Not Planting to Manage Phytophthora Blight by Reducing Disease Risk

From Clemson Plant Pathologist Anthony Keinath and Clemson Agricultural Economist Felipe Silva.

After an absence of several years, Phytophthora blight reappeared in South Carolina in July 2021 on three farms. As expected, the outbreaks were on two of the three most susceptible vegetable crops: 2 cases on pepper and 1 case on pumpkin. (Summer squash is the other very susceptible crop.)

A white powdery layer of Phytophthora capsici spores covers this pumpkin fruit. A watery rot will soon follow. Photo from Dr. Mary Hausbeck, Michigan State University

Five management options are listed in Land-Grant Press 1014: Managing Phytophthora capsici Diseases on Vegetables. One of the recommendations under Soil Management is to not plant low areas in an infested field, because that is where the Phytophthora blight pathogen, Phytophthora capsici, will first become active. Rain or overhead irrigation will spread the pathogen to the rest of the field. Remember that once a field is infested with Phytophthora capsici, The. Field. Remains. Infested. Period.

Based on the calculations shown in Table 1, a grower that considers the likelihood of disease outbreaks and plants only the well-drained 4.5 acres in a 5-acre infested field would earn an expected average of $1,600 per 5 acres per year. A grower who does not consider the disease occurrence pattern and plants all 5 acres can expect an average loss of over $2,600 per year (see the column labeled “Avg. Net Return”). These estimates consider the likelihood of disease occurrence based on the different planting sizes and areas.

The difference between planting 5.0 vs. 4.5 acres in an infested pumpkin field totals a positive net return of $4,300 per 5 acres. Why does not planting—and forfeiting yield—make more money? Assume that an outbreak of Phytophthora blight reduces the entire farm yield by 50%. Not planting the low area of the field, where disease is likely to start, will cut the number of outbreaks of Phytophthora blight in half (see the column labeled “Disease likelihood”), decreasing the chances of an outbreak from 60% to 30%. This estimate comes from vegetable pathology colleagues in other states who have worked on Phytophthora blight for many years.

Even in an infested field, by reducing the disease risk, the expected net return increases by more than $4,000. Note that over half of the gain in profit comes from reducing input costs by not planting the 10% of the field that probably will not yield anything. Although this example is calculated for pumpkin, the risk of the pathogen spreading from diseased peppers in a low spot in the field is just as likely or greater, because the pathogen produces spores readily on the fruit. Reducing disease risk is the key to increasing profits.

Fusarium Wilt in Watermelon

From Clemson Plant Pathologist Tony Keinath.

Fusarium wilt is showing up in watermelon fields and in research plots at Coastal REC in Charleston. The most telltale symptoms are wilting of a few leaves at the crown of the plant, wilting of one vine on a plant, or wilting without yellowing of a small plant.

One vine of this watermelon plant has wilted. This is a telltale sign of fusarium wilt.

A good field diagnostic trick is to cut a wilted vine close to the crown, split it open lengthwise, and look for reddish brown spots on the crosswise cut or streaks in the lengthwise cut.

Cross section of a watermelon stem showing the discolored, reddish brown spots.

The Fusarium fungus is most active when the soil temperature is below 81 degrees F. Although daytime temperatures were warm in April, the nights were still cool enough to allow infections. At this point, there is nothing that can be done to mange Fusarium wilt. All successful management practices must be done before transplanting. See: Keinath AP. Integrated Management for Fusarium Wilt of Watermelon. Land-Grant Press by Clemson Extension. 2019; LGP 1022.