Leaf Spot of Radicchio Caused by Alternaria cichorii in California.

A foliar disease of commercially grown radicchio (Cichorium intybus) was observed in 1996 and 1997 in the Salinas Valley (Monterey County), California. Symptoms consisted of circular to oblong, necrotic spots ranging in diameter from 3 to 20 mm and having concentric zones of darker tissue. A fungus identified as Alternaria cichorii Nattrass (1) was observed fruiting on the spots and was consistently isolated from the margins of the spots. Conidia from leaves were obclavate in shape with slender, unbranched beaks extending from the narrow end of the spore body. Spore body dimensions measured 56 to 78 × 14 to 20 μm, and beaks measured 36 to 81 × 1 to 2 μm. Spore bodies had 7 to 9 transverse septa. Often there were no longitudinal septa, but occasionally there were 1 or 2 such septa. For pathogenicity tests, five isolates were grown for 4 weeks on potato dextrose agar under a combination of cool white and Vita-Lite fluorescent tubes on a 12 h light/dark cycle. Conidial suspensions (4.0 × 10 conidia per ml) were sprayed onto 8-week-old radicchio (cv. Rossana Rogers). Plants were incubated in a moist chamber for 48 h and then maintained in a greenhouse. After 12 days, leaf spots similar to the original symptoms developed on all plants inoculated with the five isolates, and the pathogen was reisolated. Control plants sprayed with distilled water remained symptomless. The experiment was repeated and the results were similar. When inoculated onto endive (Cichorium endivia cv. Tres Fine Maraicchere) and two lettuce (Lactuca sativa) cultivars, the isolates caused small (1 to 2 mm in diameter), necrotic, circular leaf spots on endive and Romaine lettuce cv. Green Towers, but did not cause symptoms on the iceberg lettuce cv. Alpha. This is the first report of A. cichorii on commercially grown radicchio in California. In addition, the same disease was confirmed on commercially produced greenhouse transplants of radicchio, indicating that primary inoculum can possibly be seed-borne. Reference: (1) J. C. David. Mycopathologia 129:41, 1995.