Cabbage Integrated Pest Management : An Ecological Guide.

Major Diseases of Cabbage¤Points to remember about Alternaria leaf spot:1. Alternaria is a wet season problem.2. Alternaria leaf spot is introduced on infected seed or by wind-blown spores from nearby cruciferweeds or old crop residues.3. Leaf spot usually occurs on older leaves only and chemical control is therefore not necessary.4. Sanitation (removing crop debris and taking it (far!) away from the field) is the key to leaf spotmanagement.5. Any practice which promotes the rapid drying of leaves and soil will help reduce leaf spot disease.8.6 Clubroot - Plasmodiophora brassicaeSee plate 2 Fig. 10 and 11Causal agent: fungus – Plasmodiophora brassicaeSigns and symptomsClubroot causes wilting and yellowing of the above-ground parts. Oftenmature plants wilt during hot days but may recover during the nights.Plants stay smaller in size and often develop no heads. The best diagnosticsymptom is the presence of big spindle-shaped enlargements (the “clubs”)on the roots. This may occur on fine roots, secondary roots and the maintaproot. Sometimes, the fungus can enter through wounds in the stem atsoil level - the clubs then appear there and just below soil level. The lowestleaves of the plant may turn yellow and drop off.The fungus can attack both seedlings and mature plants. Symptoms ofseedlings may only be detected when the plants are pulled.The disease is often most severe in low-lying, poorly drained soils.Secondary invasion by other pathogens like soft rot bacteria may occur.Source and spreadThe fungus is soil-borne and enters the plant through fine hair roots or through wounds in secondary rootsor in the stem. The disease is spread by soil particles (soil easily clings to shoes or slippers and to toolsused in the field), by transplants and by drainage water. Clubroot is not seed-borne. Transplants areusually the main means of widespread distribution.Every infected plant with clubs is a little timebombM. The clubs are full of fungus spores. These sporesare released into the soil when the root decomposes. That happens when infected plants are not removedfrom the field. The spores of clubroot are very persistent and can stay alive in the soil for many years(from 7 to 20 years!). This is because the spores have a very thick skin that protects them from drying outand from high temperatures.163 Cabbage Ecological Guide - 2000