Editor's note: A special thanks to Dr. Monica Ozores-Hampton at the Southwest Florida Research and Education Center for coordinating The Immokalee Report, of which the article below is part.
Plant pathogens in the genus Phytophthora cause significant losses in vegetable and fruit crops in Florida, particularly when conditions are wet.
Late blight, caused by Phytophthora infestans, can be very difficult to manage on tomato and potato crops.
Such was the case during the past two seasons when, coupled with other production issues, tomato fields were lost or abandoned. Phytophthora blight on vegetables, caused by P. capsici, is particularly severe on summer squash since it’s highly susceptible.
The crown and fruit of squash can be infected and the entire plant dies.
In citrus, P. nicotianae affects tree health, causing foot rot and root rot.
The genus Phytophthora is classified as an oomycete and is not true fungi, but it acts like fungi. These pathogens also are called water molds because wet conditions favor their development.
When it is wet following host infection, these pathogens rapidly produce an abundance of sporangia (spores) that each release dozens of zoospores.
The zoospores are motile swimming spores that use chemotaxis—a chemical stimulus that directs their movement—to find and then infect their host.
Favorable conditions along with large rain storms can result in multiple cycles of infection and spore production.
This makes diseases caused by Phytophthora species notoriously difficult to control.
Single-site modes of action
Fungicides are an important tool for managing diseases caused by Phytophthora species. The most effective products are those that can move within plants.
Unfortunately these are prone to resistance developing because they act at a single site in the pathogen.
Selection for resistance in the pathogen population can result in the fungicide becoming less effective or even completely ineffective (full resistance).
Some of the best tools are the phenylamide fungicides, which are commercially available as mefenoxam and metalaxyl.
Characterization of mefenoxam resistance in P. infestans, P. nicotianae and P. parasitica through inheritance studies is consistent with a single gene conferring resistance, although more recent studies suggest that the genetics might be more complex, particularly in P. infestans.
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.
Pamela Roberts is a plant pathology professor at the Southwest Florida Research and Education Center in Imokalee. She can be reached at firstname.lastname@example.org. Ryan Donahoo is a plant pathologist at the Institute of Food and Agricultural Sciences, Gainesville.