Current Situation of Spinach Downy Mildew in California

Spinach Downy Mildew Peronospora farinosa  f. sp. spinaciae is a devastating disease in many regions of the world. It was first reported in 1824 when Race 1 was discovered. Since then 10 more races have been reported and officially recognized. Currently there is one more tentative race, “Race N,” which might be confirmed as Race 12. This tentative new race, thus far, has clearly been observed in two areas of the Salinas Valley (Soledad and King City) and was first identified in June 2009. From 1988 to 2008 Races 4 to 11 were reported. This disease is the most widespread and damaging spinach disease in California. The pathogen requires cool moist conditions to develop and infect plants. The high plant density in spinach which retains moisture, coupled with the fog and cool temperatures of the regions where spinach is grown, create ideal conditions for disease development. Spores are easily spread from plant to plant and from field to field by wind and splashing water. Under ideal conditions the pathogen spreads fast, and in addition to the damage caused  by leaf spots, if not noticed in the field, could break down and rot when packed in bags and cartons.  This pathogen is host-specific to spinach, but it is believed that some Chenopodium weed species might also be secondary hosts. The use of resistant cultivars is the most effective way to  control downy mildew. For the past 50 years breeders have developed new resistant varieties as the new mildew races develop.