Basil Peronospora belbahrii resistance to metalaxyl-M in Italy.

Sweet basil (Ocimum basilicum) is in Italy an economically important herb crop cultivated both in greenhouse and in field for fresh and mainly processed (pesto sauce) consumption. Downy mildew (Peronospora belbahrii) is the major threat of this crop especially because it needs of many fungicide applications. Over many years after authorization on basil, metalaxyl-M has been the most utilized and effective active ingredient, but recently many difficulties in the control of downy mildew have been pointed out. The present study reports an overview on the sensitivity of 13 P. belbahrii populations collected in 2013-2014 in different Italian areas. This study was carried out in greenhouse where seedborne plants were sprayed with metalaxyl-M (Ridomil Gold SL 10 and 20 g/100L), azoxystrobin (Ortiva 100 mL/100L) and mandipropamid (Pergado SC 50 mL/100L). Twenty-four hours after treatment, potted plants were inoculated with the sporangia suspensions (105/mL). The assessments were made as percentage of sporulated leaf surface area. Among the 13 different populations, 11 showed resistance to metalaxyl-M while completely sensitive was the profile pointed out towards azoxystrobin and mandipropamid. These results support the hypothesis that the efficacy decreases complained in open field could be due to the development of P. belbahrii resistance to metalaxyl-M.