Trouble for the Sweet Green of the South

Collard greens are a staple of southern cooking. Slow cooked with the salty flavor of ham hocks and served up with golden corn bread, collards make the perfect meal for a cool winter’s day. Collard greens belong to a group of loose-leaf cultivars (non-heading) of Brassica oleracea. Other members of Brassica oleracea include cabbage, broccoli, Brussels sprouts and cauliflower. These leafy greens are believed to have descended from Asian wild cabbages and eventually made their way into Europe where ancient Greeks and Romans grew the plants in domestic gardens over 2,000 years ago. When colonists settled in the New World, greens were a major part of any well-rounded vegetable garden. In the South, collards were abundant and cheap and quickly became a main ingredient in soul food inspired dishes that southerners still enjoy today.

Lately, we have seen quite a few collard and cabbage samples in the clinic. This season, white mold has been quite prevalent in both home gardens and commercial fields. White mold is caused by the soil-borne fungus Sclerotinia sclerotiorum. This disease is favored by cool, wet conditions and most infections occur in late winter to early spring. Early symptoms of the disease are tan, water-soaked, circular areas on the leaves or crown. These areas quickly become covered in fluffy white fungal growth, followed by a watery breakdown of infected tissue. Eventually, the fungus completely colonizes the head and produces large, black, seed-like structures called sclerotia.

White mold (Photo: E. Lookabaugh)

Sclerotia can survive in the soil for many years. Cool, wet conditions trigger the sclerotia to germinate. Sclerotia can germinate to form mycelium that directly infects plant tissue, or to produce small cup-shaped apothecia (fruiting bodies). These small, mushroom-like apothecia are most likely to form when the sclerotia have been exposed to cold, saturated soil for several weeks. They produce millions of ascospores that can be windblown to nearby plants and infect.

Germinated sclerotia with mushroom-like apothecia (Photo: H.D. Shew)

Sclerotinia sclerotiorum has a very wide host range and can attack over 350 species of plants. Common vegetable hosts include cabbage, broccoli, cauliflower, Brussels sprouts, lettuce, pepper, bean, pea, potato, and tomato. Field crop hosts include sunflower, canola (rapeseed), and soybeans. Additionally, several species of weeds are hosts, including ragweed, dandelion, wild clover, vetch, common chickweed, and pigweed.

Sclerotinia on field grown broccoli (Photo: B. Shew)

Controlling white mold can be difficult and requires an integrated approach. Maintain good air circulation by using proper plant spacing and selecting varieties that have minimal leaf overlap between adjacent plants. Maintain good soil drainage by planting in raised beds. Planting susceptible plants in areas with a history of white mold is not recommended. Rotate with non-susceptible plants such as small grains (rye, wheat), corn, beets, onion, and spinach to prevent inoculum build-up. Only very long rotations (4 years or more) will be helpful in areas with high inoculum levels. Practice good weed control strategies to remove weedy hosts from the area. Planting a small grain as a winter cover crop may help to exclude winter weed hosts. Deep plowing or turning of the soil can be useful because sclerotia typically do not germinate at soil depths greater than 5 cm. This is only effective for one year as subsequent tillage will bring the sclerotia back up to the soil surface. Avoid spreading the fungus with equipment or on infected planting material. When practical, remove infected plants by bagging and burning or disposing of offsite. Do not place infected plants in nearby cullpiles as these can become sources of inoculum. Avoid overhead irrigation and use subsurface irrigation when possible. There are no chemical control options for the home gardener. A few products are available to commercial producers, but correct timing of sprays is critical for successful control. See the Ag Chemical Manual for more information on chemical and cultural control of this disease.

For information on Sclerotinia-caused diseases of other hosts, see our previous blog post.

For information on another disease of collards, see our post on black rot.

Sample of the week: Black rot of crucifers

If you are looking forward to a nice mess of collards for Christmas or greens for the first meal of the New Year, this week’s samples are for you. We received several samples of seedlings and plants in the cabbage family that had yellow or brown yellow lesions. Some of the leaves had V-shaped lesions along the margins, while other leaves also had necrotic and blackened veins.

Black rot symptoms caused by Xanthomonas campestris pv. campestris. Note V-shaped lesions on leaf margins.

Collard leaves with blackened veins. Water droplets formed at the hydathodes can be seen along the margins of the upper leaf. Photo by Lisa Rayburn.

These symptoms are typical of black rot, a common disease of crucifers (cabbage, broccoli, kale, collards, turnips, etc.) caused by the bacterium Xanthomonas campestris pv. campestris. Bacterial ooze was visible coming out of the lesions on seedlings, supporting the black rot diagnosis.

The black rot bacterium infects plants through wounds and natural openings, such as hydathodes (water pores) found on the leaf margins. Later, the bacteria invade the vascular system and spread systemically. The disease is favored by mild, wet weather and the bacteria can rapidly spread from plant to plant in splashing rain. Bacteria can be spread when plants are handled (for example, during transplanting) and they survive on and in seed and in debris in the soil. 

Black rot is very difficult to control because it is systemic and because we have very few chemicals that are effective against bacteria.  To reduce spread, avoid cultivation or handling when plants are wet. To prevent black rot, plant clean seeds or seedlings into areas where no crucifers have been grown for 2-3 years.

For more information see:

http://www.ces.ncsu.edu/depts/pp/notes/oldnotes/vg16.htm

http://www.extension.org/pages/29925/managing-black-rot-of-cabbage-and-other-crucifer-crops-in-organic-farming-systems