Florida Citrus Disease Updates

Plant pathologist Megan Dewdney on Nov. 20 presented updates on greasy spot, greasy-green, citrus black spot (CBS) and phytophthora brown rot. Dewdney is a University of Florida Institute of Food and Agricultural Sciences associate professor and Extension specialist at the Citrus Research and Education Center.

GREASY SPOT AND GREASY-GREEN

Dewdney reported that control measures for greasy spot, caused by the fungus Zasmidium citri-griseum, include copper and the strobilurin fungicides Abound, Gem, Headline, Amistar Top and Pristine. She stated that petroleum oil provides less consistent control on fruit than copper. Petroleum oil can be mixed with all other fungicides.

Greasy-green, which has been plaguing grapefruit growers, is caused by the same fungusthat causes greasy spot. Dewdney reported that suspected fungicide resistance may contribute to insufficient greasy-green management. She said greasy-green can’t be managed with post-harvest measures; it must be managed in the grove.

CITRUS BLACK SPOT

CBS has been found in nine Florida counties, mostly in the southwest part of the state: Collier, Hendry, Lee, Charlotte, Glades, Polk, Manatee, DeSoto and Highlands. The disease is caused by the fungus Phyllosticta citricarpa, which is abundant in leaf litter.

According to Dewdney, older trees are most at risk for CBS, but resets can be infected in two to three years in high-inoculum groves. Fruit is mostly susceptible to CBS from formation to September. All citrus is vulnerable to CBS, but late-hanging sweet oranges are most susceptible in Florida, she stated.

The goal for fungicide use on CBS is to maintain coverage on fruit, Dewdney advised. Growers should use a minimum of once-a-month applications for best control. Applications should start in May if April was dry; otherwise start applications in April.

Research conducted to determine if skirting might improve CBS fungicide programs indicated it doesn’t appear to help. However, there are other good disease-management reasons to skirt, Dewdney said.

PHYTOPHTHORA BROWN ROT

Fruit loss from brown rot is mostly in older, larger trees, Dewdney reported. Hamlin orange is most affected by brown rot. Navel oranges and grapefruit are affected, with brown rot showing up in the packinghouse box.

Cultural management of brown rot includes raising tree skirts to increase air movement and promote drying of foliage.

Brown rot treatments with Orondis, ProPhyt and Revus all resulted in less diseased fruit than was found in an inoculated control, Dewdney reported. She said copper is primarily protective against brown rot but can kill the inoculum.