Corn-salad or lamb's lettuce (Valerianella locusta) is a specialty leafy green, annual vegetable that is grown commercially in California for use in salads. During the summer (June through August) of 2004, field plantings in coastal California (Monterey County) showed symptoms and signs of a powdery mildew. White, ectophytic mycelia and conidia were present on leaves and petioles. Extensively colonized leaves were slightly twisted and later developed a tan necrosis. Mycelial growth was spread out, flat, sometimes dense, and colonized both sides of the leaf. Growth was more extensive on the upper leaf surfaces. Hyphae were 5 to 7.5 μm wide with nipple-shaped appressoria. Conidiophores were straight and had foot cells that were simple, mostly curved, measured 50 to 65 × 10 to 12.5 μm, and followed by two to three cells of equal size. Conidia developed in short chains. Conidia were cylindric to doliform, measured 27.5 to 32.5 × 15 to 17.5 μm, and lacked fibrosin bodies. Conidial length-to-width ratios were usually less than or equal to 2.0. Conidia germinated at the ends (cichoracearum-type) and had germ tube lengths that ranged from short to 1.0 to 1.5 times the length of the conidium; germ tubes had swollen tips. Cleistothecia were not observed. On the basis of these characteristics, the fungus is identified as Golovinomyces (Erysiphe) orontii(Cast.) Heluta (1). To demonstrate pathogenicity, heavily colonized corn-salad leaves from commercial plantings were collected and gently pressed onto leaves of potted corn-salad plants. Plants were then maintained in a greenhouse (22 to 24°C). After 8 to 10 days, symptoms and signs of powdery mildew developed on the foliage of inoculated plants, and the pathogen morphology matched that of the originally observed pathogen. Noninoculated control plants did not develop powdery mildew. To our knowledge, this is the first report of powdery mildew of corn-salad caused by G. orontii in the United States. This corn-salad disease has been reported from a number of countries in Europe (2). Because the presence of powdery mildew on the harvested foliage made the leaves unmarketable, part of the crop could not be sold.
References: (1) U. Braun. Nova Hedwigia 89:1, 1987. (2) U. Braun. The Powdery Mildews (Erysiphales) of Europe. Gustav Fischer Verlag, Jena, Germany, 1995.
