil moisture is adequate or significant rainfall event happens.

    Normal healthy trees could have their induction boosted and differentiation interrupted by applying some drought stress. Unfortunately, with vulnerable root systems associated with HLB you shouldn’t risk heavier preharvest fruit drop of the current crop by using water stress to prevent unwanted early vegetative growth and enhance induction of flowers.

    Under current conditions, DO NOT apply GA with intention of suppressing early flowering in groves, GA application is not beneficial for suppressing early flowering (if the differentiation has already initiated).

    Flowering related management considerations for HLB-affected trees:

    • Gibberellic acid sprays (20 g a.i. per acre) can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins). DO NOT spray GA after first of January to manage flowering. GA can keep the fruit green, therefore can be a concern for fresh market fruit.
    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth, you should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm.
    • Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are very less likely to benefit because of 2 reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flower, interestingly a good branch of severe HLB trees has same flowering potential as mild HLB trees. So additional flowering promoting fertilizer is not needed. (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop therefore, pushing tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.
  • 12/30/2021

    Flower Bud Induction Advisory #3

    Season Forecast: This is a La Niña winter, second in a row, which means Florida is experiencing temperatures warmer than normal and rainfall lower than normal. We are in for a warm and dry winter weather!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a very prolonged bloom. On the positive side if dry weather prevails, not much differentiation will happen and dry conditions during the bloom period there could potentially lower the risk for postbloom fruit drop (PFD).

    Current Condition: As expected in a La Niña winter, the temperatures have been high for last month or so, the days have been pretty warm (at least above 70° F). However, due to cold temperatures in mid-end November in this year, by December 1st we had received 615 to 350 induction hours (IH) from north to south. With a little bit more accumulation of induction hours currently, all citrus producing counties have accumulated more than 650 IH which puts them in high flower bud induction period; Umatilla has accumulated 1080 IH whereas Arcadia has 800 IH. Immokalee and Indian River region both have around 650 IH which puts them on cusp of moderate and high. It seems that in grove with adequate soil moisture, the differentiation has begun. Fortunately, due to La Niña effect, next two weeks are predicted to be dry therefore, early flowering maybe suppressed. However, it is quite likely we will see prolonged and sporadic flowering this season especially in the groves that have been picked or have lighter than usual crop. It is expected that with current weather conditions, we will see the first wave of flowering in early February in north regions and mid-end of February in southern and Indian River region.

    Rainfall is going to play a significant role! Dry and cool weather will be ideal. If we get some cold weather (which is unlikely in La Niña) it will really help with flowering. Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. After moderate induction (we are in high right now), a warm spell when coincides with rainfall can initiate differentiation, which after sufficient days of warm temperatures will lead to bloom. Trees will be very vulnerable to growth stimulation by a warm period after they accumulate 300-400 hours of cool temps if soil moisture is adequate or significant rainfall event happens.

    Normal healthy trees could have their induction boosted and differentiation interrupted by applying some drought stress. Unfortunately, with vulnerable root systems associated with HLB you shouldn’t risk heavier preharvest fruit drop of the current crop by using water stress to prevent unwanted early vegetative growth and enhance induction of flowers.

    Based on weather predictions, if you are concerned about early flowering in your region, a gibberellic acid (GA) application can prevent some early flowering. GA should be applied before warm temperatures (that is before differentiation begins). Under current conditions, especially in the areas where high IH was accumulated by December 15-18, GA application may not be beneficial for suppressing early flowering (since the differentiation may have initiated). DO NOT apply GA with intention of suppressing early flowering in groves where bud swell or distinct flowering can be seen. In most of the groves, the window of applying GA is over or will be ending within next week.

    Flowering related management considerations for HLB-affected trees:

    • Gibberellic acid sprays (20 g a.i. per acre) can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins). DO NOT spray GA after first of January to manage flowering. GA can keep the fruit green, therefore can be a concern for fresh market fruit.
    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth, you should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm.
    • Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are very less likely to benefit because of 2 reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flower, interestingly a good branch of severe HLB trees has same flowering potential as mild HLB trees. So additional flowering promoting fertilizer is not needed. (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop therefore, pushing tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.
  • 12/16/2021

    Flower Bud Induction Advisory #2

    Season Forecast: This is a La Niña winter, second in a row, which means Florida is experiencing temperatures warmer than normal and rainfall lower than normal. We are in for a warm and dry winter weather!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a very prolonged bloom. On the positive side if dry weather prevails, not much differentiation will happen and dry conditions during the bloom period there could potentially lower the risk for postbloom fruit drop (PFD).

    Current Condition: As expected in a La Niña winter, the temperatures have been high in the last couple of weeks. By December 1st of this year, we had received 615 to 350 induction hours (IH) from north to south and therefore, we were in low to moderate flower induction period all across the state. Currently, all citrus producing counties have moderate (if not high) flower bud induction; Umatilla has accumulated 800 IH (which makes it high) whereas Arcadia has 612 IH which puts it in moderate. Immokalee and Indian River regions both have around 550 IH which puts them on cusp of low to moderate. The current warm spell raises the concern for flower differentiation, indeed sporadic bloom can be seen in several groves (especially the ones that are more stressed or young blocks). The next 10 days are expected to be somewhat cooler than current conditions where average temperature should be lower than 78° F in central and north citrus producing regions. In addition, it is expected to stay dry for the next 10 days therefore, a major wave of bloom may not happen. Though sporadic flowering on stressed trees should be expected. However, in southern and Indian River region slightly higher average temperatures (about 80° F day average) along with higher chance of rainfall is expected. Thus making these regions prone to an early wave of flowering. Rainfall is going to play a significant role!

    Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. After moderate induction, a warm spell when coincided with rainfall can initiate differentiation, which after sufficient days of warm temperatures will lead to bloom. Trees will be very vulnerable to growth stimulation by a warm period after they accumulate 300-400 hours of cool temps if soil moisture is adequate or significant rainfall event happens.

    Keep track of induction hours in your area and watch for projected warm periods from the weather services. Normal healthy trees could have their induction boosted by applying some drought stress. Unfortunately, with vulnerable root systems associated with HLB you shouldn’t risk heavier preharvest fruit drop of the current crop by using water stress to prevent unwanted early vegetative growth and enhance induction of flowers. Based on weather predictions, if you are concerned about early flowering in your region, a gibberellic acid (GA) application can prevent some early flowering. GA should be applied before warm temperatures (that is before differentiation begins).

    Flowering related management considerations for HLB-affected trees:

    • Gibberellic acid sprays (20 g a.i. per acre) can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins). Previous research on HLB-affected trees in 2017-2018 (a La Niña winter) has shown that when GA was applied monthly in the fall, early flowering was suppressed. Therefore, if you have a weak crop load and are forecasted to have warm spells, GA application can be considered to suppress off season flowering. DO NOT spray GA after the first of January to manage flowering. GA can keep the fruit green, therefore can be a concern for fresh market fruit.
    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth, you should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm.
    • Flowering enhancing fertilizer to increase the number of flowers is NOT suggested for severely HLB-affected trees as they are very less likely to benefit because of two reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flower, interestingly a good branch of severe HLB trees has same flowering potential as mild HLB trees. So additional flowering promoting fertilizer is not needed. (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop therefore, pushing the tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.
  • 12/2/2021

    Flower Bud Induction Advisory #1

    Season Forecast: This is going to be a La Niña winter, second in a row, which means Florida will experience temperatures warmer than normal and rainfall lower than normal. We are in for a warm and dry winter weather!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a very prolonged bloom. On the positive side if dry weather prevails during the bloom period there could potentially be lower incidence of postbloom fruit drop.

    However, like last year we have been receiving good cold weather so far and if this continues along with dry weather we should expect to see one major concise bloom like last year (Spring 2021).

    Current Condition: Currently, all the citrus producing regions have low to moderate flower bud induction, which means some inductive conditions have already occurred. So far Umatilla has accumulated about 618 IH, Lake Alfred about 560 IH, Immokalee about 374 IH, and Indian River has experienced 444 IH. It is still early in the induction but a warm spell can push some of those inducted-buds to flower. Next 10 days will be intermediate for cool temperature accumulation with about 60 to 150 IH, south to north. Few days of above average warm temperatures (75-80° F) is predicted, however, in absence of rainfall (it is predicted to stay dry for next several days) early flowering should not be the concern.

    Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. After moderate induction, a warm spell when coincides with rainfall can initiate differentiation, which after sufficient days of warm temperatures will lead to bloom. Trees will be very vulnerable to growth stimulation by a warm period after they accumulate 300-400 hours of cool temps if soil moisture is adequate.

    Keep track of induction hours in your area and watch for projected warm periods from the weather services. Normal healthy trees could have their induction boosted by applying some drought stress. Unfortunately, with vulnerable root systems associated with HLB you shouldn’t risk heavier preharvest fruit drop of the current crop by using water stress to prevent unwanted early vegetative growth and enhance induction of flowers. Based on weather predictions, if you are concerned about early flowering in your region, a gibberellic acid (GA) application can prevent some early flowering.

    Flowering related management considerations for HLB-affected trees:

    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth, you should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm.
    • Gibberellic acid sprays can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins). Previous research on HLB-affected trees in 2017-2018 (a La Niña winter) has shown that when GA applied monthly in fall, early flowering was suppressed. Therefore, if you have a weak crop load and are forecasted to have warm spells, GA application can be considered to suppress off season flowering. DO NOT spray GA after first of January to manage flowering. GA can keep the fruit green, therefore can be a concern for fresh market fruit.
    • Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are very less likely to benefit because of two reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flower, interestingly a good branch of severe HLB trees has same flowering potential as mild HLB trees. So additional flowering promoting fertilizer is not needed. (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop therefore, pushing tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.

 

2020-21

  • 1/19/2021

    Flower Bud Induction Advisory #5

    Current Condition: Even though this year is predicted to be La Niña, that is warmer than average temperatures are expected, so far we have received good amount of cold weather and the next 10 days are also expected to be less than 80°F for most of the citrus producing region.

    Currently, all citrus producing regions in Florida (from Umatilla to Immokalee) have accumulated more than 900 inductive hours (IH), the flower bud induction is high in all cases. High induction levels indicate that sufficient inductive conditions have occurred to cause flower bud differentiation of economic importance. Therefore, a warm spell (temperatures above 80°F) of 5-8 days and adequate soil moisture should be sufficient to sprout flowering buds. Next 10 days in all the regions are still expected to have temperatures less than 80°F, the days will be warm and nights are expected to be cold, this can delay the bud differentiation. It is expected that we will have a single wave of bloom in mid-March in most of the citrus producing region. Visit http://disc.ifas.ufl.edu/bloom/ to get precise predicted bloom date for your county.

    Under current predicted weather conditions, no additional flowering management strategy is recommended.

    Suggestions for Upcoming Season

    This is a good time to start thinking about next year’s crop and management strategy to improve fruit set, growth, and development.

    Historically, February to April are the driest months in Florida. Coincidentally, this is the period when flowering, fruit set, and fruit growth occurs. Water deficiency has been reported to increase with HLB symptoms during this period and can affect fruit set and growth. It is highly recommended to irrigate the trees during the time of flowering and fruit set, March to May before rains start. Frequent irrigation with small amount of water is recommended, emphasis is on frequency of irrigation and NOT increasing the total amount of water applied.

    Another scenario is where growers have reported good bloom but low yields. Low yield can be due to:

    1. Poor fruit set- in this case you will see good flowering but low number of fruit on the tree and not much fruit drop.

    2. Fruit drop- during fruit development or preharvest

    In both cases, a GA application can be beneficial. To improve fruit set, GA can be applied once 80% of bloom has occurred and petal fall has started to happen. In regards to reducing fruit drop, GA and HLB research on Valencia has shown that when GA is applied monthly from September to January, fruit drop can be reduced. This work is done on Valencia, however, in the case of Hamlin, it is speculated that GA sprayed before color break has happened can be beneficial in minimizing fruit drop. GA sprays on mature fruit can delay color break, this can be problematic especially for fresh fruit varieties.

    Bottom line- For improving fruit set and growth, a good irrigation strategy is the key. In addition, the adoption of GA can be beneficial, however, timing of application is the essence for efficacy of plant growth regulators. Contact Tripti Vashisth (tvashisth@ufl.edu) if you have additional questions.

  • 1/4/2021

    Flower Bud Induction Advisory #4

    Season Forecast: This is going to be a La Niña winter which means Florida will experience temperatures warmer than normal and rainfall lower than normal. We are in for a warm and dry winter weather!

    Under these conditions, enough hours below 68°F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a very prolonged bloom. On the positive side if dry weather prevails during the bloom period there could potentially be lower incidence of postbloom fruit drop.

    Current Condition: Even though this year is predicted to be La Niña, that is warmer than average temperatures are expected, so far we have received decent amount of cold weather and the next 10 days are also expected to be less than 75°F for most of the citrus producing region.

    Currently, all citrus producing regions of Central and North Florida (Umatilla) have accumulated above 800 inductive hours (IH) and Southwest and Indian River regions have accumulated about 650 IH, the flower bud induction is moderate-high in all cases, which is sufficient for flower bud induction in most cases, both early and late varieties. Therefore, a warm spell (temperatures above 80°F) of 7-10 days can initiate the differentiation of flower buds resulting in the first wave of flowering to occur early (second half of February). Fortunately, next 10-14 days are predicted to be lower than 78°F although some rainfall is expected but overall major flower bud differentiation is unlikely to happen in the next couple of weeks. Even if we receive some rainfall, cool temperatures will not be conducive for bud differentiation. If these weather conditions continue, it is expected to see a major single flowering wave in early-mid March. Under current predicted weather conditions, no additional flowering management strategy is recommend. However, we need to closely monitor weather predictions as this season is supposed to be La Niña, it is expected to have warm temperatures. Prolonged warm temperatures after sufficient flower bud induction can accelerate bud differentiation and since it is recommended to keep irrigating throughout the fall, these conditions can be conducive for flowering (may shorten time required for flower formation).

    Flowering related management considerations for HLB-affected trees:

    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth, you should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm.
    • Flowering enhancing fertilizer to increase the number of flowers is NOT suggested for severely HLB-affected trees as they are less likely to benefit because of two reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flowers, interestingly a good branch on severe HLB trees has the same flowering potential as mild HLB trees. So additional flowering promoting fertilizer is not needed. (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop therefore, pushing tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.
    • GA sprays can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins).

    GA and HLB: Previous research on HLB-affected trees in 2017-2018 (a La Niña winter) has shown that when GA applied monthly in the fall, early flowering was suppressed. Therefore, if you have a weak crop load and are forecasted to have warm spells, GA application can be considered to suppress off season flowering. DO NOT spray GA after the first of January to manage flowering. GA sprays will reduce the total number of flower buds, however, the current literature shows that reduction in the number of buds in HLB trees with use of GA does not affect final yield.

    Suggestions for Upcoming Season

    This is a good time to start thinking about next year’s crop and management strategy to improve fruit set, growth and development. This season is predicted to be La Niña, that is warmer than average temperature and lower than average rainfall.

    Recent research on HLB has shown:

    • HLB trees have poor water uptake due to reduced feeder root density.
    • Small fruit are more likely to drop than large fruit.
    • With increasing symptoms of HLB, fruit size decreases significantly.
      Fruit drop is directly related to HLB symptoms, where small fruit are more likely to drop.
    • Fruit size starts to differ within first couple of months of fruit set. Fruit size is affected by water availability.
    • Fruit set and retention is poor in HLB symptomatic trees.
    • Historically, February-April are driest months in Florida. Coincidentally this is the period when flowering, fruit set, and fruit growth occurs. Water deficiency has been reported to increase with HLB symptoms during this period.

    Therefore, it is highly recommended to irrigate the trees during time of flowering and fruit set, March-May before rains start. Frequent irrigation with small amount of water is recommended, emphasis is on frequency of irrigation and NOT increasing the total amount of water applied.

    Another scenario is where growers have reported good bloom but low yields. Low yield can be due to:

    1. Poor fruit set-in this case you will see good flowering but low number of fruit on the tree and not much fruit drop.

    2. Fruit drop- during fruit development or preharvest

    In both cases GA application can be done. To improve fruit set, GA can be applied once 80% of bloom has occurred and petal fall has started to happen. In order to reduce fruit drop, GA and HLB research on Valencia has shown that when GA is applied monthly from September to January, fruit drop can be reduced. This work is done on Valencia, however, in the case of Hamlin, it is speculated that GA sprayed before color break has happened can be beneficial in minimizing fruit drop. GA sprays on mature fruit can delay color break, this can be problematic especially for fresh fruit varieties.

    Bottom line- For improving fruit set and growth, a good irrigation strategy is the key. In addition, the adoption of GA use is beneficial, however, timing of application is the essence for efficacy of plant growth regulators. Contact Tripti Vashisth (tvashisth@ufl.edu) if you have more questions.

  • 12/22/2020

    Flower Bud Induction Advisory #3

    Season Forecast: This is going to be a La Niña winter which means Florida will experience temperatures warmer than normal and rainfall lower than normal. We are in for a warm and dry winter weather!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a very prolonged bloom. On the positive side if dry weather prevails during the bloom period there could potentially be lower incidence of postbloom fruit drop

    Current Condition: Currently, citrus producing regions of Central and North Florida (Umatilla) have accumulated above 650 inductive hours (IH), which is sufficient for flower bud induction in many cases, especially if the early cultivars have been harvested. At this point, most of the growers have decided to pick Hamlins due to extensive fruit drop, before all the fruit rains down! Therefore, a warm spell of a few days in Central and North Florida can initiate the differentiation of flower buds resulting in the first wave of flowering to occur early (second half of February). Fortunately, next 10-14 days are predicted to be lower than 78°F and low rainfall is expected therefore, major flower bud differentiation is unlikely to happen in the next couple of weeks. If these weather conditions continue, it is expected to see a major single flowering wave in early-mid March. Under current weather conditions (if they remain as predicted), no additional flowering management strategy is recommend. However, we need to closely monitor weather predictions as this season is supposed to be La Niña, it is expected to have warm temperatures. Prolonged warm temperatures after sufficient flower bud induction can accelerate bud differentiation and since it is recommended to keep irrigating throughout the fall, these conditions can be conducive for flowering (may shorten time required for flower formation).

    In Southwest and Indian River regions currently, approximately 450 IH have been accumulated, the flower bud induction is low-moderate. It is predicted that this region will accumulate additional 100-200 IH in the next 14 days. Therefore, flower bud induction will be in the moderate category, a warm spell (temperatures above 80°F) of 7-10 days at that time can then initiate differentiation, leading to bloom. For these regions, the major concern is the possibility of an early warm spell with sufficient soil moisture that can initiate the differentiation of easily induced flower buds resulting in some flowering to occur early, therefore extending the bloom period in the spring. Pre-HLB, under healthy conditions, imposing drought stress on the trees would have been a good strategy to minimize flower bud initiation this early under warm conditions however, for HLB-affected trees it is strongly recommended to NOT induce drought stress.

    Overall, prevalent cold weather for the next 2-3 weeks will be favorable to result in synchronized and concise bloom in March. However, the trees are now very vulnerable to growth stimulation by a warm period. Based on weather predictions, if you are concerned about early flowering in your region, DO NOT apply drought stress; however, a gibberellic acid (GA) application can be applied to prevent some early flowering. Though, the GA application should be done before differentiation begins.

    Flowering related management considerations for HLB-affected trees:

    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth, you should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm,
    • Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are less likely to benefit because of two reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flowers, interestingly a good branch on severe HLB trees has same flowering potential as mild HLB trees. So additional flowering promoting fertilizer is not needed. (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop therefore, pushing tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.
    • GA sprays can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins).

    GA and HLB: Previous research on HLB-affected trees in 2017-2018 (a La Niña winter) has shown that when GA applied monthly in fall, early flowering was suppressed. Therefore, if you have a weak crop load and are forecasted to have warm spells, GA application can be considered to suppress off season flowering. DO NOT spray GA after first of January to manage flowering. GA sprays will reduce the total number of flower buds, however, the current literature shows that reduction in the number of buds in HLB trees with use of GA does not affect final yield.

  • 12/7/2020

    Flower Bud Induction Advisory #2

    Season Forecast: This is going to be a La Niña winter which means Florida will experience temperatures warmer than normal and rainfall lower than normal. We are in for a warm and dry winter weather!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a very prolonged bloom. On the positive side if dry weather prevails during the bloom period there could potentially be lower incidence of postbloom fruit drop

    Current Condition: Currently, citrus producing regions of Central and North Florida (Umatilla) have accumulated above 350 inductive hours (IH), which is low-moderate flower bud induction. For these regions, the major concern is the possibility of an early warm spell that will initiate the differentiation of easily induced flower buds resulting in some flowering to occur early, therefore extending the bloom period in spring. Pre-HLB, under healthy conditions, imposing drought stress on the trees would have been a good strategy to minimize flower bud initiation this early under warm conditions; however, for HLB-affected trees it is strongly recommended to NOT induce drought stress. Fortunately, next 10-14 days are predicted to be lower than 78° F. Since the accumulated cold weather is in the low-moderate category, bud initiation is expected to be minimum to none; therefore, early flowering is not yet a concern for these regions. Nonetheless, it is expected that North Florida and Central Florida regions will accumulate sufficient inductive hours by Christmas time to result in major bloom (dependent on weather) around end of February and early March.

    In Southwest and Indian River regions approximately 250 IH have been accumulated, therefore the flower bud induction is low. It is predicted that these regions will accumulate additional 100-200 IH in the next 14 days. Therefore, flower bud induction will be in the moderate category, a warm spell (temperatures above 80° F) of 7-10 days at that time can then initiate differentiation, leading to bloom.

    Prevalent cold weather for the next month will be favorable to result in synchronized and concise bloom in March. Trees can be very vulnerable to growth stimulation by a warm period after they accumulate 300-400 hours of cool temps if soil moisture is adequate. Based on weather predictions, if you are concerned about early flowering in your region, DO NOT apply drought stress; however, a gibberellic acid (GA) application can prevent some early flowering. Though, the GA application should be done before differentiation begins.

    Flowering related management considerations for HLB-affected trees:

    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppresses vegetative growth. You should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm.
    • Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are less likely to benefit because of two reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flower, interestingly a good branch on a severe HLB tree has same flowering potential as mild HLB trees. So additional flowering promoting fertilizer is not needed. (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop; therefore, pushing trees to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.
    • Gibberellic acid (GA) sprays can be used to suppress early spring flowering, but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins). Previous research on HLB-affected trees in 2017-2018 (a La Niña winter) has shown that when GA is applied monthly in the fall, early flowering was suppressed. Therefore, if you have a weak crop load and are forecasted to have warm spells, GA application can be considered to suppress off season flowering. DO NOT spray GA after first of January to manage flowering.
  • 11/23/2020

    Flower Bud Induction Advisory #1

    Tripti Vashisth, Horticulturist, Citrus Research & Education Center, Lake Alfred, FL

    This is a service to our citrus growers posted on the CREC website. The indicated Expert System on intensity and time of bloom can be accessed at the designated Web Site: http://disc.ifas.ufl.edu/bloom 

    Season Forecast: This is going to be a La Niña winter which means Florida will experience temperatures warmer than normal and rainfall lower than normal. We are in for a warm and dry winter weather!

    Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds but intermediate warm periods during the winter can lead to multiple flower cohorts and a very prolonged bloom. On the positive side if dry weather prevails during the bloom period there could potentially be lower incidence of postbloom fruit drop.

    Current Condition: Currently, citrus producing regions have low flower bud induction, which is low accumulation of inductive hours (IH), temperatures below 68° F or 20° C. So far Umatilla has accumulated about 175 IH, Lake Alfred about 150 IH, Immokalee about 80 IH, and Indian River has experienced very low IH. Next 10 days will be intermediate for cool temperature accumulation with about 50 to 120 IH, south to north. Accumulation of more cool temperatures and hopefully a quick turn to cooler temperatures is needed to prevent early flushing, off-season flowering and good spring flowering for 2021-2022 production.

    Cool weather stops growth and then promotes induction of flower buds as more cool weather accumulates. A warm spell can then initiate differentiation, which after sufficient days of warm temperatures will lead to bloom. Trees will be very vulnerable to growth stimulation by a warm period after they accumulate 300-400 hours of cool temps if soil moisture is adequate. Keep track of induction hours in your area and watch for projected warm periods from the weather services. Normal healthy trees could have their induction boosted by applying some drought stress. Unfortunately, with vulnerable root systems associated with HLB you shouldn’t risk heavier preharvest fruit drop of the current crop by using water stress to prevent unwanted early vegetative growth and enhance induction of flowers. Management of flower bud induction is limited at the low induction level.

    Flowering related management considerations for HLB-affected trees:

    • DO NOT drought stress HLB-affected trees even though drought stress promotes flower induction and suppress vegetative growth, you should not risk current crop due to additional drought stress. Drought stress can exacerbate fruit drop. Daily, lower volume irrigations to minimize fall drought stress is suggested, especially when the weather is warm.
    • Flowering enhancing fertilizer to increase the number of flowers are NOT suggested for severely HLB-affected trees as they are very less likely to benefit because of two reasons: (1) HLB-affected trees have more dead wood therefore, there are fewer buds available to become flowers, interestingly a good branch of severe HLB trees has the same flowering potential as mild HLB trees. So additional flowering promoting fertilizer is not needed. (2) High twig dieback and low fruitlet retention is the main concern with severe HLB trees in regards to fruit set. Only 2% of the total flowers turn into harvestable crop therefore, pushing the tree to flower more is not advisable as that is likely to waste trees’ energy and resources in extra flowers.
    • Gibberellic acid (GA) sprays can be used to suppress early spring flowering but the timing of application is critical for GA to be effective. GA should be applied before warm temperatures (that is before differentiation begins). Previous research on HLB-affected trees in 2017-2018 (a La Niña winter) has shown that when GA applied monthly in the fall, early flowering was suppressed. Therefore, if you have a weak crop load and are forecasted to have warm spells, GA application can be considered to suppress off season flowering. DO NOT spray GA after first of January to mange flowering.

 

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