Reduce Fruit Drop From Phytophthora-Induced Brown Rot

By Chris Oswalt

During this time of the year, Florida growers should be aware of the potential for the development of citrus brown rot disease. It can infect fruit at color break. If severe, the disease can cause a significant amount of infected fruit to drop. For this reason, groves with a history of brown rot in areas that recently received a significant amount of rainfall should consider an application of an effective fungicide for the control of citrus brown rot now.

The disease is caused by Phytophthora nicotianae and Phytophthora palmivora, the same fungal-like organisms that cause foot and root rot. Early-maturing sweet orange varieties (like Hamlin) are the most susceptible. The disease can be problematic from mid-August to October and is usually associated with poor drainage and/or limited air movement under the tree canopy. P. nicotianae infects fruit in the lower third of the tree canopy, while P. palmivora infects fruit throughout the tree canopy. At this point, growers with brown rot don’t care what species of Phytophthora they have. Instead, they are concerned with how to stop the fruit drop.

DIFFERENT DROP CAUSES

It is extremely important to correctly diagnosis brown rot from other causes of citrus fruit drop. Today, HLB-induced pre-harvest fruit drop is a significant concern.

During past seasons, some fruit drop has been attributed to scale insects feeding on the fruit calyx. Excessive rainfall in the late fall and early winter could lead to fruit splitting and fruit drop. In the fall, plant bugs can also cause a certain amount of fruit drop on tangerine varieties.

However, brown rot symptoms are unique and include fruit drop in late summer and early fall. The peel will appear leathery with an olive brown discoloration, and the fruit will have a distinct rancid odor. If it is humid at ground level, fluffy white fuzz can grow on the fruit surface.

PREFERRED PRODUCTS

Current University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) brown rot recommendations call for the application of an effective fungicide. Copper, Alliette, Phostrol, ProPhyt, Revus and Orondis Ultra are currently recommended for disease control.

In general, a single application of the systemic Alliette, Phostrol and ProPhyt fungicide prior to the onset of visual symptoms in late July will control the disease. They will provide fruit protection through the normal infection period (60 to 90 days).

If you missed that application or have the onset of symptoms, then copper applied in August will provide protection for 45 to 60 days. Revus and Orondis Ultra will protect fruit for at least 30 days and can be applied after symptom appearance but work best when applied before the onset of symptoms.

Chris Oswalt is a UF/IFAS citrus Extension agent for Polk and Hillsborough counties.

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