Phytophthora Root Rot Management

Tacy CalliesDiseases, Tip of the Week

By Evan G. Johnson

Phytophthora root rot

As September winds down, the fall root flush is starting and will 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. Phytophthora spores are attracted to new root growth, stopping the growth and quickly spreading through the new growth, drastically reducing its 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 getting late to make a threshold-based decision from soil samples. 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. Find more information about phytophthora root rot 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. Fruit drop would be in the bottom third of the canopy if Phytophthora nicotianae is the main problem but could be canopy-wide in areas with a history of P. palmivora.

If you didn’t apply phosphite in late July or August and have a problem developing, copper sprays can prevent additional spread to protect the rest of the crop. The fall is expected to have normal rainfall, but it is hard to predict rainfall for the next couple of months, especially with the active tropical storm season.

If you are concerned about a possible extended rainy season, now is a good time for a second phosphite application. This recommendation also applies to grapefruit, but it is likely already under a copper-based canker management plan, which will protect against brown rot, too. Get more information about brown rot in the Citrus Production Guide.

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.