Host Preference of Mating Type in Pseudoperonospora cubensis, the Downy Mildew Causal Agent of Cucurbits.

The A2 mating type of Pseudoperonospora cubensis was first discovered in Israel in May 2010 on butternut gourd (Cucurbita moschata) (1). We monitored the occurrence of the A2 mating type of P. cubensis in isolates collected during May 2010 through September 2012 from downy mildew-infected cucurbit crops growing along the coastal plain of Israel. Mating type was determined by oospore production in melon leaf discs co-inoculated with sporangia of a test isolate mixed with sporangia of A1 or A2 tester isolates (2). The A1 and A2 tester isolates were maintained at 14°C (14 h light/day) by repeated inoculation of detached leaves of cucumber and pumpkin, respectively. The 29 isolates that were collected from cucumber (Cucumis sativum) were all A1. Of the 33 isolates collected from pumpkin (Cucurbita maxima), squash (C. pepo), or butternut gourd (C. moschata), 88% were A2 and 12% were A1. The host preference of mating type in P. cubensis was monitored at Bar-Ilan University farm during April to July 2012, among about 800 plants of eight cucurbit species (~100 plants per species) that were grown side-by-side in three adjacent net-houses (two 6 × 50 m and one 6 × 100 m) and exposed to natural infection. Downy mildew developed on cucumber, melon, pumpkin, squash, and butternut gourd, but not on watermelon, sponge gourd (Luffa cylindrica), or Momordica balsamina. Three-hundred and three isolates of P. cubensis were collected and tested for mating type: 123 from cucumber, 53 from melon, 30 from pumpkin, 48 from butternut gourd, and 41 from squash. The cucumber isolates expressed A1, A2, and A1A2 at a ratio of 94.3%, 3.3%, and 2.4%, respectively; the melon isolates 58.5%, 26.4%, and 15.1%; the pumpkin isolates 0%, 96.7%, and 3.3%; the butternut isolate 7.3%, 87.3%, and 5.5%; and the squash isolates 2.4%, 97.6%, and 0%, respectively. A1A2 isolates produce oospores when crossed with either A1 or A2 tester isolates. This is the first evidence suggesting a preference of A1 isolates to Cucumis spp. and of A2 isolates to Cucurbita spp. similar preference was recently observed among Chinese isolates of this pathogen (unpublished data). The mechanism(s) controlling this preference is not known. Classical genetics is currently employed to P. cubensis in order to understand if it derives from true linkage. The practical implication for downy mildew management is that growing cucumber/melon in close proximity to pumpkin/squash/butternut gourd should be avoided as it may enhance oospore production in nature. Oospores in soil were recently shown to serve as a primary source of downy mildew infection in cucumber (3). References: (1) Y. Cohen, A. E. Rubin, and M.Galperin. Plant Dis. 95:874, 2011; (2) Y. Cohen and A. E. Rubin. Eur. J. Plant Pathol. 132:577, 2012; (3) Y. J. Zhang et al. J. Phytopathol. 160:469, 2012.