Diseases of lettuce (
Downy mildew
Downy mildew is a foliar disease caused by Bremia lactucae, an obligate fungal-like parasite. Cool, moist conditions are necessary for disease development. Free moisture on the leaf surface is essential for spore germination and infection, but not growth of the pathogen within the leaf. Initial symptoms are pale yellow regions on the upper side of older leaves with corresponding white fluffy growth, the spores of the pathogen, on the lower leaf surface (photo 1). Infected areas are limited by leaf veins. Infected tissue turns brown (photo 2). Infection stops when temperatures rise above the disease optimum of 50° - 72° F and free moisture from rain, irrigation or dew is absent.
Disease is managed by planting varieties of lettuce with tolerance
and preventative application of fungicides. Fungicides should be applied
when there is a period of cool, moist weather, such as those periods
of winter storms in Arizona, before disease is detected. Available
fungicides include Azoxystrobin, maneb, fosetyl-Al and metalaxyl, however
insensitivity to metalaxyl has been reported for some time. It
is very important to alternate use of different chemistries to avoid
development of resistance to other fungicides. More information on downy
mildew of lettuce can be obtained from:
http://edis.ifas.ufl.edu/VH044
1 
2 
February 20, 2013