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.

Weekly Update – 9/14/20

Coastal

Zack Snipes reports, “The talk of the Lowcountry this week is transplant die off.  We see lots and lots of transplant die off and the culprits are usually mole crickets, ants, or cutworms.  Tunneling near the base of the plant is very common when mole crickets are present. Dr. Ayanava Majumdar from Auburn University has done some trials with parasitic nematodes and has seen very good results. For more information on the parasitic nematode visit https://www.arbico-organics.com/category/mole-cricket-controls?gclid=CjwKCAjw19z6BRAYEiwAmo64LSCtnQy6Dd1mx7TLTiNwB0-o7BBZ-XZd54LdBCtYuf2Jm6abINhyKRoChFIQAvD_BwE.  Ants are very common culprits of plant die off as well.  Timely applied baits are the best method for control.  Drench treatments make growers feel better but are not as effective as the baits.  Baits should be applied a few times a year between April and October. A few options to check out are Seduce Bait, Monterey Ant Control Pellets, Come and Get It, and PayBack.”

Transplant die off can occur from cutworms, mole crickets, ants, or disease pathogens.  Be sure to correctly identify before treatments are made.

Pee Dee

Tony Melton reports, “Harvesting sweet potatoes as fast as processing plant can handle them.  Getting good yields.  Army worms are bad!!!  Collards, turnips, and mustard are up and hauling butt.  Dry in some areas and wet in others.  Fall peas and butterbeans are doing well except where damaged by too much rain.  Cucumber for pickles are yielding well except where they were not sprayed for downy mildew.  Watermelons are yielding well where farmers where able to control the gummy stem and drowning during all the rain even then some plants were lost.”

Upstate

Andy Rollins reports, “Fall pepper crop is looking very strong.  Cool milder weather conditions have set us up for a very high yielding and high quality pepper crop.  I found some plants on this farm that died from being waterlogged in the lowest spot of one field.  I also found some fruit with side wall issues.  It was worse in the larger fruited varieties as is normal.  Many times this is misdiagnosed as sunscald but if you examine fully by doing leaf tissue tests you may find out as I did that this is a slight calcium deficiency or imbalance in the plant.  Yes, it is also called blossom end rot in tomato.  In pepper it shows up on the sidewall not just the bottom or blossom end.  Supplying the right amount of calcium is only part of the treatment as normally the problem is more often caused by the plant not being able to move the calcium not that it is missing.  Calcium is a large molecule and requires energy on the plants part to be taken up and distributed.  Careful frequent and regular watering has helped this farm keep this problem to a minimum.”

Pepper fruit with sidewall issues.
Pepper field with waterlogged area causing dieback.