عنوان مقاله [English]
نویسنده [English]چکیده [English]
In order to identify the Alternaria species involved in cabbage black spot disease, samples of infected leaves were collected from Urmia cabbage fields during the summer and autumn of 2012. Each sample was included in a separate paper bag and the samples were transferred to laboratory. Isolation of the fungi was carried out with culturing the samples on Potato Dextrose Agar (PDA), Potato Carrot Agar (PCA) and Water Agar (WA). The growing fungi with Alternaria characteristics were purified by single spore method. Identification to species levels was based on Simmons’ recommended method (Simmons 2007) under controlled conditions at 25°C, 8/16 light/dark cycle (cool white fluorescent) and PCA medium after 5-7 days. Alternaria interrupta was identified based on the followingcharacteristics. Colonies exhibit concentric rings of growth and sporulation. Vegetative mycelium grows on and within the culture medium. Primary conidiophores are relatively long and simple. Conidial chains on PCA are usually simple, with 8-17 conidia, of which 3-4 initial conidia are relatively large and the remainder abruptly smaller in size. Initial conidia are narrow ellipsoid, 30-45×7-8 µm, have 4-6 transepta and 1 longiseptum in the widest part. Conidia in the upper part are ovoid, 10-20×4-6 µm and have 0-3 transepta and rarely 1 longiseptum in a few conidia of a chain. Conidiophore color is light brown and conidium color is dark brown. The outer wall of conidia is smooth or punctuate (see Fig. 1 in Farsi section). Pathogenicity tests were done on detached, 3 month- old cabbage leaves. Healthy leaves were first washed under tap water, then surface sterilized with 70٪ ethanol and placed in a plastic container containing a wet filter paper. A 5 mm- diameter mycelial plug from the edge of actively growing, 4-5 day- old fungal colony was placed on the leaves. The treated leaves were kept in a growing chamber at 25°C for 1 week. Control leaves received agar plugs without fungus. Characteristic symptoms of the disease including chlorotic and necrotic areas around the inoculated sites developed only on the inoculated leaves (see Fig. 2 in Farsi section). Reisolation of inoculated fungus from infected leaves was done and Koch's postulates were fulfilled. Presence of Alternaria brassicicola, A. brassicae and A. raphani inassociation with infected cabbages has been previously reported in Iran (Ershad 2009), but this is the first report of isolation and pathogenicity test of A. interrupta on cabbage in this country.