By: Anthony Keinath, Vegetable Pathologist, Clemson Coastal REC
Originally posted on SC Grower: https://scgrower.com/
“Damping-off” refers to root and stem rot on young seedlings of direct-seeded crops. “Stem and root rot” is used to describe the same disease on transplanted crops. I use the term “seedling disease” when information below applies to both types of crops.
Stem and root rot on a watermelon transplant. Note the brown, water-soaked, discolored area on the stem just above the soil line.
Stem and root rot on a tomato transplant. Note the tan, collapsed area on the stem just above the soil line.
In South Carolina, the water mold Pythium is the most common cause of seedling diseases, especially on cucurbits, tomato, and pepper. The fungus Rhizoctonia also occurs on seedlings of certain crops, including beet, onion, radish, arugula, and head and stem brassicas (broccoli, cabbage, cauliflower) along with collard and kale. (Note that head and stem brassicas, collard, and kale are, botanically, Brassica oleracea, which is more susceptible to Rhizoctonia than mustard, Brassica juncea). Most vegetable crops can be infected by both pathogens. For example, 49% of diseased arugula seedlings were infected by Pythium and 54% by Rhizoctonia on an organic farm in the Lowcountry in an on-farm study in 2011.
At a recent vegetable grower meeting, summer squash was voted the crop most susceptible to damping-off out of a list of direct-seeded crops that included beets, onions, parsley, and cilantro. Squash and other cucurbits are susceptible to at least 9 species of Pythium, 3 warm-season species and 6 cool-season species. Thus, squash and other cucurbits are susceptible to seedling diseases year-round.
Recovery of Pythium species from transplants of cucumber, watermelon, Hubbard squash or bottle gourd, 2017 to 2019, Charleston and Lexington counties.
Fungicides and Biofungicides
The standard conventional fungicide Ridomil Gold applied via drip irrigation is recommended to reduce seedling diseases. Research in another state showed that Ridomil Gold is more effective applied as soon as possible after transplanting rather than several days or weeks later. Ridomil Gold will not completely prevent seedling disease, but, typically, it reduces the number of diseased plants. Biopesticides and NOP/OMRI organic-approved products generally have little effect on seedling disease in the field. They are useful in the greenhouse to prevent damping-off in seedling flats.
Cultural Practices
Plastic mulch is such a common production practice that I almost forgot how much it significantly reduces stem and root rot on transplants. I was reminded, in dramatic fashion, in an experiment in cooperation with the University of Georgia in spring 2015 that included mulched and not mulched plots of transplanted seedless watermelon. It’s very possible that some growers are seeing so much damping-off on summer squash because it is typically planted on bare ground.
Stem and root rot of seedless watermelon transplants in a field naturally infested with multiple species of Pythium at Coastal REC, Spring 2015
Resistance
No cultivars of any vegetable crop have genetic resistance to seedling diseases. As the graph below shows, however, there may be some differences among cultivars that could be called “field resistance,” a term used when certain cultivars have less disease under field conditions than is expected based on greenhouse testing.
Seedless watermelon cultivar Melody had significantly less Pythium stem and root rot than Citation and Fascination, Coastal REC, Spring 2015
Recommendations
- Transplants generally have less seedling disease issues than direct-seeded crops, because all vegetable crops become more resistant to stem and root rot as they age. This is not to say that stem and root rot doesn’t affect older plants, but it’s less likely.
- If you replace transplants that died from stem and root rot, do not set the new transplant in the same hole. Move the planting hole at least 6 inches away, because there might be fewer pathogen propagules in a different spot. The numbers of pathogens in soil is very variable.
- Overseeding at a higher rate of seed/foot or seeding a bit more shallowly than normal can improve stands of direct-seeded crops.
