Based on preliminary research findings, it appears that yet another new race of downy mildew has occurred on spinach grown in coastal California. In July 2008, outbreaks of downy mildew appeared on several cultivars that previously had been resistant to races 1 through 10. Throughout the late summer period, downy mildew disease was frequently found on these cultivars, and in some cases significant losses were experienced. The disease continues to be a concern as late season (September and October) downy mildew flare-ups are presently taking place.
Downy mildew, caused by the pathogen Peronospora farinosa f. sp. spinaciae, continues to be the most important disease of spinach in California. This fungus-like organism causes the familiar light green to yellow spots and patches on leaves, rendering the spinach leaf unmarketable.
The characteristic purple sporulation occurs mostly on the undersides of leaves. The pathogen needs cool, humid conditions to grow and develop. Managing downy mildew relies on the use of resistant spinach cultivars and application of fungicides.
To distinguish downy mildew races, a set of 10 international differential spinach cultivars are grown and inoculated with the sample mildew. The race is identified based on which cultivars are susceptible and develop disease. Inoculations with multiple field isolates collected in the summer of 2008 indicated that the current outbreak of downy mildew was caused by strains infecting four of the 10 international differential cultivars: Viroflay, Resistoflay, Bolero, and Lazio. This disease “fingerprint” is unique from all 10 previously characterized races.
Strains with this reaction profile have been found in multiple locations in California and Yuma, Ariz.
To more effectively communicate worldwide information about downy mildew on spinach and facilitate screening and characterizing of resistance, the International Working Group Peronospora committee (IWGP) was formed several years ago to establish criteria for naming new races. Jim Correll (University of Arkansas) has been in communication with the IWGP to discuss the current California situation. Presently, the general consensus of the committee is that the new race should not yet be officially named until further information is available regarding disease development and stability of the race.
The spinach industry should continue to be vigilant and aware of any unusual downy mildew developments. The UC Cooperative Extension diagnostic lab continues to analyze spinach downy mildew isolates submitted from growers, PCAs, and others. This service acts as a gauge of the downy mildew situation and an early warning system should new races occur. Research on spinach downy mildew in California is a joint project between the University of California Cooperative Extension and the University of Arkansas.