Cabbage Integrated Pest Management : An Ecological Guide.

Major Diseases of CabbageþStudy of spread of downy mildew:Take a number of leaves with white downy mold on the underside. Put these leaves in a smallquantity of water and stir firmly. Remove the leaves from the water. The spores on the white mold willbe released into the water. The water has now become a source of infection. Take a few pots containinga bundle of cabbage seedlings each and sprinkle the water over the plants. Treat a similar pot withseedlings with clean (uncontaminated) water as a control. Cover the pots with a plastic bag to keepa high humidity inside. Keep plants at a cool place for a few days. Depending on temperature andhumidity, symptoms will appear on the leaves.Management & control practicesPreventive activities:· Use a crop rotation plan: do not grow any type of cruciferous crop for at least 2 years.· Use a resistant/tolerant variety. Disease resistant varieties are not available for most cruciferouscrops. However, differences in varietal susceptibility have been reported. Some hybrid varieties ofbroccoli are resistant or tolerant to downy mildew. It is advisable to test varieties under localconditions. See section 3.2.2.· Use clean seed that has been hot-water treated and/or coated. See section 3.3.· Practice sanitary measures such as the use of clean seed beds away from other crucifer productionand the destruction of plant debris and cruciferous weeds.· Use a planting site and plant spacing pattern that expose plants to full sun throughout the day.· When planting, orient rows in the direction of prevailing winds for better circulation of air throughthe foliage.· Thin out seedlings to about 2-3 cm apart. Closely planted seedlings will result in a high humidityand may stimulate infection.· In order to reduce disease spread by hand or machinery, avoid working in fields while the plantsare wet.Once the disease is present in the field:· When seedlings show symptoms of downy mildew, try applying some extra nitrogen. Seedlingstend to outgrow the disease if they are top dressed with a nitrogenous fertilizer.· If severe disease pressure is expected, timely application of a registered fungicide before theonset or at the very beginning of the disease may reduce the severity of the disease. This isusually only necessary to protect young seedlings. Repeated applications may be required, dependingon weather. Consult local extension agencies for current fungicide recommendations.· When symptoms appear on mature plants at a late growth stage, usually no control is necessary.· Plowing immediately after harvest helps eliminate the sources of airborne downy mildew sporesand encourages the rapid decomposition of crop residues.¤Points to remember about downy mildew:1. Downy mildew is destructive only in seedbeds and on young transplants during cool, rainy weather.2. The pathogen survives between seasons in crucifer weeds, seed and crop residues. Spores arespread to new crops primarily by wind and splashing rain.3. Any practice which promotes the rapid drying of foliage and soils will help minimize diseaseincidence.Cabbage Ecological Guide - 2000168