First Report of the Occurrence and Resistance to Mefenoxam of Peronospora belbahrii, Causal Agent of Downy Mildew of Basil (Ocimum basilicum) in Israel

Abstract

Downy mildew in basil was first reported from Uganda in 1933 (4). In 2004, it was reported from Italy (3) and, thereafter, from other countries around the world. In Israel, the disease was first observed in November 2011 in two greenhouses located in the northern part of the Jordan Valley. Within a month, second and third outbreaks of the disease occurred simultaneously near the southwest and southeast borders of Israel, 250 km from the initial disease outbreak. By the summer of 2012, the disease had appeared throughout the country, causing major economic damage. The causal agent, identified as Peronospora belbahrii (see below), produced chlorotic lesions on leaf blades with sporangia developing on the lower leaf surfaces. Lesions gradually turn necrotic, and infected leaves abscised. Sporangia were dark purple, oval, 30.4 ± 2.9 μm long × 21.4 ± 1.7 μm wide. Sporangiophores emerged from stomatal openings in a saturated atmosphere, were hyaline, 400 to 600 μm long, dichotomously branched, with three to five branches per sporangiophore, and bore a single sporangium on each branchlet tip. Oospores, seldom seen, were brown, round, and 46.2 ± 2.8 μm in diameter. Sporangia germinated directly, each producing a single germ tube that penetrated the periclinal wall of epidermal cells. PCR assays using sporangia and infected leaves as the template, and specific BAZ primers (1), produced a 134-bp band typical of P. belbahrii (1,2). Twenty isolates, collected from 12 locations in Israel from December 2011 to September 2012, were all sensitive to mefenoxam as the isolates did not cause symptoms on 15-leaf, potted basil plants (cv. Peri, Volcani Center, Israel) that were sprayed with 10 μg mefenoxam/ml (Ridomil Gold 48%, Syngenta, Basel, Switzerland) prior to inoculation. However, one isolate collected in early October 2012 from a severely infected plant in a greenhouse at Rehov in Bet-Shaan Valley, in which the plants had been treated with mefenoxam, was resistant to mefenoxan, showing abundant sporulation on leaves of potted basil plants that had been sprayed with 1,000 μg of mefenoxam/ml prior to inoculation. To our knowledge, this is the first report of the occurrence of downy mildew in basil in Israel. This is also the first global report of resistance to mefenoxam in P. belbahrii. References: (1) L. Belbahri et al. Mycol. Res. 109:1276, 2005. (2) R. Djalali et al. Mycol. Progress 11:961, 2012. (3) A. Garibaldi et al., Plant Dis. 89:683, 2004. (4) C. G. Hansford. Rev. Appl. Mycol. 12:421, 1933.