First Report of Powdery Mildew Caused by Erysiphe cruciferarum on Brassica juncea in Australia.

In Australia, Brassica juncea (L.) Czern & Coss (Indian mustard) has the potential as a more drought-tolerant oilseed crop than the B. napus L., with the first canola-quality B. juncea varieties released in Australia in 2006 and first sown for commercial production in 2007. Increased production of B. juncea is expected to result in the appearance of diseases previously unreported in Australia. In the spring of 2007 at the University of Western Australia field plots at Crawley (31.99°S, 115.82°E), Western Australia, plants of B. juncea genotypes from Australia and China had extensive stem colonization by powdery mildew at the end of the flowering period, with whitish patches ranging in size from 3 mm to 3 cm long. These patches coalesced to form a dense, white, powdery layer as they expanded. Pathogenicity was demonstrated by gently pressing infected stems containing abundant sporulation onto leaves of potted B. juncea seedlings of variety JM-18, incubating the plants in a moist chamber for 48 h, and then maintaining the plants in a controlled-environment room at 18/13°C for day/night. Signs of powdery mildew appeared at 7 days after inoculation, and by 10 days, it was well developed. Uninoculated control plants did not have powdery mildew. When symptomatic plants were examined, abundant conidia were typical of Erysiphe cruciferarum Opiz ex Junell, with cylindrical conidia borne singly or in short chains as described previously (2). Mycelia were amphigenous, in patches, and often spreading to become effused. Conidiophores were straight, foot cells were cylindrical, and conidia were mostly produced singly and measured 21.2 to 35.4 (mean 26.7 μm) × 8.8 to 15.9 μm (mean 11.9 μm) from measurements of 100 conidia. The spore size that we measured approximated what was found for E. cruciferarum (2) (30 to 40 × 12 to 16 μm), since we found 35 and 50% of spores falling within this range in terms of length and width, respectively. Conidia were, however, generally smaller in size than that reported on broccoli raab in California (1) (35 to 50 × 12 to 21 μm). We confirmed a length-to-width ratio greater than 2 as was found previously (1,2). Infected leaves showed signs of early senescence. While powdery mildew caused by E. cruciferarum is an important disease of B. juncea in India where yield losses as much as 17% have been reported (4), its potential impact in Australia is yet to be determined. To our knowledge, this is the first record of E. cruciferarum on B. juncea in Australia. In Western Australia, E. cruciferarum has been recorded on B. napus (oilseed rape) since 1986 and on B. napus L. var. napobrassica (L.) Reichenb. (swede) since 1971 (3). In other regions of Australia, it has been recorded on B. rapa in Queensland since 1913 and on B. napus (oilseed rape) in South Australia since 1973. References: (1) S. T. Koike and G. S. Saenz. Plant Dis. 81:1093, 1997. (2) T. J. Purnell and A. Sivanesan. No 251 in: Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, UK, 1970. (3) R. G. Shivas. J. R. Soc. West. Aust. 72:1, 1989. (4) A. K. Shukla et al. Manual on Management of Rapeseed-Mustard Diseases. National Research Centre on Rapeseed-Mustard, Bharatpur, India, 2003.