- Author: Steven T. Koike
Along with researchers in Europe, Jim Correll (Univ. Arkansas) and Steven Koike (UC Cooperative Extension, Monterey County) report here another new race, the 14th, of the downy mildew pathogen (Peronospora farinosa f. sp. spinaciae) of spinach. First identified in November 2010 from spinach in Ventura County, California, this race breaks the resistance of several important cultivars. The isolate was initially designated as UA4410 and was characterized with a standard set of differential varieties. Isolates with the same disease reaction as UA4410 were subsequently found in locations throughout California and Arizona in 2011 and 2012. This race has not been reported in Europe. After careful evaluation of the significance of this development to the spinach industry, the International Working Group on Peronospora (IWGP) has designated this isolate as race Pfs: 14. Isolate UA4410 will be the type isolate (or official isolate) of Pfs: 14. The IWGP is located in The Netherlands and is administered by Plantum NL.
Race Pfs: 14 poses a threat to the spinach industry because it is particularly well-adapted to modern hybrids with resistance to races 1-13. Similar developments have taken place when races Pfs: 5 (1996), Pfs: 6 (1998), Pfs: 7 (1999), Pfs: 8 and 10 (2004), Pfs: 11 (2008), Pfs: 12 (2009), and Pfs: 13 (2011) were identified and named. The occurrence of Pfs: 14 will encourage development and eventual use of Pfs: 1-14 resistant spinach cultivars.
A collaboration of researchers with the IWGP, University of Arkansas (Correll), and University of California (Koike) is monitoring the development of new races of spinach downy mildew on a global scale by continuously collecting and testing suspected new isolates. Collected field samples are tested for race identification using a fixed, standardized host differential set of varieties that contains the full range of available resistances. New race designations will be mutually agreed upon by this collaboration based on persistence of the race over several years, occurrence in a wide area, and significant economic impact. In this way it is hoped that research findings and conclusions will be agreed upon and better communicated between the researchers, seed industry, spinach growers, and other interested parties.
For California and Arizona, the Correll-Koike team will continue to receive and test spinach downy mildew samples for growers, pest control advisors, and seed companies. Industry is encouraged to continue to submit downy mildew outbreak samples to Correll-Koike, as such samples facilitate the discovery of additional new races. The Correll-Koike research is made possible by support from the California Leafy Greens Research Board and by active participation from the agricultural industries in California and Arizona.
The IWGP consists of spinach seed companies (Pop Vriend, Monsanto, Rijk Zwaan, Nunhems, Takii, Sakata, Bejo, Enza, Syngenta, Vilmorin, and Advanseed) and Naktuinbouw (the Inspection Service for Horticulture in The Netherlands), and is supported by researchers at the University of Arkansas and the University of California Cooperative Extension (Monterey County) in the USA. 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 Steven Koike (firstname.lastname@example.org), Jim Correll (email@example.com), Diederik Smilde (firstname.lastname@example.org), or IWGP chairperson Jan de Visser (JandeVisser@popvriendseeds.nl).
- Author: Steven T. Koike
The cucumber crop in central coast California is a minor crop, with only a modest acreage planted annually. However, a major disease threatens this commodity that is grown both out in the field and inside greenhouses. In recent years, a very aggressive, destructive strain of downy mildew (the pathogen is Pseudoperonospora cubensis) has devastated cucumber plantings. Leaves first develop angular shaped lesions that turn yellow. Later, the tissue in these lesions dies and becomes brown (photo 1). In most cases the diagnostic purple gray mycelium and spores develop on the leaf undersides (photo 2). As disease progresses, entire leaves decline and the plants collapse due to severe infection. Downy mildew also infects squash and watermelon, though this current problem is most problematic on cucumber.
California growers are hardly alone in this situation. Last year the aggressive downy mildew damaged cucumber crops in various parts of the USA, along the eastern seaboard stretching from New York down to Florida, and from there extending west as far as Wisconsin, Illinois, Missouri, Louisiana, and Texas. For California, downy mildew was reported on production cucumber in the central coast and other regions, and on seed cucumber crops in the upper San Joaquin Valley.
Management of this apparently new cucumber strain is difficult. Organic producers have few options because protectant sprays do not appear to help, and suitable resistant cultivars have not yet been identified. For conventional growers, early preventative sprays should be made (see the UC IPM website: http://www.ipm.ucdavis.edu/PMG/r116101611.html). This cucumber situation is yet another case illustrating how this group of pathogens is able to change and cause problems for growers. Central coast growers are already very familiar with the new races and aggressive outbreaks of lettuce and spinach downy mildews.
Plant Pathologist Steven Koike is monitoring the California cucumber situation and is collaborating with researchers in Michigan and North Carolina. He is interested in hearing about downy mildew outbreaks on cucumber in California (phone 831-759-7350; email@example.com).
Photo 1: Angular lesions on cucumber caused by downy mildew.
Photo 2: Downy mildew lesions support the purple growth of the pathogen.