Diseases of spinach (
Downy mildew
Downy mildew of spinach is caused by Peronospora farinosa f. sp. spinaceae (formerly called Peronospora effusa), a fungal-like organism that is a member of the oomycete group, a group of organisms often called the water molds. All downy mildew organisms belong to this group, and they require cool, wet conditions to cause disease. Peronospora farinosa f. sp. spinaceae has a very limited host range, and infects only spinach and a few Chenopodium weed species such as lamb's quarters. It is the most important disease of spinach in Arizona.
The pathogen, Peronospora farinosa f. sp. spinaceae, causes lesions on cotyledons and leaves that are initially light green to yellow (photo 1) then turn brighter yellow. In the early morning when substantial dew is present or when moisture remains on leaves after rainfall or irrigation, the pathogen produces a blue-gray growth on the bottom of the leaves consisting of microscopic asexual reproductive structures, sporangia, and characteristic sporangiophores (http://www.apsnet.org/education/IllustratedGlossary/default.htm).
Because of the layered growth habit of spinach and the density of plants in most fields, conditions may remain conducive for disease development even during seemingly dry conditions. The sporangia are dispersed by wind, and new infections and disease occur rapidly in cool, moist conditions. As infections age and dry, the dense sporangia and sporangiophores may look purple to tan, and lesions appear as dead necrotic areas on the leaves (photo 2).
Downy mildew of spinach is a chronic problem because it has many races. Races of Peronospora farinosa f. sp. spinaceae are identified by their pathogenicity on different spinach cultivars. Host resistance is the most important means of disease control, but as new races of the pathogen develop, spinach cultivars without resistance to the new race of the pathogen become infected. Seedlings can be protected by seed treatments with metalaxyl. Chemicals applied to developing and mature plants to prevent infection include: mefanoxam (Ridomil Gold formulations), phosphorous acid salt (Fosphite), fosetyl-Al (Aliette), copper hydroxides (Kocide), azoxystrobin (Quadris).
More information:
http://www.ipm.ucdavis.edu/PMG/r732100111.html
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April 30, 2008