Putting Down Brown Rot

By Megan Dewdney

Sometimes there is an unfortunate coincidence between the timing of fruit changing color and rain events. This can set up a situation where brown rot occurs unexpectedly and growers need to stop the disease after it has begun.

CONTROL OPTIONS

By this time of year, the traditional brown rot control measures are less effective. These are primarily applications of the phosphite-containing products [Fungicide Resistance Action Committee (FRAC) code P 07]. Their main mode of action is stimulation of tree defenses.

One option is copper applications. Copper has the advantage of killing inoculum that is in the fuzz that forms on the outside of the fruit. The protective copper coating is generally effective for 45 to 60 days. But if rainfall is heavy, copper may need to be reapplied at a half rate.

Alternate products are Revis (FRAC code 40) and Orondis Ultra (FRAC codes 40 and 49). These products are well placed in the later season if you have brown rot despite a treatment plan earlier in the season or you were surprised by a later occurrence of the disease. Fruit protection lasts at least 30 days.

Whatever product you choose, it is important to rotate among modes of action, including those used for root rot/foot rot. Please see the Florida Citrus Production Guide for usage patterns of fungicides and rates.

PHYTOPHTHORA DISEASES

It is important to remember that all of the phytophthora diseases are related. While there is not a direct relationship between brown rot and the root propagule counts, high counts increase the likelihood of brown rot when color breaks in susceptible cultivars. Fruit drop increases the amount of inoculum in the grove as the organisms can quickly colonize them and sporulate. The resulting zoospores and sporangia then are splashed into the canopy.

There are two organisms responsible for brown rot in Florida, Phytophthora palmivora and P. nicotianae. Only P. palmivora can infect the canopy 3 feet above the soil. This is why tree skirting is recommended if P. nicotianae is the primary cause, but it will also help slow P. palmivora.

Megan Dewdney is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.