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Flower Bud Induction Overview and Advisory

NOTICE FOR CITRUS EXTENSION AGENTS & SPECIALISTS AND GROWER NEWSLETTERS
The following information has been developed as part of the Decision Information System for Citrus. (http://disc.ifas.ufl.edu/bloom)

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

Flower Bud Induction Advisories

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.