By Evan Johnson
The Florida citrus spring leaf flush is hardening off, which means the first root flush of the year is beginning. With the spring root flush comes thoughts of phytophthora foot rot. Spring roots are at particular risk in groves that had high phytophthora pressure last fall because many of the resting spores will start activating with the warmer weather.
So far this spring, Florida has not had much rain. A dry spring could be a good thing for phytophthora management. Depending on irrigation cycles, roots may have been exposed to severe wet-dry cycling that causes roots to leak sugars. The swimming zoospores of phytophthora are attracted to these sugars and are activated during the wet periods.
Therefore, it is time to start sampling for phytophthora propagule counts to see if a damaging population is present. Ten to 20 propagules per cubic centimeter is concerning, and treatment should be considered if there is a history of phytophthora problems in the block. More than 20 propagules per cubic centimeter of soil is considered damaging and should be treated.
Phytophthora root rot management now will reduce the pressure later as the wet season arrives and when the major root flush of the year occurs in the fall. Root rot management is particularly important on varieties susceptible to brown rot, which includes Hamlin and Navel sweet oranges and grapefruit. Managing root rot early will reduce inoculum pressure for brown rot later in the season.
For the spring root flush, phosphite-based materials are low cost and provide sufficient efficacy to keep infection down and reduce propagule buildup early in the season. However, it is best to rotate phosphites with other active chemistries as the season continues.
It’s recommended that groves with damaging populations of phytophthora get four treatments per year. Spring and midsummer treatments are usually phosphite-based because the rotation allows protection against early brown rot with the midsummer phosphite treatment. For early wet season and fall (after the last leaf flush) treatments, another active ingredient, such as mefenoxam, fluopicolide, or oxathiapiprolin, is recommended. It is best to determine your root rot pressure now and start planning for the season before it grows out of control.
Evan 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.