During the spring and fall of 1999, commercial plantings of the cut flower stock (Matthiola incana) in coastal regions of California developed a severe foliar disease. Abaxial leaf surfaces supported the extensive white fungal growth typical of a downy mildew. Adaxial surfaces of newly infected leaves did not show symptoms, but with time, leaves exhibited extensive, diffuse chlorotic areas and slight twisting. Infected plants were not acceptable for the cut flower market and, therefore, were not harvested. Sporangiophores had main trunks with dichotomous branches ending in slender curved tips that were 2.8 μm long. Sporangia were ovoid, measuring 22 to 25 μm long and 19 to 22 μm wide. The fungus was identified as Peronospora parasitica. Pathogenicity was established by gently pressing diseased leaves onto stock plants (cv. Avalanche), incubating plants in a humidity chamber (20 to 22°C, 100% relative humidity) for 48 h, and keeping plants in a greenhouse. Diffuse chlorotic leaf symptoms developed after 10 days, and the same fungus later developed on abaxial surfaces. Uninoculated plants did not develop downy mildew. Inoculations were conducted a second time, and the results were the same. This is the first documentation of P. parasitica on stock in California.
