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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

1/19/2021

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.