By Evan Johnson
The Florida summer rainy season is in full swing, and there have been one or two root flushes by now. This is a good environment for increases in soil Phytophthora propagule counts. High counts have been seen this year, especially in Southwest Florida.
While much of the focus may be on root rot, it is important to remember brown rot as well. Depending on variety, brown rot can cause total yield loss. Early sweet orange varieties (like Hamlin) and grapefruit are becoming susceptible to brown rot.
Both Phytophthora palmivora and P. nicotianae can cause brown rot, but P. palmivora is the largest concern because of its ability to splash from fruit to fruit, infecting fruit all the way to the top of the tree. P. nicotianae can still be a significant problem on unskirted trees.
Knowing which species is building up in your groves is important when considering your management aggressiveness. Most grapefruit are well protected by copper applications for citrus canker and will not need additional brown rot management. However, Hamlin canker management normally stops in June. Last year, multiple growers faced near total yield loss from brown rot in their Hamlin blocks.
July is an ideal time for a phosphite application. Phosphites need to be applied in advance of infection but provide the most durable brown rot protection. One July application of phosphite normally gives good control through an average rainy season and helps keep root rot in check. If rains continue into late September/October, another brown rot treatment may be needed.
Copper can reduce brown rot but needs to be reapplied every 21 days. However, it can protect healthy fruit once rot has already occurred on some fruit in the canopy. The Revus label was recently expanded to include a foliar spray for brown rot and offers some control up to 30 days, but there is limited experience with these new modes of action in Florida citrus.
Evan Johnson is a research assistant scientist at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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