Screening of Brassica oleracea var. capitata cultivars against Peronosporaparasitica

Cabbage seeds were screened for the presence of the downy mildew pathogen P eronospora parasitica (Pers. ex Fr.) Fr. Seeds from both open-pollinated an d hybrid varieties were tested. The alkaline maceration technique revealed the presence of mycelia in different parts of the seed and scanning electro n microscopy (SEM) was used to confirm the seed-borne nature of the fungus. Percentage seed infection and germination rates varied from cultivar to cu ltivar. infection levels in the seed coal and endosperm varied between 15-7 0%, whilst in the embryo percentage infection varied between 5-65%. Hybrid seeds showed extremely high and low rates of germination, whilst the open p ollinated seeds showed intermediate to high rates of germination. Maceratio n of host tissues and SEM revealed severe infection of the stem and the roo ts by P. parasitica. Germinating oospores, intact oospores, mycelia and con idia of P. parasitica were observed in the tissues. The detection of the my celium in the seed is significant, since the use of clean seed material is an important sanitary measure against the downy mildew disease. The use of the maceration technique in Seed Health Testing Stations in KwaZulu-Natal a nd other parts of South Africa is discussed.