Gauging Phytophthora resistance
Our lab collected isolates of Phytophthora from tomato, potato, squash and citrus grown in South Florida. These isolates were examined for several key characteristics, including sensitivity to mefenoxam.
In 2012 and 2013, more than 150 isolates of P. infestans were collected from tomato and potato plants in Florida.
In 2012, isolates ranged from sensitive to full resistance, with the latter more common compared with previous years.
This occurred even though mefenoxam-based fungicides were not reported to be widely applied for this disease.
In 2013, on the other hand, only sensitive and intermediate sensitive isolates of P. infestans were identified.
Therefore, mefenoxam sensitivity cannot be reasonably predicted from previous season results so isolates during the epidemic must be tested for resistance.
Isolates of P. capsici recovered during the initial disease outbreak in squash fields were sensitive to mefenoxam.
But a shift toward intermediate sensitivity (able to grow on media amended with mefenoxam but reduced in growth by greater than 30 percent but less than 60 percent compared to growth of the same isolate on non-amended media) was detected among the more than 100 isolates recovered at the end of the season after mefenoxam applications.
Resistance found in citrus, too
Soil sampling in citrus groves at several locations for characterization of P. nicotianae populations also revealed a full range of mefenoxam sensitivity, including full resistance found in soil from blocks of young trees (3 to 4 years old), blocks containing mature plants and resets, and also blocks containing only mature trees (more than 20 years old).
All levels of mefenoxam resistance were characterized in populations of P. infestans, P. capsici and P. nicotianae occurring on tomato and potato, squash and citrus, respectively, in the past two years.
In vegetables, several fungicides with diverse modes of action are labeled for Phytophthora diseases that can be used to manage late blight and Phytophthora blight (see Vegetable Production Handbook for Florida).
In citrus, choices of fungicides are limited to mefenoxam/metalaxyl (Fungicide Resistance Action Committee Group 4) or phosphite (FRAC 33) for control of soilborne Phytophthora.
Management of diseases caused by oomycetes requires cultural control measures. such as management of soil moisture for the soil-borne phase as well as specific fungicides.