I recently received a call from a commercial vegetable producer in east central Missouri about continued problems that the farm experiences every year: phytophthora. Phytophthora can infect all cucurbits including pumpkins, squash, melons, cucumber, as well as beans and solanaceous crops like peppers, tomatoes, eggplants. The farm has a history of phytophthora infections because it can remain in the soil a long time and is driven by moisture. It travels through water, so planting in well-drained fields and raised beds can improve drainage. Phytophthora blight causes fruit rot, rapid wilting, and plant death. At my farm visit, we saw plant death in squash and peppers. This problem was more prominent in low lying areas that can experience flash flooding from a creek that wanders through the entire farm.

Healthy Butternut squash (left) and empty rows of acorn squash (center and right)

The progression of symptoms of phytophthora is visible in this row

Phytophthora blight is caused by the water mold Phytophthora capsici. Spores from the disease resemble powdered sugar on the plants. The “powdered sugar” is made up of millions of spores that release smaller swimming spores in water. The spores can be moved long distances through water. Movement can be through standing water from heavy rain with poor drainage, flash flooding, irrigation, or even between plants from wind-blown rain. The swimming spores are attracted to plant roots as they move through the water, spreading the infection. They can also be moved from field to field in soil on tillage equipment or tires. We can also aid in its spread by transporting infected plant material from place to place. I made sure I cleaned my boots and washed my hands before I went home after that farm visit!

Close-up of infected acorn squash plants

Distribution of infection visible across the entire field

What should you do to prevent phytophthora from becoming a problem on your farm? Never dump cull fruit or plants in production fields, and if you use compost, obtain it from a trusted source. Promote good drainage and do not over irrigate, used raised beds, plant tolerant varieties, and rotate plantings. If you have confirmed that phytophthora is present on your farm, prevent it from spreading around the farm and into irrigation sources, dispose of infected plants, harvest early from infected fields, and use fungicides according to the label.

Interesting observations from the farm visit include that butternut squash plants (Cucurbita moschata) did not show signs of severe infection evident in acorn squash (Cucurbita pepo). It also was more of a problem in hot peppers than bell peppers.

You can contact your local MU Extension horticulture specialist for assistance or submit a plant sample to the MU Plant Diagnostic Clinic.