Category: Vegetable Pathology

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

1. 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.
2. 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.
3. Overseeding at a higher rate of seed/foot or seeding a bit more shallowly than normal can improve stands of direct-seeded crops.

Contributing Author: Dr. Anthony Keinath

Black spot, also known as Alternaria leaf spot, is considered to be the most important fungal disease on organic kale in South Carolina. It tends to be more prevalent in the fall than in the spring. The primary cause is the fungus Alternaria brassicicola (AB), found worldwide on brassica crops. In South Carolina and Georgia, a second species, Alternaria japonica (AJ), also is present. AJ is more cold tolerant than AB, so it is more likely to be found on fall crops or overwintered kale than on spring crops.

In October 2021, two fields at Coastal REC were each transplanted to 14 kale cultivars representing four types of kale: curly, Tuscan (lacinato), Siberian, and Portuguese. One field was organic certified, and one was conventional. The same rates of organic and conventional fertilizer were applied to each field. Transplants were spaced 1 ft apart in single rows on a 3-ft-wide bed with drip irrigation and white-on-black plastic mulch. Leaves were cropped in early to mid-December and again in late December to early January. Leaves were sorted into healthy and diseased categories, then counted and weighed by category. The diseased category included leaves with any leaf spots on them, even very small ones.

Average yields differed by the type of kale. Summed over both harvests, Siberian kale grew the fastest and yielded the most, followed by Portuguese kale, curly kale, and finally Tuscan kale. Cultivars did not differ much within kale type, except White Russian yielded more than Red Russian and Russian Royale when the yields from both harvests were added together.

Kale type also differed in susceptibility to black spot when plants were sprayed with spores of Alternaria. Portuguese was the most susceptible and had more diseased leaves, 70%, than any other type or cultivar. Tuscan was the least susceptible, while curly and Siberian were intermediate. Curly kale cultivars didn’t differ in susceptibility at the first harvest, but when regrowth was harvested, Oldenbor had more disease (57% diseased leaves) than the other four curly kale cultivars. The red curly kale curly Roja had less disease than the four green curly kales, but it also had low yields. Among Siberian kales, Red Russian had less disease than White Russian. Toscano Tuscan kale had more disease than cultivar Lacinato.

Another potential way to increase kale yields is to grow plants in double rows rather than single rows. At Coastal REC we did this in fall 2020 by transplanting two rows of kale on a 6-ft-wide bed with drip irrigation and white-on-black plastic mulch. The popular cultivars Darkibor and Winterbor were used. One plant at the end of each plot was inoculated with spores of AJ. Three plants in the middle non-inoculated portion of each plot were harvested, and the inoculated plant plus the plant next to it were also harvested. Leaves were sorted as described above.

Yield of healthy leaves was increased for both cultivars in double-row plots compared to single-row plots. This yield increase was observed on both inoculated and on non-inoculated plants. Black spot decreased yields of Winterbor in double rows but not single rows and not with cultivar Darkibor at either spacing. Double rows did not increase disease as much as might be expected because the hours kale leaves stayed wet was similar in both row spacings: 17.3 in single and 18.8 in double. This study demonstrates the importance of type and cultivar on disease prevalence. Selecting a type based on the balance of yield potential and disease resistance is an important part of an integrated management program for Alternaria black spot in kale.

This work was in part supported by the Organic Transitions Program, National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2021-51106-35495.