January 8, 2024
Flowering Bud Induction Advisory #4
-
Flowering Bud Induction Advisory #4
Current Condition: Currently, all the citrus producing regions of Florida are in high flower induction levels which means that sufficient inductive conditions have occurred to cause flower bud differentiation of economic importance. Warm conditions along with adequate soil moisture should be sufficient to sprout flowering buds. It is anticipated that we will see the first wave of flowering in end of February in north Florida. Currently, the flowering model predicts two waves of flower in Umatilla, February 19 and March 4; and one wave in Central Florida, March 3; one wave in Immokalee, March 5. However, the cooler weather that is expected for next 10 days will slow down the differentiation of flower. Thus, the major wave of flowering is expected in first week of March for most of the citrus producing counties. It is expected that the flowering will be concise in most of the central Florida. In southwest Florida and east Florida, the flowering is expected to be slightly more prolonged but now very bad as we saw in 2022.
Some areas are already showing pockets of sporadic flowering however, these are the areas where crop load was low, and trees were injected. It is too early to speculate the effect/interaction of injections on flowering, but we continue to monitor.
Now it is too late to make Gibberellic Acid (GA) application with goal of suppressing flowering. The growers are highly recommended to not use GA in their groves from now until June Drop
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.
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. Therefore, a GA spray in second or third week of December can be effective in reducing floral bud induction. Once the 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.