First Report of Powdery Mildew Caused by Erysiphe cruciferarum on Camelina sativa in Montana.

Camelina sativa (L.) Crantz, also known as false flax, is an annual flowering plant in the family Brassicaceae and originated in Europe and Asia. In recent years, it is cultivated as an important biofuel crop in Europe, Canada, and the northwest of the United States. In June of 2021, severe powdery mildew was observed on C. sativa 'Suneson' plants under greenhouse conditions (temperature 18.3°C/22.2°C, night/day) in Bozeman, Montana (45°40'N, 111°2'W). The disease incidence was 80.67% (150 pots, one plant per pot). White ectophytic powdery mildew including mycelia and conidia were observed on the upper leaves, usually developed from bottom tissues to top parts, also present on stems and siliques. Mycelia on leaves were amphigenous and in patches, often spreading to become effused. These typical symptoms were similar to a previous report of powdery mildew on Broccoli raab (Koike and Saenz 1997). Appressoria are lobed, and foot cells are cylindrical with size 18 to 26 × 7 to 10 μm. Conidia are cylindrical and produced singly, with a size of 35 to 50 × 12 to 21 μm and a length : width ratio greater than two (Koike and Saenz 1997). No chasmothecia were observed under the greenhouse conditions. The symptoms and fungal microscopic characters are typical of Pseudoidium anamorph of Erysiphe (Braun 1995). The specific measurements and characteristics are consistent with previous records of Erysiphe cruciferarum Opiz ex L. Junell (Braun and Cook 2012; Vellios et al. 2017). To identify the pathogen, the partial internal transcribed spacer (ITS) region of rDNA of sample CPD-1 was amplified using primers ITS1 and ITS4 (White et al. 1990). The amplicons were sequenced, and the resulting 559-bp sequence was deposited in GenBank (CPD-1, Accession number: OK160719). A GenBank BLAST search of the ITS sequences showed an exact match (100% query cover, E-value 0, and 100% identity 559/559 bp) with those of E. cruciferarum on hosts Brassica sp. (KY660929.1), B. juncea from Vietnam (KM260718.1) and China (KT957424.1). A phylogenetic tree was generated with the CPD-1 ITS sequence with several of ITS sequences of close species with different hosts obtained from the GenBank. Isolate CPD-1 was grouped with pathogens from Brassica hosts rather than the holotype strain (KU672364.1) from papaveraceous hosts. To fulfill Koch's postulates, pathogenicity was confirmed through inoculation by dusting conidia onto leaves of seven healthy, potted, 14-day-old C. sativa seedlings (cv. Suneson). Seven non-inoculated plants served as a control treatment. The plants were incubated in a greenhouse with a temperature of 18°C (night) to 22°C (day). The inoculated plants developed similar symptoms after 7 days, whereas the control plants remained symptomless. The fungus on the inoculated plants was morphologically identical to that was originally observed on the diseased plants. Though many Brassica spp. have been known to be infected by E. cruciferarum throughout the world, powdery mildew of C. sativa cultivar Crantz in natural conditions by E. cruciferarum has been reported only in the province of Domokos in Central Greece (Vellios et al. 2017). To our knowledge, this is the first report of powdery mildew caused by E. cruciferarum on C. sativa in Montana. Though the powdery mildew on C. sativa was observed in the greenhouse conditions in this work, it poses a potential threat to the production of this biofuel crop in the northwest of the United States.