First report of pathogenicity group 2 of Leptosphaeria maculans causing blackleg of oilseed rape in Iran

Rapeseed (Brassica napus) is one of the most important oilseed crops in Iran with more than 200,000 ha planted in 2008. Phoma blackleg (Leptosphaeria biglobosa), pathogenicity group 1 (PG-1) or non-aggressive type, has been reported on rapeseed from Golestan province (Fernando et al., 2007). Recently, in some regions of Mazandaran and Golestan provinces of northern Iran,typical stem canker symptoms were observed with incidence ranging from 20 to 60%. During October and November 2008, ascospores were isolated from pseudothecia on infected rapeseed debris and cultured on V8-agar medium (Mengistu et al. 1993). Based on the description of Punithalingam & Holliday (1972), sporulating colonies were identified as Leptosphaeria maculans (Desmaz.) Ces. & de Not. (Anamorph Phoma lingam (Tode: Fr.) Desmaz.).

Eleven isolates of L. maculans were used for determining the pathogenicity group according to phenotypic interaction (PI) on rapeseed cultivars including Westar, Quinta and Glacier. Nine seven-day old seedlings of each cultivar were inoculated for each isolate and the test was repeated three times. Wounded cotyledons were each inoculated with 10µl of conidial suspensions at 2 x 10⁷ spores/ml. All plants were maintained in a growth chamber at 21° C (light) to 16° C (dark), with a 16-hour photoperiod and relative humidity of 95%. After 10 days, disease severity wasrated on a 0-9 scale (Williams, 1985) (Fig. 1).

Two isolates (Es-5 and Es-7) were classified as belonging to pathogenicity group PG2 and nine isolates as PG1. PG2 isolates showed PI reactions 0 to 2, 7 to 9 and 7 to 9 on Glacier, Quinta and Westar, respectively. In addition, cultivars Hyola401 and Okapi were highly sensitive to PG-2 isolates in the cotyledon assay. This is the first report ofthe occurrence of Leptosphaeria maculans PG-2 in Iran.