Corn-salad or lamb's lettuce (Valerianella locusta) is a specialty leafy green, annual vegetable grown commercially in California as a fresh market commodity used in salads. In the spring (February through April) of 2008, fields in coastal California (Monterey County) showed symptoms and signs of a downy mildew. Initial symptoms consisted of irregularly shaped, light green patches observed on adaxial leaf surfaces; these lesions later turned yellow. As disease progressed, patches became brown and necrotic. The abaxial sides of affected leaves were heavily colonized by an extensive, purplish growth characteristic of a downy mildew pathogen. Symptomatic leaves were unmarketable and diseased portions of plantings were not harvested. Crop loss was estimated to be between 1 and 15% for any one particular planting. The purple growth consisted of hyaline, branched conidiophores that emerged from stomata and had branches ending in slender, curved branchlets that did not have swollen tips. Conidia were slightly brown, ovoid, mostly nonpapillate, and measured 23.7 to 29.4 μm long × 15.9 to 22.2 μm wide. When necrotic leaf tissue was examined microscopically, thick-walled, yellow brown oospores were abundant within leaf tissues. Oospores measured 33.2 to 36.9 μm in diameter. On the basis of disease symptoms and morphology of the organism, the pathogen was identified as Peronospora valerianellae (1,3). To prove pathogenicity on corn-salad, 24 3-week-old seedlings were sprayed until runoff, using a hand-held spray bottle, with a conidial suspension (1.0 × 104 sporangia/ml), incubated for 24 h in a dew chamber (18 to 20°C), and then maintained in a greenhouse (22 to 24°C). Inoculum was obtained from one section of an affected commercial field. After 10 to 12 days, symptoms and signs of downy mildew occurred on inoculated plants, and the pathogen morphology matched that of the pathogen originally observed. Twelve untreated control plants did not develop downy mildew. To test for seedborne inoculum, 10 g of seed of each of two corn-salad cultivars were added to 100 ml of a dilute (0.05%) Tween 20 solution. The suspension was agitated for 3 h, filtered through cheesecloth to remove seed, and centrifuged. The resulting pellet was examined microscopically and found to contain low numbers of oospores that were similar in morphology to those observed in necrotic leaf lesions. To my knowledge, this is the first report of downy mildew caused by P. valerianellae on corn-salad in California and the United States. The pathogen has been reported on corn-salad in England, France, Germany, Scotland, and the Ukraine (1--3). Seedborne oospores of downy mildew have been reported on corn-salad seed tested in France (1).
References: (1) R. Champion and H. Mecheneau. Seed Sci. Technol. 7:259, 1979. (2) D. F. Farr et al. Fungal Databases. Systematic Mycology and Microbiology. Online publication. ARS, USDA, 2008. (3) G. Pietrek and V. Zinkernagel. Advances in Downy Mildew Research. Kluwer Academic Publishers, Dordrecht, the Netherlands, 2002.
