What’s Up With Citrus Fruit Drop?

Photo by Frank Giles

Photo by Frank Giles

Over the past two seasons, the Florida citrus industry has seen an unprecedented amount of preharvest fruit drop. Data from the National Agricultural Statistical Service (NASS-USDA) has determined that previous annual preharvest drop rates averaged about 11%. Drop rates were 18% and 27% and 23% and 19%, respectively, for early-mid and Valencia fruit in the past two harvest seasons. In our most recent studies in 2013-2014, individual blocks were found to have from 10% to 60% preharvest fruit drop. The annual value of the additional fruit loss was estimated at $150 million in 2013. A major question is what is causing this excessive fruit drop? It is likely that a complex of factors, including tree HLB status, is contributing to the preharvest fruit drop problem.

The general or classical physiology of fruit abscission is that both the formation of the abscission zone between the fruit and stem (peduncle) at the button and also the weight of the fruit with minimal fruit movement (as from light wind), is sufficient to cause the fruit to drop. The abscission zone is a natural tissue layer, and its final development occurs when enzymes that dissolve cell wall components are up-regulated, usually by ethylene. Ethylene is produced by the plant in response to stress. Physical injuries, freeze damage, water stress, and other stresses such as decay organism development in the fruit lead to ethylene production. We have not determined that this is the process by which preharvest fruit drop occurs on HLB infected trees, but the typical abscission zone does develop.

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The Usual Suspects

While many factors can lead to fruit drop, the following are the most likely culprits from problems the past two seasons.

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Water Stress: In the late spring, we have measured midday water potentials of -22 to -25 bars for declining trees compared to the healthiest adjacent trees which were -12 to -16 bars on irrigated Valencia trees. Water stress stimulates the production of the ethylene precursor aminocyclopropane-1-carboxylic acid (ACC) in the roots, which travels via the xylem to the fruit and leaves. There the ACC is transformed to ethylene, which triggers the adverse enzyme up-regulation. Both excessive fruit drop and leaf drop have been observed during the preharvest period.

An important factor in the development of water stress is the poor root systems observed on HLB-infected trees. Even though severely HLB declining trees have much fewer leaves than healthier trees, the higher water stress in HLB trees indicates that the root system is even more severely affected than the canopy.

Carbohydrate Limitations: Another possible factor that results in stressed fruit is the reduced carbohydrates available for normal fruit maintenance and growth of HLB-affected fruit. The area around the abscission zone is likely to have lower carbohydrates with HLB infection than healthy fruit because of the restricted phloem movement from leaves and the reduced photosynthates produced by mottled leaves. Necrosis around the button of many of the preharvest dropping fruit may be a symptom of low carbohydrate availability. Easy development of decay organisms in fruit and the peduncle also indicate that carbohydrate levels are too low for normal plant resistance to weak pathogens like diplodia.

Bacterial Toxicity: Although no one has demonstrated any toxic factors produced by the HLB bacteria, still there could be a direct adverse response from bacteria living in the phloem near the fruit abscission zone through a toxin or
other mechanism.

Other Stresses: Although additional stresses that might be associated with HLB infection may exist, they are hard to quantify. Perhaps HLB trees are more sensitive to high temperatures in the fall or spring when early or late maturing varieties are near harvest. It is probable that any stress the tree is under, coupled with HLB stresses, exacerbates the preharvest drop problem.

Breaking The Fall

Some growers are having better success than others in stemming yield losses to fruit drop. While everybody has their own formula, the following actions may help reduce drop in citrus groves.

Herbicide Interaction: Several growers reported avoiding a herbicide application in the fall to clean up the under-tree floor for harvest reduced preharvest drop. Several growers have seen some drop associated with glyphosate application, but it may be due to equipment hitting low limbs or direct spray on the fruit. We have no confirmation of this effect yet, but a test on Hamlin trees is under way.

Apply Plant Growth Regulators (PGRs): Although PGRs were not successful in the vast majority of tests last season, two tests significantly reduced preharvest fruit drop. In Hamlins, ProGibb (gibberellic acid, Valent) reduced drop about 5% in one test. In a Valencia test, Citrus Fix (2, 4-D, IPE, AMVAC) reportedly reduced drop when applied in December. Earlier and later applications of a variety of PGRs in our tests did not significantly reduce drop, but Citrus Fix and gibberellic acid in combination often gave numerically lower fruit drop values than the control. The tree to tree variation was very high. So even though the numerical values were occasionally 5% to 10 % lower, they were not significantly different from unsprayed controls.

Minimize Or Eliminate Stresses: It appears that a major factor with Swingle- and Carrizo-rooted trees is susceptibility to high pH in the soil and water and irrigation water carbonates. Correcting these problems will presumably improve root health and density. This may be a major factor where nutritional enhancement sprays are not working because the trees are under water stress and needed fertilizer uptake from the soil is not complementing foliar applications.

Improve Irrigation Scheduling: Since water stress appears to be a major component of the stresses associated with HLB, more frequent light irrigation in the dry warm fall and spring periods is suggested. The root system is greatly reduced and the period when sufficient root contact with moist soil occurs will be shorter. With more frequent irrigation, duration of each application must be shorter to stay within water-use guidelines. Several growers have gone to more frequent, lighter irrigation schedules and some report preharvest fruit drop appears to have been reduced.

Headline Effect: Last season, one production company applied the strobilurin Headline (pyraclostrobin, BASF) to 20 acres of Valencia trees and compared it to 20 untreated acres. They reported a large reduction in fruit drop when Headline was applied in March and fruit drop was measured over one month into April. Pounds solids also appeared to be higher in the Headline-treated area. This was a single test, but a number of growers have treated Hamlin blocks and replicated trials also are in place. Of great concern is the potential development of fungal resistance to such products if they become more widely used.

Word Of Caution

A general concern with reducing preharvest fruit drop is whether the tree can manage a larger crop and sustain itself against the debilitating effects of HLB. Enabling a declining tree to retain fruit that normally would have been dropped may hasten the decline of the tree. During the preharvest period, there is little additional fruit growth although sugar accumulation still occurs until harvest. Additional fruit may not be a large sink during this time period but adding more stress factors could contribute to tree debilitation. Still, it is likely any successful treatment that improves tree health will lead to more leaf retention and better root growth as well as reduced preharvest fruit drop.

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Avatar for Dot Remington Dot Remington says:

I have a backyard juice orange tree. This year it is dropping fruit early. I have noticed that there are tiny rust colored spots on the inside of the peel. It makes me think an insect has “stung” the oranges. Often there is small dry area of pulp next to this rusty dot. I have been eating these oranges but I would like to know what is causing this premature drop and what I can do next year to prevent this problem. Thanks for any recommendations.

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