Denomination of Pfs: 15, a new race of downy mildew in spinach
Jim Correll, Steven Koike, Diederik Smilde, Jan de Visser
A new race of the downy mildew pathogen (Peronospora farinosa f. sp. spinaciae) on spinach was first identified in November 2012 in Imperial Valley, CA, U.S. This race overcomes the resistance of several important varieties. The isolate was designated UA4712 and was characterized with a standard set of differential varieties. Subsequently, isolates with the same reaction pattern on the differential set have been found in numerous locations. After careful evaluation of the significance of this development to the spinach industry, the International Working Group on Peronospora farinosa (IWGP) has denominated isolate UA4712 as race Pfs: 15.
Race Pfs: 15 gives the same disease reactions as Pfs: 4 on the standard set of differentials, but is able to overcome the resistance of race 1-14 resistant varieties such as Caladonia (see chart below). Also, Whale is susceptible to Pfs: 15 whereas it is resistant to Pfs: 4. Although this is a new race, a number of commonly used resistances are effective on this race.
New races and deviating strains of the Pfs: pathogen continue to appear in many spinach growing areas of the world. The IWGP is continuously monitoring the appearance of strains of the pathogen that deviate in virulence from the known races. In this way the IWGP aims to promote a consistent and clear communication between public and private entities, such as the seed industry, growers, scientists, and other interested parties about all resistance-breaking races that are persistent enough to survive over several years, occur in a wide area, and cause a significant economic impact.
The IWGP is located in The Netherlands and is administered by Plantum NL. The IWGP consists of spinach seed company representatives (Pop Vriend, Monsanto, RijkZwaan, Nunhems, Takii, Sakata, Bejo, Enza, Syngenta, and Advanseed) and Naktuinbouw, and is supported by research centers at the University of Arkansas and the University of California Cooperative Extension (Monterey County) in the U.S. Researchers all over the world are invited to join the IWGP initiative and use the common host differential set to identify new isolates.
For more information on this subject you can contact Jim Correll (jcorrell@uark.edu), Steven Koike (stkoike@ucdavis.edu), Diederik Smilde (d.smilde@naktuinbouw.nl), or the IWGP chairperson Jan de Visser (JandeVisser@popvriendseeds.nl).
Disease reactions of race 15 (UA4712) observed on spinach differentials by the IWGP compiled July 2014.
aDisease reactions observed in controlled inoculation tests by 11 different participants of the IWGP.
“ + ” indicates susceptible disease reaction; “ - “ indicates resistant disease reaction.
