Crucifer Diseases
 

Downy Mildew
Peronospora parasitica


Worldwide

Damage Symptoms

Most crucifers are susceptible to downy mildew. It is particularly damaging to young cabbage seedlings and is considered to be oen of the more destructive foliar diseases of Chinese cabbage in Asia. Plants can be affected at any stage of development.

The distinctive feature of the disease is production of a fluffy white growth found mostly on the underside of leaves. A yellow irregular shaped area appears on the upper side of the leaf opposite where sporulation occurs. On the surface of cabbage heads, the pathogen causes numerous sunken black spots ranging in size from necrotic flecks to 2 cm in diameter, with little or no sporulation. Cauliflower and broccoli floral heads develop dark brown internal streaks.


Conditions for Development

The fungus survives between crops as oospores or on cruciferous crop and weed plants. The fungus produces an abundance of sporangia that can be disseminated by wind and rain splash. Extended periods of leaf wetness caused by fog, rain, or dew are ideal for disease development. Downy mildew develops most rapidly when night temperatures are between 10 and 15°C. When night temperatures exceed 24°C, disease development is greatly restricted.