Manage fungicides to cut chances of Phytophthora resistance

07/10/2013 10:07:00 AM
Pamela Roberts and Ryan Donahoo

Phytophthora capsici infests a wide host range, including squash. Sampling of fields showed some populations were shifting to become less sensitive to mefenoxam.

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.


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