Sally A. Miller
Randall C. Rowe
Black rot is the most serious disease of crucifer crops world wide when environmental conditions (relatively high temperature and humidity) are favorable. The disease affects primarily aboveground parts of plants at any stage of growth and causes high yield and quality losses, especially in tropical and subtropical regions during the rainy season. All vegetables in the crucifer family, including broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kale, mustard, radish, rutabaga, and turnip, are susceptible to black rot. Many cruciferous weeds such as Shepherd's Purse, wild mustard, and yellow rocket may also be hosts of this pathogen.
The characteristic black rot symptom on most cultivated crucifer plants is the appearance of yellow, V-shaped lesions along the margins of leaves. The point of the V-shaped lesion is directed toward a vein (Figure 1). When lesions enlarge, wilted tissue expands toward the base of leaves. Eventually the diseased areas become necrotic and the veins turn black or brown. The infection may move down the vascular tissue of petioles and then spread up and down the stems. When stems and petioles of an infected plant are cut crosswise or lengthwise, the black-brown vascular tissue with yellowish bacterial slime is observed (Figure 2). These symptoms may be confused with Fusarium yellows, except that Fusarium causes brown vein discoloration without bacterial slime. Moreover, symptoms of black rot may vary according to age of host, host genus, species, and cultivar and even environmental conditions. For example, symptoms on cauliflower may appear as numerous black or brown specks, scratched leaf margins, black veins, and discolored curds (Figure 3). Many cruciferous weed species do not exhibit any of these characteristic symptoms even when infected.
|Figure 1. V-shaped black rot lesions on cabbage.||Figure 2. Discolored vascular tissue in cabbage stem and petiole.|
|Figure 3. Black rot symptoms|
on cabbage transplants.
Black rot of crucifer is caused by a bacterium, Xanthomonas campestris pv. campestris (Xcc). The bacteria can overwinter in plant debris, in and on seeds from diseased plants, and in and on weeds. The pathogen may survive in diseased crop residue buried in soil for up to 2 years, but not more than 60 days free in soil. The major source of these bacteria is infected seeds, which enable long-distance spread of the disease. The pathogen is spread within and between fields by splashing water, wind, insects, machinery, and irrigation or drainage waters. The bacteria infect the cotyledons and young leaves through natural plant openings (stomata, hydathodes) or wounds and then migrate between cells until they reach the xylem tissue where they spread throughout the plant. Free moisture is required for infection by the pathogen. After infection, symptoms may appear on plants within 7 to 14 days under optimum conditions (25 to 30 degrees C).
Effective management of black rot of crucifers depends on the application of the following practices in combination:
a. If source of the seeds is unknown, or infested seedlots must be used, treat seed with hot water to eradicate pathogenic bacteria. Cabbage, broccoli, and Brussels sprouts can be treated at 50 degrees C for 25 minutes, while seeds of cauliflower, kale, turnip, and rutabaga are treated for 15 minutes. However, this treatment may reduce the viability of seed. Therefore, some other chemical seed treatments, including, sodium hypochlorite, hydrogen peroxide, and hot acidified cupric acetate or zinc sulfate can be applied to eliminate the bacteria from crucifer plant seeds.
a. Eliminate all volunteer crucifer plants from previous crops and alternative wild host plants within and around the field.
b. Do not apply manure that may contain crucifer residues.
c. Do not use sprinkler irrigation.
d. Avoid working in the field when plants are wet.
e. Do not allow machinery and equipment movement from infested areas to non-infested fields.
f. Deep plow to bury all crucifer residues after harvest.
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