Introduction
Downy mildew is generally considered the most important disease in spinach. Genetic resistance always used to be considered the most effective control method, but new strains have increasingly emerged over recent years which threaten production.
Geographic distribution
Downy mildew causes problems worldwide. Strains may vary slightly in different countries, but it is practically inevitable that new strains will spread throughout the world.
Symptoms and diagnosis
The first signs of downy mildew are irregularly shaped, pale green or yellow spots. A purplish spore formation becomes visible on the underside of the leaf and sometimes also on the top, in damp weather conditions. The lesions in the leaf become larger and turn paler. In severe cases, the leaves curl up and then wilt altogether.
Conditions for disease development
Downy mildew can be caused by soil-borne oospores and may also be seed-transmitted, given that oospores have been detected in seeds. The pathogen can survive on spontaneously emerging plants and may, to a lesser degree, spread via white goosefoot (Chenopodium album). During the main growing season, airborne spores from infected crops are the most obvious source of inoculation. Disease development is enhanced by cold, wet conditions, with a rapid rate of dispersion at temperatures between 15 and 25 °C. Irrigation is regularly applied to boost crop growth; the residual moisture on the leaves can worsen problems with downy mildew.
Impact and importance
In severe cases, downy mildew can render products unsaleable. Resistant varieties have traditionally been an essential part of disease control, but this strategy is currently being undermined by the evolution of new strains. Recent research in the United States indicates that isolates exist that can defeat resistance genes R1-14. Few new sources of resistance to downy mildew are available to growers and losses due to this disease are consequently expected to increase. Seed treatment with metalaxyl-M offers some protection against downy mildew, but other active ingredients should be used in spray treatment to restrict the evolution of fungicide-resistant strains.
