Best Practices for Brown Rot Management

By Megan Dewdney

The Florida rains have finally come this summer, just in time for early-season fruit to start the maturation process. This means there is some risk of brown rot in early sweet oranges and grapefruit. Much of Florida has been getting summer storms, wetting the soil. While this is excellent in general, the downside is that it allows phytophthora to move more easily and multiply in moist soil conditions.

Brown rot is most problematic in early harvested cultivars because of the coincidental timing of the Florida summer rains and the ripening process or color break. For brown rot management, it is important to maintain records of whether this disease has occurred in previous seasons in particular blocks and also whether you have a history of root rot.

Phytophthora root rot, crown rot and brown rot are caused by the same two organisms, Phytophthora nicotianae and P. palmivora. While there is not a direct relationship between the number of soil propagules of these organisms, higher counts increase the likelihood that there will be brown rot in sensitive cultivars. Both species are capable of causing brown rot, but usually P. palmivora is considered more serious because of its ability to splash from fruit to fruit, high into the canopy. P. nicotianae can be very destructive in the lower canopy, but infection stops at approximately 3 feet from the soil because it does not move as easily in the air and relies on soil splash to get into the canopy. This is the rational for skirting problematic blocks.

There are multiple management options for brown rot available. However, timing is important. If you are regularly applying copper (every 21 days) for other diseases like canker throughout the summer, it is unlikely that brown rot will be a problem. If your canker program stops early in the season, then other management practices are needed.

Phosphites and Aliette [Fungicide Resistance Action Committee (FRAC) P 07] are best applied in July as they need time to stimulate the plant defenses. If you are applying a phosphite for algal spot in late June or early July, this will likely help with brown rot as well. It is also well timed for protecting your root system from root rot.

In an average year, one phosphite application is often sufficient for brown rot management. If the rains extend into the fall, then another treatment is likely to be needed.

When possible, rotate fungicides. Two newer products are available, Revus (FRAC 40) and Orondis Ultra (FRAC 40 + 49). Their efficacy is at least 30 days but likely longer. These products work very well in the later season. They can be used earlier if so desired, but application of another product may be needed at the end of the season.

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.