Wire stem and bottom rot diseases of crucifers (cole crops) are both caused by the soil-borne fungus Rhizoctonia solani. These diseases can be easily overlooked in the plant bed. They may cause losses in stand, yield, or quality, depending on the time of the season in which they occur.
Wire stem gets its name from symptoms that occur on the stem at the soil level. A dark, watersoaked lesion initially appears on the stem. Later stems become wiry and slender at the point of the lesion. Diseased crucifer plants transplanted to the field grow poorly, are stunted, and may eventually die, especially if there is inadequate moisture shortly after transplanting. If infected plants remain alive, the stem becomes tough and woody (Fig. 1). Plants that survive usually mature late and fail to produce a marketable head.
Bottom rot is a disease of mature cabbage. After cabbage transplants become large enough to begin to shade the ground, the disease appears on the lower side of the head leaves that are in contact with the soil. The midrib is often the first part of the leaf attacked. Resulting lesions are sunken, black, and sharply elliptical with the long side of the lesion parallel to the side of the midrib. Lesions may dry out and become papery brown in appearance if the weather becomes dry. The surface of the lesion may be covered with a sparse, weblike mycelium. Eventually a general black decay occurs at the base of the leaf. The tip of the leaf then turns yellow and then the entire leaf wilts. Infected leaves are shed and only a naked stalk, capped with a small head, remains.
Bottom rot becomes a head rot if temperatures are warm and the relative humidity is high. A weblike mycelium develops between diseased leaves. Infected leaves are eventually covered with small, brown, fungal structures called sclerotia, which can persist on plant debris in the soil for long periods of time. The stem remains unaffected, thus, the head remains upright.
|Refer to the current Virginia Pest Management Guide for Home Grounds and Animals (VCE Publication 456-018), http://pubs.ext.vt.edu/456/456-018/456-018.html, for details on the proper use of pesticides.|
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Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009