November 25, 2024
Flowering Bud Induction Advisory #1
Season Forecast: This winter and spring (2025) are predicted to be a weak La Niña which means Florida will experience temperatures warmer than normal and rainfall lower than normal. Drought conditions are also likely to develop late in the winter and early spring as numerous fair and warm days are anticipated. Overall, 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 post bloom 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 260 IH, Lake Alfred about 190 IH, Immokalee about 161 IH, and Indian River has experienced about 178 IH. The 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 and prolonged flowering and good spring flowering for upcoming season. However, later interruption by warm weather is going to be an issue.
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 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 is 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 flowers, 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.
- 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 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.
With current weather conditions and predictions, GA spray to suppress early flowering is advisable; especially, if the crop has been harvested in December. Suppressing early flowering will synchronize flowering and will improve leaf production (thus better source to sink ratio). Better canopy = better fruit production on HLB-affected trees.
DO NOT spray GA after first of January to manage flowering.