Perenospora brassicae

This fungus is a member of the Zygomycota group of fungi.

Damage. This serious disease causes a white bloom mainly on the under surface of leaves (see Figure 15.6) which present a more favourable humid microclimate for infection and spore production than the upper leaf surface. Ornamental cruciferous plants such as stocks and wallflowers, brassicas such as cabbage and occasionally weeds such as shepherd's purse are attacked by this fungus. The disease is most damaging when seedlings are germinating, particularly in spring when the young infectable tissues of the host plant and favourable damp conditions may combine to kill off a large proportion of the developing plants.

Life cycle and spread. Asexual spores (zoospores) are produced on microscopic structures on the lower leaf surface, mainly in spring and summer, and are spread by wind currents. Thick-walled sexual spores (oospores) produced within the leaf tissues fall to the ground with the death of the leaf and survive the winter to initiate the spring infections, when rain splash carries the spores up to the lower leaf surface of seedlings and young plants.

Control. It is not advisable to grow successive brassicas in the same field, and particularly not to sow in spring next to overwintered crops.

The amateur grower can use a product containing the protectant fungicide, mancozeb. The professional horticulturist uses a protective chemical such as chlorothalinal at the seedling stage to kill off spores on the leaf. A combination of a systemic fungicide metalaxyl plus chlorothalinal gives better control, while reducing the development of fungus strains resistant to the systemic ingredient.

Other crops such as lettuce and onions are attacked by different downy mildews (Bremia lactucae and Perenospora destructor respectively) and no cross-infection is seen between crops belonging to different plant families.

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