Timely Diagnosis

From Clemson Plant Pathologist Tony Keinath.

Plants are a rich source of food for microorganisms—for aggressive plant pathogens, for weak pathogens, and for common saprophytes (the 90% of microorganisms that feed only on dead plant parts). Once a pathogen kills parts of leaves or side roots and we see dark brown spots, things change. The normal ways plants defend themselves from pathogens shut down, and it’s a “free for all” for any microorganisms on the leaf, fruit, or root. These weak pathogens and saprophytes suddenly can grow on parts of the plant that weren’t accessible to them when the plant was healthy. It’s part of the natural succession of the phytobiome, the community of microorganisms that live in, on, and around plants.

Although Alternaria leaf spot can still be recognized by the round, brown spots with concentric rings, saprophytic Alternaria growing on the yellowed areas makes diagnosis more difficult.

For a diagnostician, it’s infinitely easier to find a pathogen at the early stages of disease, when suspicious yellow spots appear on a leaf, rather than on a dead leaf, because you’re not “looking for a needle in a haystack,” or more accurately, a pathogen in a forest of microorganisms. Cucurbit leaves, for example, tend to have the same three saprophytes on them, Alternaria, Epicoccum, and even Fusarium. An experienced diagnostician knows to ignore them or use them as forensic clues that the tissue has been dead for some time. I tend to suspect spray burn when I see these fungi growing in distinct spots on leaves.

Every diseased plant sample collected for diagnosis, whether it is for in-person delivery or mailing, should be carefully selected and handled by following these guidelines. Samples collected during the summer or on sunny days at any time of the year should be placed in a cooler with an ice pack after collection. This usually means taking the cooler into the field. Proper handling of samples will help the diagnostician provide an accurate answer or avoid asking for another (better) sample.

As another saying goes, “early is on time.” An early diagnosis is a timely diagnosis.

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.