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First Report of Leaf Blight Caused by Alternaria longipes on Carrots in Israel

Alternaria leaf blight, caused by Alternaria dauci (Köhn) Groves & Skolko, is one of the most devastating foliar pathogens of carrots (Daucus carota L.). Lesions appear as minute, necrotic, dark brown spots often initiated on the edge of the leaflet blade. They later enlarge in size and may merge into a large necrotic area, causing shriveling of the entire leaflet (1). In summer 2000, observations made in several carrot fields in the northwestern part of the Negev Region in Israel revealed infections that were atypical for A. dauci because they were initiated primarily in the middle section of the leaflet blade and were surrounded by a large yellowish area. A. longipes (Ellis & Everh.) E. Mason was consistently isolated from the lesions. Occasionally both A. longipes and A. dauci developed on the same leaves. The two pathogens differed in conidial morphology (size and shape of spore and beak) when cultured on potato dextrose agar medium. One hundred conidia of each species were measured. A. dauci conidia were 100 to 450 μm long and 6 to 15 μm wide, with a beak of up to 3 times the length of the conidium; A. longipes conidia were 35 to 110 μm long and 11 to 21 μm wide, and the beak measured one-third to one-half the length of the conidium. These measurements corresponded to the sizes listed previously (2). Inoculation of greenhouse-grown plants and completion of Koch's postulates confirmed that A. longipes is pathogenic to carrots. Conidia of both species germinated at temperatures from 5 to 36°C. In vitro tests revealed that A. longipes was less sensitive than A. dauci to fungicides commonly used in Israel in carrot fields. A fifty percent effective dose of chlorothalonil and difenoconazole was 3.0 and 0.2 μg a.i./ml, respectively, for mycelia growth of A. dauci, whereas the corresponding values for A. longipes were 10.5 and 3.0 μg a.i./ml, respectively. The prevalence of A. longipes in carrot fields and the influence of this pathogen on yields are currently not known.

References: (1) I. Barash et al. Physiol. Plant Pathol. 19:7, 1981. (2) M. B. Ellis. Dematiaceous Hyphomycetes. CMI. Kew, Surrey, England, 1971.