Protecting Trees From Phytophthora Disease

Tacy Callies Diseases, Tip of the Week

Phytophthora root rot

By Evan G. Johnson

It is time to start thinking about protecting the fall root flush that will start soon and continue through November/December. This is the largest root flush of the year that stores carbohydrate reserves for next spring. Coming out of the rainy season, phytophthora has had the chance to build up plenty of inoculum, especially in groves with a history of root rot problems.

Of particular concern this year is the high prevalence of Phytophthora palmivora across Florida, but especially in the western flatwoods to the southern ridge. Phytophthora spores are attracted to new root growth, stopping the growth, quickly spreading through the new roots and drastically reducing fall root lifespan.

There is still time to protect the largest root flush of the year that will provide the water and nutrient uptake for the spring leaf flush and flowering. If you haven’t been monitoring your phytophthora populations in your groves, it is time to take soil samples for threshold-based management decisions.

Normally, products with higher efficacy than phosphite are recommended for protecting the fall root flush because of the inoculum buildup that happens over the summer. More information about phytophthora root rot can be found in the Citrus Production Guide.

For susceptible early-maturing varieties like Hamlin and Navel, brown rot might have already shown up and could be causing a fruit drop problem. Infection and fruit drop would be in the bottom third of the canopy if P. nicotianae is your main problem. However, it can spread from fruit to fruit to the top of the canopy with P. palmivora propagules in the soil.

If you didn’t apply phosphite in late July or August or have a problem developing, copper sprays can prevent additional spread to protect the rest of the crop. If you have high soil inoculum, a copper or Revus spray could help protect the fruit as phosphite efficacy drops.

The fall is expected to have normal rainfall, but it is hard to predict rainfall for the next few months, especially with the active tropical storm season. If you are concerned about a possible extended rainy season, you may want to plan a second phosphite application 60 days after the July application.

While grapefruit is also susceptible to brown rot, it should already be protected by a copper-based canker management program if it is for fresh market. More information about brown rot can be found here.

Evan G. 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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