Disease Management

Broccoli Xanthomonas

Xanthomonas of Brassicas

Black Rot caused by Xanthomonas campestris pv. campestris is one of the most devastating diseases of cruciferous crops which can result in high losses of yield and quality. It occurs worldwide and infects all species of Brassica. Symptoms can appear at any growth stage as yellow, V-shaped lesions that extend toward the base of the leaf resulting in wilt and necrosis. The pathogen may move into the petiole and spread up the stem or into the roots and become systemic. As the disease progresses, the veins of infected tissues turn black and the normal flow of water and nutrients is impeded. Symptoms on root crops may not be visible on foliage, but blackened veins appear in the roots. On leaf crops, infection may spread into the leaves of the head. Black Rot is often followed by invasion of soft-rotting organisms.

Xanthomonas campestris pv. campestris, a bacterial disease, plugs the water-conducting tissue of the plant with xanthan, a mucilaginous sugar. Its most important means of transmission is on seed and as little as 0.03% infection can cause epidemics. The bacteria can persist in infected plant debris for up to two years; it survives in the soil for 40-60 days. It is favored by warm temperatures and symptoms may not appear in the seedbed, allowing infected plants to be transplanted into the field. It is spread within the field by splashing water, wind, equipment, people, and insects. X. campestris pv. campestris can be spread long distances by infested seeds and transplants.

Management:

• Select seed that has been certified as disease-free.
• Treat seed with hot water to eradicate the bacteria. Treat seed for 15- 30 minutes at 50° C, dry, and test for germination. This process must be done carefully and it is recommended that a small sample of seed be tested for the effect on germination first.
• Fumigate or steam sterilize soil in seedbeds and use clean, sterilized seed flats.
• Locate seedbeds where cruciferous crops have not been grown for 4 years and avoid areas that receive run-off from areas previously planted to crucifers.
• Avoid dense seeding rates which can prolong periods of leaf infection and favor pathogen spread.
• Monitor transplants and promptly remove and destroy infected seedlings.
• Do not trim seedlings as the bacteria are easily spread by contaminated tools.
• Practice a three year rotation and control cruciferous weeds.
• Do not work fields when they are wet and avoid overhead irrigation.
• Do not locate cull piles near fields or storage areas.
• Promptly incorporate crop residues after harvest to speed decomposition.

Chemical recommendations:

acibenzolar-S-methyl (Actigard 50 WG): 1 oz/A (7 dh, REI 12h). Suppression only. Apply preventively in sufficient water to ensure adequate coverage. Do not apply Actigard to plants that are stressed by drought, excessive moisture, herbicide injury, etc.

cupric hydroxide (Kocide 4.5LF): 0.6 to 1.3 pt/A (0 dh, REI 24h). Apply as soon as disease appears on a 7-10 day schedule. Tank mixes with Maneb or manex may improve disease control, although not all crucifers are on the Maneb or manex label. Do not apply in a spray solution of less than 6.5 as phytotoxicity may occur.

cuprous oxide (Nordox 75WG): 2/3 to 2 lb/A (0 dh, REI 24h). Apply as soon as disease appears on a 7-10 day schedule.

Prepared by M. Bess Dicklow, UMass Extension, 108 Holdsworth NRC, University of Massachusetts, Amherst, MA 01003-9320. Tel. 413-545-3209 Fax. 413-545-4385. mbdicklo@umext.umass.edu

Revised December 2005