Leaf spot, downy mildew and verticillium wilt: New spinach disease problems surface

Growers, pest control advisors, and others involved with the growing and harvesting of spinach should take note of increased severity of Stemphylium leaf spot and proliferation of new downy mildew races. The spinach industry should also be aware of verticillium wilt has also been reported as a problem on spinach seed crops.

Stemphylium leaf spot (Stemphylium botryosum). This disease is usually quite minor and has not been a serious concern for spinach growers.

However, in spring 2004 there were disease outbreaks that were more severe than usually experienced. Outbreaks occurred in both Salinas and Santa Maria valleys.

Initial symptoms on mature leaves consist of small, one-eighth to one-fourth inch, circular to oval, gray-green leaf spots. As the disease progresses, leaf spots enlarge, remain circular to oval in shape, and turn tan in color. Older spots coalesce, dry up, and become papery in texture.

Visual signs of fungal growth are generally absent from the spots; hence this problem is readily differentiated from foliar diseases in which purple growth (downy mildew), green spores (Cladosporium leaf spot), or acervuli (black fruiting bodies of the anthracnose fungus) develop. If young expanding leaves are infected, the symptoms are similar to those described for mature leaves. Overall, symptoms resemble the tan, circular to oval spots caused by pesticide or fertilizer damage.

Recent research from Washington State University shows this pathogen is commonly seedborne. Therefore, California growers may continue to see this disease occur in spinach crops.

New mildew races

New races of downy mildew (Peronospora farinosa f. sp. spinaciae) have been implicated on recent outbreaks of downy mildew on previously resistant cultivars are in California and Europe.

Preliminary research indicate several new developments:

  • A race that similar to race 7 has been documented in California. Previously, race 7 was present only in Europe. The new California 7, however, is not identical to European 7, so further research will be needed to clarify the relationship between these two isolates.

  • A new race, yet to be named, has been identified from Europe. This race has not yet been detected in California. This isolate infects a wide range of new cultivars having resistance to races 1 to 7.

  • Two new and apparently unique races have tentatively been identified from California. Work is currently under way to determine the responses of differential spinach cultivars to these new isolates compared to the known races. One of the isolates can overcome the resistance in several race 1-7 resistant cultivars. The second isolate apparently can overcome the resistance in the differential cultivar Lion.

Prior to the detection of this isolate, Lion was resistant to all California isolates tested on this cultivar. The results of these findings are now being reconfirmed.

Work is continuing in Monterey County on identifying and developing additional fungicide tools to use for spinach downy mildew. The UC Cooperative Extension lab in Salinas will continue to collaborate with the plant pathology lab at the University of Arkansas in typing isolates and identifying the races.

Verticillium wilt (Verticillium dahliae) is not a new disease on spinach and has been reported previously by researchers in the U.S., Canada, and Europe. Recent work at Washington State University documented that this pathogen can be seedborne in spinach. The disease appears to be a problem only on spinach crops being grown for seed.

Thus far, there are no reports of verticillium wilt on production spinach in California. Spinach growers, PCAs and processors are ask to notify UC Cooperative Extension if they see symptoms (lower leaf yellowing, poor growth, slightly discolored vascular tissue) on California spinach.

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