By Megan Dewdney
It is summer in Florida, and the early fruit are maturing. This is always a risky time for brown rot in early sweet oranges and grapefruit cultivars. The summer rains have been happening, but they have been intermittent depending on location. This means that phytophthora propagule counts may vary depending on how much rain has occurred in your area.
So, what does this mean for brown rot? If your phytophthora root propagule counts are high, you are more likely to have brown rot on cultivars at color break. If you are unlucky, and conditions are perfect, brown rot can cause 100% yield loss.
Both Phytophthora palmivora and P. nicotianae can cause brown rot. P. palmivora is a greater concern for brown rot because of its ability to splash from fruit to fruit, causing infection all the way to the top of the canopy. However, P. nicotianae can be very destructive in unskirted trees, but infection stops at approximately 3 feet above the soil.
Management is going to depend on species present in your soil and what other problems you are actively treating. Most grapefruit are aggressively managed for citrus canker with a copper program. If this is the case, grapefruit should not need to be managed for brown rot as the copper will be effective for both diseases. In early oranges, canker management stops earlier in the season, so other treatments will be needed. Copper needs to be reapplied for brown rot management every 21 days. The advantage of copper is that it can protect healthy fruit in the canopy if there is infection already present.
July is the best time for phosphite applications, but you may be able to sneak one in if the rains hold off for a few weeks. This is because phosphites need to be applied in advance for the best brown rot protection. If you were treating in late June or early July for algal spot, this is likely to also help with brown rot. 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.
Two other possible products to use are Orondis Ultra and Revis. There is trial work to show efficacy for up to 30 days. University of Florida Institute of Food and Agricultural Sciences researchers are hoping to complete some trial work to give these products full recommendations in the near future.
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.
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