The pathogen phytophthora has long presented problems for Florida citrus growers. But the HLB disease that has plagued Florida since 2005 reduces the efficacy of phytophthora management, citrus pathologist Ozgur Batuman reported in a recent virtual seminar. Batuman, who works for the University of Florida Institute of Food and Agricultural Sciences, said improved fungicide timing, number of applications and rates are needed to achieve better control.
Batuman reported that prior to HLB, phytophthora was estimated to reduce citrus yields in Florida from 3 to 6% per year, causing approximately $20 million in crop loss.
The pathogen species causing phytophthora diseases in Florida citrus are Phytophthora nicotianae, which is the common cause of foot rot and root rot, and Phytophthora palmivora, which causes brown rot of fruit as well as root rot in poorly drained soils with high water tables.
A slide in Batuman’s presentation indicated that wet conditions favor root infection cycles. It added that the susceptibility of roots to phytophthora is highest during very wet to very dry cycles.
Batuman addressed control measures for three types of phytophthora diseases in citrus:
Foot rot can be controlled with proper rootstock selection and by avoiding wounding and soil exposure of the scion. The graft union should be more than 6 inches above the soil. Foot rot kills bark and can girdle a tree.
Growers can control root rot with good drainage and irrigation, use of clean nursery material and by monitoring disease pressure.
Brown rot control includes eliminating fruit under the tree to reduce inoculum, raising tree skirts to increase air movement and promote drying of foliage, and application of preventive sprays of phosphite or copper. Time phosphite sprays in July. Spray in October if the fall rainy season is prolonged.
Brown rot mainly damages early citrus varieties. Fruit symptoms include light brown leathery decay and white fungal growth on the surface under humid conditions. Infected fruit have a sharp, pungent odor.
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