Symptoms and Management of Brown Rot and Greasy Spot

University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) presenters on Aug. 30 discussed symptoms and management of the fungal citrus diseases brown rot and greasy spot. Plant pathologist and associate professor Megan Dewdney, the featured speaker, focused on management. Multi-county citrus Extension agent Mongi Zekri, who hosted Dewdney’s talk, described disease symptoms.

BROWN ROT

Brown rot is usually most severe on early-maturing sweet-orange cultivars, Zekri said. It commonly appears from mid-August through October on mature or nearly mature fruit following extended periods of high rainfall.

Lesions on rinds are brown or tan and firm or leathery. If later colonized by secondary decay organisms, the diseased fruit soften. At high humidity, brown rot-infected fruit become covered with white mycelia. Zekri said infected fruit usually drop and have an unpleasant odor.

Dewdney added that most fruit loss due to brown rot occurs in older, larger trees. Hamlin oranges are most affected. She said in navel oranges and grapefruit, infection often shows up in the packinghouse.

Cultural management of brown rot includes raising tree skirts to increase air movement and promote drying of foliage, and to lower the risk of organism movement into the canopy, Dewdney said.

For fungicidal management, Dewdney said only a single spray is often needed in late July prior to disease symptoms. Recommended plant defense-stimulating products are Aliette, Phostrol and ProPhyt, which provide 60 to 90 days of control.

Growers using copper can apply it before or after symptoms are visible, Dewdney suggested. She said copper is primarily protective against brown rot but can kill the inoculum and provide 45 to 60 days of control. An October copper spray is needed in very rainy conditions.

Revus or Orondis Ultra are good choices for late-season application if growers missed the earlier application window. They provide at least 30 days of protection, Dewdney said.

GREASY SPOT

Greasy spot lesions appear yellow or dark brown to black on the underside of mature citrus leaves, Zekri reported. Affected leaves fall prematurely from the tree during the fall and winter, resulting in reduced tree vigor and yield.

Zekri said full spray coverage of the underside of leaves is necessary for best control of greasy spot. Full coverage can be achieved with higher spray volumes and slower tractor speeds.

Dewdney said Enable is a good fungicide substitute for copper if growers are concerned with stippling. Other fungicides effective against greasy spot are Double Nickel and Miravis Top.

Zekri and Dewdney also discussed algal spot symptoms and management during their Aug. 30 presentation. Learn more here.

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