Bacterial Diseases of Cruciferous Crops

Agdex#:	114/632
Publication Date:	01/80
Order#:	86-046
Last Reviewed:	01/97
History:	Replaces Factsheet "Bacterial Leaf Spot of Cauliflower," October 1970
Written by:	R.T. Wukasch - Department of Environmental Biology/University of Guelph; B.N. Dhanvantari - Agriculture Canada

Table of Contents 

1. Introduction
2. Black Rot
3. Bacterial Leaf Spot of Cauliflower
4. Bacterial Soft Rot
5. Controlling Crucifer Diseases

Introduction

Three bacterial diseases of cruciferous crops such as cabbage, cauliflower, broccoli and rutabaga are known to occur in Ontario. The biology, management and control of black rot, bacterial leaf spot, and soft rot are discussed in this Factsheet. A separate Ontario Ministry of Agriculture, Food (OMAF) Factsheet, "Fungal Diseases of Cruciferous Crops", Order No. 85-043, describes diseases caused by fungi.

| Top of Page |

Black Rot

Black rot, caused by the bacterium, Xanthomonas campestris, is the most serious disease of cruciferous crops worldwide. All brassica vegetables are equally susceptible to this disease, except that some varieties of radish and kale are less easily infected. The crucifer seed is contaminated with bacteria in the seed production process. Seedlings grown from contaminated seed are often systemically infected, turning pale yellow and dying under warm conditions. Older plants are infected through vein endings at the leaf margin, initiating yellow, V-shaped lesions which turn brown and necrotic as they expand towards the base of the leaf (Figures 1,2). The veins of infected leaves, stems, or roots turn black as the multiplying bacteria plug the normal flow of water and nutrients (Figures 3,4). Black rot infection is often followed by soft rot organisms, which further reduce the quality and storage life of vegetable brassicas (Figure 5).

Figure 1 Brown and yellow, V-shaped necrotic lesions of black rot on the edge of a cabbage leaf. Note the blackened veins.	Figure 2 Entire cabbage leaf showing several black rot lesions.

Figure 3 Veins of plant systemically infected with black rot turn dark brown to black in colour.	Figure 4 Blackened vascular system of black rot infected rutabaga.

The bacteria persist in diseased crop refuse until that refuse is completely decayed - up to two years. Black rot organisms also can survive free-living in soil for about 40-60 days. Cruciferous weeds such as the following, are also capable of harboring the bacteria: Brassica campestris, bird rape; Brassica kaber, Indian mustard (Figure 8); B. nigra, black mustard; Capsella bursa-pastoris, shepherd's purse; Cardaria pubescens, globe-podded hoary cress; Lepidium spp., pepper-grasses; Raphanus raphanistrum, wild radish.

Thus, black rot can be spread into the crop from diseased crop residue, infested soil, diseased cruciferous weeds, as well as within the crop by wind-blown and splashing rain, in irrigation water and soil, and on insects, machinery and workers. As few as three infected seeds in 10,000 (.03%) are reported to cause black rot epidemics in the field. Because the black rot bacteria thrive best in wet weather at high temperatures, symptoms are obvious at 20 - 30º C. However, symptoms are not always obvious in infected crucifers at temperatures of 15-20º C. Since transplants are often grown at low temperatures, extra care must be taken to detect black rot early before plants are field-set.

| Top of Page |

Bacterial Leaf Spot of Cauliflower

Another bacterial disease of crucifers has on occasion been a severe problem in cauliflower. Pseudomonas maculicola especially attacks cauliflower but will also infect broccoli, Brussels sprouts, and other crucifers. Usually in July, leaves become covered with small (1 mm) irregularly shaped brown spots with yellow halos. This stage is difficult to see on the upper leaf surface, whereas the leaf underside clearly is "peppered" with water-soaked brown spots (Figures 6,7). Eventually the many small spots coalesce into large brown papery areas with yellow borders which tear, giving the plant a ragged appearance (Figure 9). Like those which cause black rot, leaf spot bacteria survive in diseased crop residue for at least two years, are carried by seed, and have similar methods of dispersal. However, P. maculicola is favored by cool weather, developing optimally at 22-23ºC. Control measures for black rot are also effective for bacterial or peppery leaf spot of cauliflower.

Figure 5 Bacteria from black rot lesions in the tied cauliflower leaves have infected the curd, followed by soft rot, causing pockets of decay.	Figure 6 Bacterial leaf spot of cauliflower, caused by Pseudomonas maculicola, is not obvious in early stages, especially when only the upper leaf surface is viewed.	Figure 7 The same leaf as in Fig. 6, when turned over, reveals the brown, water-soaked spots on the underside.

Figure 8 A leaf of Indian mustard, Brassica kaber, showing black rot symptoms. Note the blackened veins.	Figure 9 Advanced stages of bacterial leaf spot of cauliflower showing brown, papery areas surrounded by yellow leaf tissue. Note the holes in the leaf blade, giving the plant a ragged appearance.

| Top of Page |

Bacterial Soft Rot

Bacterial soft rot is caused by Erwinia carotovora and other secondary bacteria. Fleshy tissue is invaded through wounds caused by insects, man, machinery, cold temperature, wind-blown soil, hail, or other diseases and is rapidly decayed into a watery, stinking mess at high temperatures (Figure 10). Cauliflower curd is sometimes invaded by both seed corn maggots and soft rot bacteria, although it is uncertain which agent is primary (Figure 11). Broccoli heads having depressions in which water collects often become infected by soft rot bacteria in the field. Tip burn in cabbage heads can lead to subsequent breakdown by soft rot bacteria. Soft rot is a severe problem in storage and transit as well as in the field. Every effort should be made to minimize wounding during harvest and to avoid rinsing or chilling crucifers with unclean water.

Figure 10 Brown, mushy, soft-rotted tissue on this cabbage head is caused by bacterial soft rot organisms.	Figure 11 Cauliflower head on right has bacterial soft rot bacteria and is also infested with seed corn maggot larvae in the brown, decayed pockets.

| Top of Page |

Controlling Crucifer Diseases

A generalized control program for bacterial as well as fungal diseases of cruciferous crops is as follows:

(A) For producing transplants or direct-seeding:

1. Be sure to use resistant cultivars as much as possible. Breeding cabbage for resistance to black rot is in progress, based on Asian material such as Early Fugi. Resistant inbred lines are now available to seedsmen from the University of Wisconsin, and many tolerant cultivars of cabbage are marketed commercially. Consult your seed catalogues, seed company representatives, and crop specialists for further information on available resistant cultivars.
2. Use only new vigorous seed with high-percent germination. Old, improperly stored seeds germinate more slowly, producing weak plants which are more susceptible to disease.
3. Use seed which is certified free from black rot bacteria and Phoma, and has been treated with hot water or streptomycin and fungicides to control seed-borne disease. Efforts to rid the seed of black rot bacteria have in the past involved mercuric chloride and hot water treatment. However, with mercurials no longer available and with problems of reduced seed germination in hot water treatment, especially of old seedlots, research has explored the use of antibiotics.

   Streptomycin seed treatment is recommended for Brussels sprouts, broccoli, cauliflower, cabbage and rutabaga. Seeds are placed in cheesecloth bags and soaked in 500 ppm streptomycin solution for an hour, rinsed for two minutes in running tap water, re-soaked in 0.5% solution of a freshly prepared sodium hypochlorite (household bleach) solution for 30 minutes and rinsed again in running tap water for two minutes. The seeds are then dried. With this treatment, the seedlings may develop yellow or purple discoloration, but recover later.

   Hot water treatment, if carefully done, is still a very effective method of controlling not only seed-borne Xanthomonas, but also black leg, bacterial leaf spot of cauliflower, Alternaria spp., and downy mildew. Refer to Seed Treatment section of OMAF's publication 363, Vegetable Production Recommendations. Even though hot water seed treatment may reduce germination, simply conduct your own germination test. Calculate the percentage germination after seed treatment, and sow extra seed to supply enough plants for your needs. Canola seed treatments are listed in OMAF Publication 296, Field Crop Recommendations.

4. Sow seed in soil which has been fumigated (seedbeds) or sterilized (greenhouse). If fumigation is uneconomical, sow seedbeds in land which has not grown crucifers for at least two years.
5. Use only new or sterilized transplant flats; disinfect greenhouse equipment.
6. Be sure seedlings are not too densely planted, and provide optimum conditions of ventilation, watering, fertility, temperature, and light for growth.
7. Locate field-grown seedbeds away from existing crucifer crops and avoid runoff from the latter or land which was recently in cruciferous crops to prevent introduction of disease.
8. Maintain weed-free seedbeds, cold frames, and greenhouses.
9. Use well-timed applications of insecticides and fungicides, according to recommendations in OMAF Publication 363 and 296.
10. Inspect the seedlings regularly, removing and destroying localized infections ("hot spots") early.
11. Avoid dipping plants in water or trimming them before transplanting in the field, as this can easily spread bacteria and fungi.

(B) In addition, transplant or direct seed into fields which have:

1. warm, well-drained soil, with pH greater than 7.2 if clubroot has been a problem;
2. adequate, balanced fertility;
3. adequate weed control in field headlands, especially of volunteer cruciferous crops, and cruciferous weeds, such as wild mustards, shepherd's purse, pepper-grasses, wild radish [see OMAF Publication 505, Ontario Weeds, for identification of weeds in the Mustard (Cruciferae) family;
4. not grown crucifers for at least three crops, and which do not have evidence of former cruciferous crop residue on or in the soil;
5. continued use of pesticides as necessary for insect and disease control; and
6. adequate sanitation practices such as removing infected plants if practical, incorporating diseased crop refuse promptly, burying cull piles of rutabagas or other crucifers remotely from growing areas, working in the field only when the foliage is dry and avoiding sprinkler irrigation of the diseased fields.

(C) When buying transplants, insist on the following precautions in the contract:

1. written verification of seedlot # and source, dates of pulling, shipping and receipt; pest-control schedule used in the crop, and the transit conditions;
2. certification of disease-free transplants from area where transplants are inspected by regulatory authorities prior to and at pulling time, e.g. transplants from southern U.S.A. should have certification;
3. written statement that transplants were not "topped" with mowing machinery which could spread disease and that only new packaging material was used.

By using the above management tools appropriately, disease problems in cruciferous crops can be minimized. For further information, consult the book, Diseases and Pests of Vegetable Crops in Canada, ISBN 0-9691627-3-1, the OMAF publication, Integrated Pest Management for Crucifers in Ontario, Order No. 701, or your local Horticultural Crops Advisor.

| Top of Page |