First Report of Powdery Mildew, Caused by Erysiphe cruciferarum, on Broccoli Raab in California

Broccoli raab (Brassica rapa subsp. rapa), also known as rappini, is a leafy vegetable that is widely grown in Europe and has in recent years become an important crop in coastal California. During summer months in the Salinas Valley (Monterey County) in California, powdery mildew was observed on commercial field and greenhouse-grown plants. White ectophytic mycelial and conidial growth was present on leaves, causing chlorosis, and also developed on stems. Mycelia were amphigenous, in patches, often spreading to become effused. Appressoria were lobed and conidiophores were straight. Foot cells were cylindrical, measured 18 to 26 × 7 to 10 μm, and were followed by one to two cells of variable length. Conidia were produced singly, were cylindrical, and measured 35 to 50 × 12 to 21 μm with a length-to-width ratio greater than 2. No fibrosin bodies were observed in the conidia, and conidia germinated at the ends. Cleistothecia were not observed. The fungus was identified as Erysiphe cruciferarum Opiz ex Junell (1). Pathogenicity was demonstrated by gently pressing infected leaves having abundant sporulation onto leaves of potted broccoli raab (cv. Spring Raab), incubating the plants in a moist chamber for 48 h, and then maintaining plants in a greenhouse. After 10 to 13 days, powdery mildew colonies developed on the test plants. Uninoculated control plants did not develop powdery mildew. With the same inoculation procedure, potted broccoli (Brassica oleracea subsp. botrytis cv. Greenbelt) also became infected by the broccoli raab isolates. This is the first report characterizing this pathogen on broccoli raab in the state. Disease incidence and severity in field-planted crops were low, but greenhouse-grown plants became severely infected. Reference: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987.