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Ministry of Agriculture, Food and Rural Affairs

Black Rot

Black rot on Brussels sprouts Black rot on cabbage Black rot on cabbage seedling
Click to enlarge.

Beginner

Scientific name
Xanthomonas campestris

Identification

  • V–shaped yellow or brown lesions or patches along the margins of leaves
  • As the lesions expand, the tissue turns brown, and the small veins become blackened

Often Confused With
Fusarium wilt

Period of Activity
Black rot is spread rapidly during warm, humid weather, with an optimal temperature range of 27- 30°C (80- 86°F) at 80- 100% humidity. Once in the soil, the bacteria are spread by splashing rain and wind. Bacteria enter plants through wounds or natural openings at the leaf margins called hydathodes.

Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf. The blackened veins and vascular ring may be seen when affected stems are cut lengthwise or crosswise, respectively.

Thresholds
None established.

Advanced

Scientific name
Xanthomonas campestris

Black rot is the most serious disease of crucifers world-wide. As little as one infected plant in 10,000 can result in a field epidemic. Black rot attacks all crucifers, but cabbage and cauliflower are most readily infected.

Identification
Symptoms of black rot vary considerably depending on the host, cultivar, plant age and environmental conditions.  Although some infected plants may appear healthy, cutting across infected stems will reveal the characteristic blackened vascular tissue.

The most obvious symptom of black rot in older plants is the presence of “V”-shaped yellow or brown lesions or patches along the margins of leaves. As the lesion develops, the tissue turns brown and the small veins become blackened. If the leaf is held up against the light and the veins stand out black, it’s probably black rot.

Infected seedlings grown in the greenhouse under cool conditions (below 15–18°C, 60- 65°F) frequently do not show any symptoms of the disease. When infected seedlings are transplanted in the field and temperatures rise to 25– 35°C (77- 95°F) during periods of high relative humidity (80– 100%), they may become stunted with dead spots on the cotyledons and will eventually wilt, and die.

In cauliflower, the water conducting vessels of the stem and head can become blackened, leaving the head small and unmarketable.

Often Confused With
Fusarium wilt

Biology
Black rot is usually introduced to a farm through infected seed or transplants. Once in the soil, the bacteria spread by splashing rain and wind. Bacteria enter plants through insect wounds or natural openings at the leaf margins called “hydathodes”.

The bacteria that cause black rot produce a sticky polysaccharide called xanthan that eventually plugs the vascular tissue inside the veins causing them to collapse and turn black. The tissue above the plugged, collapsed xylem eventually turns yellow, wilts and dies. During hot and humid weather, the bacteria can move from the leaf into the stem through the xylem. Once inside the stem, the bacteria can move up or down to other parts of the plant including the roots. Systemically infected plants may produce chlorotic areas anywhere on the leaf. Severely infected leafy brassica crops such as kale and cauliflower tend to shed their leaves from the bottom up leaving only a tuft of distorted leaves separated from the root system by a scarred barren stem.

Infected seedlings grown under cool conditions may ooze bacteria from pores and lesions, which then serve as a source of pathogen for neighbouring plants. In older plants, contaminated water droplets that exude out of hydathodes of infected leaves can then be rain- splashed to other plants

The black rot bacteria can survive in the soil as long as there is un-decomposed, infected plant debris, usually for 2- 3 years.

Period of Activity
Cool weather will sometimes check the development of black rot in the field. Warm, humid weather is conducive to rapid spread of the disease and little can be done to save a plant that has been infected with black rot. The disease can cause significant yield losses when warm, humid conditions follow periods of rainy weather during early crop development. Late infections can provide wounds that other rot organisms use to enter the plant, causing significant damage during storage.

Scouting Notes
Inspects 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf.

Thresholds
None established.

Management Notes

  • Follow a 2– 3 year crop rotation to non-crucifers and control all cruciferous weeds.
  • The single most important preventative measure for this disease is the use of clean seed. Seed is considered clean or “free” of black rot if no more than 1 in 30,000 seeds is infected. The certification level is usually printed on the outside of the seed packet.
  • Hot water seed treatment can be used to reduce the number of bacteria in seed. Hot water treatment may reduce germination in some varieties. Soaking seeds in hot water at 50°C (122°F) for 25– 30 min. is the most effective treatment for seedborne black rot control. Weak seed, seed stored for several years and seed of certain crucifer crops; such as, cauliflower, kohlrabi, kale, rutabaga and summer turnip, may be damaged by hot water treatment; soak for 15 min. at 50°C (122°F) only.
  • Greenhouse sanitation is essential to produce seedlings that are free of black rot. Thoroughly disinfect all surfaces. Where black rot has been present in the greenhouse, discard and disinfect used trays.
  • Work all crop debris deeply into the soil to aid in decomposition of the plant tissue. Take care to bury all debris.
  • Avoid soaking crates or bundles of transplant seedlings in tubs of water before transplanting. The black rot bacteria can spread from diseased to healthy seedlings by infecting leaf scars and wounds on roots when soaked in water.
  • Select fields that are well drained and will not receive run-off water from areas or fields where crucifers have been grown previously.
  • Well drained, light soils are best for crucifer production because they can be worked early in the season and facilitate earlier planting of transplants. Planting early can help avoid disease because environmental conditions are usually not conducive for the development and spread of black rot bacteria.
  • The pathogen can spread up to 30 m (98 ft.) from infected plants (including weed hosts) to healthy plants. The pathogen not only infects and spreads from weeds to cruciferous crops; it can also survive on weed seeds and can grow and multiply on weed leaves without infecting or causing disease. Good weed control within fields will aid disease management; however, careful attention to weed control in ditches and along fencerows is also important.