Downy mildew of spinach is caused by the oomycete pathogen Peronospora farinosa f. sp. spinaciae = P. effusa. In an initial effort to manage downy mildew, spinach breeders developed spinach that was genetically resistant to the pathogen. Over time, a population of the pathogen was able to overcome this resistance. Breeders again went to work and developed spinach that was resistant to this new ‘race’ of the pathogen. Again, over time, another subset of the spinach pathogen population was able to overcome this genetic resistance, leading to yet another race of the pathogen. This stepwise development of resistant spinach varieties followed by the emergence of pathogen populations able to overcome plant resistance has resulted in the current list of 17 races of the spinach downy mildew pathogen. The newest race, designated Pfs 17 by the International Working Group on Peronospora, was first identified in 2014 in Yuma, AZ. It was characterized by its interaction with a standard set of differential spinach varieties. Isolates with the same differential interaction pattern subsequently were found each year and at numerous locations since 2014. For more information on this new race, please contact Dr. Jim Correll (jcorrell@uark.edu). With new pathogen races emerging at an increasing frequency in recent years, growers are relying more on fungicides for management of downy mildew of spinach. We conduct yearly field trials at the University of Arizona Yuma Agricultural Center 1) to evaluate spinach breeding lines and varieties for resistance to downy mildew races and 2) to assess effectiveness of existing and new conventional and organic fungicides for disease management. These trials were recently completed and results will be able soon, once data is analyzed.

To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.