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: Jim Correll
- Author: Steven T. Koike
Yet another new race of downy mildew (Peronospora farinosa f. sp. spinaciae) on spinach has been identified in California’s Salinas Valley. The type, or original, strain was initially designated as UA2209 and was first detected in May 2009. Subsequently, it was found in an increasing number of locations throughout California in 2009 and 2010. This race breaks the resistance of several important cultivars. The race has been characterized on a set of differential cultivars and was designated as race Pfs 12 by the International Working Group on Peronospora (IWGP). The working group is located in the Netherlands and is administered by Plantum NL.
Race Pfs 12 poses a threat to the spinach industry because it is particularly well-adapted to most modern hybrids with resistance to race 1-11, which have been widely planted in the past few years. Race 12 is distinct from race 11 because of its virulence on the differentials Campania and Avenger. The appearance of a new race is not completely unexpected because hybrids with resistance to races 1-11 have been planted on a large scale. Similar developments have taken place when races Pfs 5 (1996), Pfs 6 (1998), Pfs 7 (1999), Pfs 8 and 10 (2004), and Pfs 11 (2009) were identified and named. The occurrence of Pfs 12 will create strong interest for Pfs 1-12 resistant spinach cultivars from both growers and breeders.
The IWGP is a working group of Plantum NL consisting of spinach seed companies (Pop Vriend, Monsanto, Rijk Zwaan, Nunhems, Takii, Sakata, Bejo, Enza, Syngenta, Advanseed), Naktuinbouw, and the University of Arkansas. The efforts of the group are supported by research activities at the University of Arkansas and the University of California Cooperative Extension—Monterey County. The aim of the IWGP is to monitor and designate new races of downy mildew in spinach, and to promote a consistent and clear communication between the seed industry, researchers, and growers 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.
IWGP is monitoring new races continuously by testing field isolates on a fixed, common host differential set of cultivars that contains the full range of available resistances. Researchers all over the world are invited to join the IWGP initiative and use the common host differential set to identify new isolates. For California, the Correll-Koike team will continue to receive and test spinach downy mildew samples for growers, pest control advisors, and seed companies.
For more information on this subject you can contact Steven Koike (email@example.com), Jim Correll (firstname.lastname@example.org), Diederik Smilde (email@example.com), or IWGP chairperson Jan de Visser (JandeVisser@popvriend.nl).
Downy mildew is the most damaging disease of spinach in California and causes yellow and tan leaf lesions.
To identify downy mildew races, a series of spinach cultivars is grown and inoculated; races are identified based on which cultivars become diseased.
Downy mildew of lettuce, caused by Bremia lactucae, is the very common foliar disease that results in the familiar yellow to brown leaf lesions and accompanying white sporulation on the lesions. However, the systemic phase of lettuce downy mildew may be less familiar to growers and pest control advisors. In the spring of 2009, systemic downy mildew was very common in coastal California. Currently in 2010, systemic downy mildew is not as serious but is still being observed in some coastal plantings.
Symptoms of systemic downy mildew may be seen on both lettuce leaves and the central, internal core of the lettuce plant. For leaf symptoms, examine the plant for large, elongated regions of the leaf that are discolored and turning dark green to brown. Such regions often develop along the midrib of the leaf and extend into the flat, outer leaf panels (photos 1, 2). White sporulation is often not present on these infected areas until late in disease development. Note that for many systemically infected lettuce plants, these leaf symptoms are absent and the only evident symptoms are in the internal core.
To check for systemic infections in the plant core, cut open and examine the central part of the plant; these tissues will show a dark brown to black streaking and discoloration (photos 3, 4). In some cases, systemically infected plants may be slightly stunted or late in maturing. Exercise caution, however, before concluding that internal core discoloration is due only to systemic downy mildew. Other important lettuce problems (Verticillium wilt, Fusarium wilt, ammonium toxicity) can cause similar internal discolorations.
Confirmation of systemic downy mildew requires laboratory testing. Affected tissues can treated with biological stains and then examined using a microscope. Such procedures can show the presence of the characteristically thick mycelium that lacks cell cross walls (photo 5). In addition, incubating pieces of affected lettuce tissue can result in sporulation of the pathogen (photo 6, showing systemic downy mildew of cauliflower), again enabling confirmation of systemic downy mildew.
Systemic downy mildew of lettuce has not been studied extensively, so researchers do not know exactly what triggers this less common phase of the disease. Some suggest that early infection of young plants may allow the pathogen to infect the inner foliage of lettuce, resulting in pathogen access to the plant growing point. Field personnel also report that some lettuce cultivars are more severely affected than others.
|Photo 1: Brown discoloration due to systemic downy mildew infection in a lettuce leaf|
|Photo 2: Brown discoloration due to systemic downy mildew infection in a lettuce leaf.|
|Photo 3: Internal discoloration of lettuce core due to systemic downy mildew infection|
|Photo 4: Internal discoloration of lettuce core due to systemic downy mildew infection.|
|Photo 5: Blue-stained mycelium of downy mildew that has systemically infected lettuce tissues.|
|Photo 6: Sporulating downy mildew from a systemically infected piece of cauliflower stem.|
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; firstname.lastname@example.org).
Photo 1: Angular lesions on cucumber caused by downy mildew.
Photo 2: Downy mildew lesions support the purple growth of the pathogen.