In 1997, a foliar disease was detected on leek (Allium porrum) grown as transplants in California greenhouses. Initial symptoms consisted of small (less than 5 mm in diameter), circular, white leaf spots. Spots later enlarged, and became elliptical to oblong in shape and purple with tan borders in color. When spots coalesced, the leaf tips desiccated and wilted. Singly borne, brown conidia from leaves were obclavate in shape with slender, unbranched beaks extending from the narrow end of the spore body. Spore body dimensions measured 81 to 120 (96 mean) µm × 14 to 19 (16 mean) µm, and beaks measured 11 to 56 (30 mean) µm × 3 to 6 (4 mean) µm. Spore bodies had 6 to 9 transverse septa and occasionally 1 longitudinal septum. The fungus was identified as Alternaria porri (Ellis) Cif. (1). The same fungus was also consistently isolated from the margins of the spots. For pathogenicity tests, isolates were grown for 6 weeks on potato dextrose agar under a combination of one cool white and one Vita-Lite fluorescent tube on a 12 h light/dark cycle. Conidial suspensions (1.0 × 10 e 4 conidia/ml) were sprayed onto 2-month-old leek (cvs. Broad London and Gavilan). Plants were incubated in a moist chamber for 48 h and then kept in a greenhouse. After 14 days, leaf spots similar to the original symptoms developed on inoculated plants, and the pathogen was reisolated. Control plants sprayed with distilled water remained symptomless. The experiment was repeated and the results were similar. The isolates also caused leaf spots on onion (Allium cepa cv. Stockton Early Yellow) and chives (Allium schoenoprasum). This is the first report of purple blotch caused by A. porri on commercially grown leeks in California. The occurrence of this disease on leeks in enclosed greenhouses strongly suggested that the primary inoculum was seed-borne.
Reference: (1) M. B. Ellis and P. Holliday. C.M.I. Descriptions No. 248, 1970.