November 21, 2023
Flowering Bud Induction Advisory #1
Season Forecast: This is predicted to be a El Niño–Southern Oscillation (ENSO) winter, which means Florida will experience more rainfall pattern than normal. The daily temperatures are predicted to be average for this time of year however, due to higher than usual rainfall (associated overcast) slightly cooler temperatures can be predicted.
Under these conditions, enough hours below 68° F are likely to accumulate to induce an economic level of flower buds, but excessive rainfall can interfere with bud induction as both low temperature and drought stress are needed for strong floral bud induction.
Current Condition: Currently, majority of the citrus producing regions have low flower bud induction. So far Ummatila has accumulated about 385 IH and rest of the areas surrounding Lake Alfred, Immokalee, and Indian River are still under 300 IH which mean low bud induction. It is still early in the induction. Management of flower bud induction is limited at the low induction level. 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 next couple of weeks will be good and push the trees toward low to moderate flower bud induction period, which is good and needed. However, the rainfall is also predicted at this time. Generally, once the tree is in low to moderate induction an intermittent warm period along with rainfall can be sufficient to induce bud break thus leading to prolonged bloom however, under current predicted conditions day temperature should not be conducive for significant bud break.
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 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 (over 75 F) 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 carbohydrate 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, within next 10-14 days we are expected to be in moderate flower bud induction period therefore, a GA spray in second or third week of December can be effective in reducing floral bud induction. If induction level is above 600 or 650 hours the spray will not completely stop all of the flowering, but a more concentrated flowering should occur after the second warm period.
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 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.