Cabbage Integrated Pest Management : An Ecological Guide.

Major Diseases of CabbageSource and spreadThe black rot bacteria are carried over with infected seed and in the soil on diseased plant residues aslong as they are not decomposed. In addition, many cruciferous weeds can harbor the black rot bacteria.The bacteria can persist in residue for up to two years.When symptoms appear early in the season or even in the nursery, this may be a sign of the seed beinginfected with the bacterium. Infected seed will germinate and from their seed coats, the bacteria gainaccess to the cotyledon and into the young leaves. Leaf infection takes place through water pores(natural openings in the leaf) at the leaf margins, through stomata (another type of natural openings inleaves) and sometimes via wounds in the roots and leaves (for example due to insect feeding). Thebacteria further spread through the veins of the plant.Bacteria are spread by splashing or running water, wind-blown rain, by blowing of detached leaves,cultivating implements and infected seedlings. Insect transmission, for example by larvae of the cabbageworm, is known but not important. Overhead irrigation also spreads the bacteria, particularly in closeplantedconditions such as in the seedbed.Role of environmental factorsHot, wet conditions favor disease development. Under warm, humid conditions, symptoms appear 10 to14 days after infection. The optimum temperature for growth is 30-32 o C, maximum is 38-39 o C. Temperatureseems to be more critical than moisture.Importance - plant compensation - physiological impactThe damage to the yield and quality of the cabbage caused by black rot can be important when theinfection occurs early in the season (it then probably started in the nursery) and when the weather is hotand wet. Poor head formation and reduced yields are a result of the black rot infection. In the absence ofrain or overhead irrigation, which spread the disease, losses are much less. When infection occurs latein the season, usually no yield loss occurs.Once entered the plant, the bacteria travel through the veins of the plant. When a plant is infected, thebacteria can be present in the whole plant, even in the parts that show no symptoms. It is therefore noteffective to prune infected leaves to try to reduce the disease.Natural enemies/antagonistsUnknownManagement & control practicesPrevention activities :· Grow a resistant variety when you have had serious black rot problems before or know that blackrot is a problem in your area. Some varietal resistance has been reported but it is advisable to testdifferent varieties at local conditions.· At least three years’ rotation of the seedbed and the transplanted field with non-cruciferous cropsis advisable.· It is advisable to sterilize seed in water at 50 o C for 30 minutes before planting. That will kill bacteriathat stick to the seed. See section 3.3.Cabbage Ecological Guide - 2000156