Sweet basil (Ocimum basilicum L.) is an economically important herb crop in Italy, cultivated both in the greenhouse and field (about 1,000 ha) for fresh and mainly processed (pesto sauce) consumption. Downy mildew (Peronospora belbahrii) (Thines et al. 2009) was first reported in Italy in 2003 (Garibaldi et al. 2004). Currently, downy mildew is the major threat for covered and open field sweet basil, requiring several and continuous fungicide applications. Mefenoxam (metalaxyl-M) plus copper has been the most widely used and effective product against P. belbahrii, since its registration on basil in Italy in 2004. Recently, downy mildew control became problematic, most likely due to the efficacy reduction of mefenoxam. From 2012 to 2014, in the growing periods from June to September, 13 P. belbahrii samples (five in 2012, two in 2013, and six in 2014) were collected from three of the most important basil production areas of Italy (about 800 ha): Emilia-Romagna (nine samples), Liguria (three samples), and Veneto (one sample) regions, where disease control failures by mefenoxam-based programs were widely reported since 2012. Each sample consisted of 10 to 20 plants (O. basilicum type “Genovese”) with sporulating leaves. Field isolates were maintained on living plants of the same type in a greenhouse. Potted plants (O. basilicum type “Genovese”) at the stage of 8 to 10 leaves were sprayed with mefenoxam (Ridomil Gold SL, 43.88% [465 g/liter], Syngenta Crop Protection SpA, Milano, Italy) at a field rate of 22.8 g/100 liters. Mandipropamid (Pergado SC, 23.4% [250 g/liter], Syngenta Crop Protection SpA) at 50 ml/100 liters, and azoxystrobin (Ortiva, 23.2% [250 g/liter], Syngenta Crop Protecion SpA) at 100 ml/100 liters, both registered for use on basil in Italy, were sprayed as standard references, while untreated plants were kept as negative controls. Four plants per product and four untreated plants were used per field isolate. Twenty-four hours after treatment, each plant was spray-inoculated with a sporangia suspension of 5 ml/plant (105 sporangia/ml) of each collected field isolates. Plants were stored in plastic boxes overnight at 24 ± 1°C and 100% relative humidity (RH), in order to enhance disease infection. Hereafter, plants were kept in a greenhouse at 24 ± 1°C and 50 to 60% RH. At the appearance of symptoms on the untreated control (i.e., about 10 to 12 days after inoculation), the plants were placed overnight in plastic boxes at 100% RH. Disease severity was visually assessed by estimating the percentage of sporulated leaf area, according to Mersha et al. (2012). All assays were repeated at least twice. Among the 13 tested field isolates, 11 were resistant to mefenoxam (with an average sporulated leaf surface varying from 20 to 60%). Only two field isolates (collected in 2012 and 2014, both from two different areas in Emilia-Romagna) proved sensitive to mefenoxam while all isolates were sensitive to mandipropamid and azoxystrobin. Similar to a report from Israel (Cohen et al. 2013), our results confirm that poor control of P. belbahrii in areas of Italy with intensive use of mefenoxam (2 to 3 per cycle of production) is due to resistance development.

First Report of Resistance of Peronospora belbahrii, Causal Agent of Downy Mildew of Basil, to Mefenoxam in Italy

COLLINA, MARINA;MERIGHI, MARTINA;TURAN, CEREN;PIRONDI, ALESSANDRO;BRUNELLI, AGOSTINO
2016

Abstract

Sweet basil (Ocimum basilicum L.) is an economically important herb crop in Italy, cultivated both in the greenhouse and field (about 1,000 ha) for fresh and mainly processed (pesto sauce) consumption. Downy mildew (Peronospora belbahrii) (Thines et al. 2009) was first reported in Italy in 2003 (Garibaldi et al. 2004). Currently, downy mildew is the major threat for covered and open field sweet basil, requiring several and continuous fungicide applications. Mefenoxam (metalaxyl-M) plus copper has been the most widely used and effective product against P. belbahrii, since its registration on basil in Italy in 2004. Recently, downy mildew control became problematic, most likely due to the efficacy reduction of mefenoxam. From 2012 to 2014, in the growing periods from June to September, 13 P. belbahrii samples (five in 2012, two in 2013, and six in 2014) were collected from three of the most important basil production areas of Italy (about 800 ha): Emilia-Romagna (nine samples), Liguria (three samples), and Veneto (one sample) regions, where disease control failures by mefenoxam-based programs were widely reported since 2012. Each sample consisted of 10 to 20 plants (O. basilicum type “Genovese”) with sporulating leaves. Field isolates were maintained on living plants of the same type in a greenhouse. Potted plants (O. basilicum type “Genovese”) at the stage of 8 to 10 leaves were sprayed with mefenoxam (Ridomil Gold SL, 43.88% [465 g/liter], Syngenta Crop Protection SpA, Milano, Italy) at a field rate of 22.8 g/100 liters. Mandipropamid (Pergado SC, 23.4% [250 g/liter], Syngenta Crop Protection SpA) at 50 ml/100 liters, and azoxystrobin (Ortiva, 23.2% [250 g/liter], Syngenta Crop Protecion SpA) at 100 ml/100 liters, both registered for use on basil in Italy, were sprayed as standard references, while untreated plants were kept as negative controls. Four plants per product and four untreated plants were used per field isolate. Twenty-four hours after treatment, each plant was spray-inoculated with a sporangia suspension of 5 ml/plant (105 sporangia/ml) of each collected field isolates. Plants were stored in plastic boxes overnight at 24 ± 1°C and 100% relative humidity (RH), in order to enhance disease infection. Hereafter, plants were kept in a greenhouse at 24 ± 1°C and 50 to 60% RH. At the appearance of symptoms on the untreated control (i.e., about 10 to 12 days after inoculation), the plants were placed overnight in plastic boxes at 100% RH. Disease severity was visually assessed by estimating the percentage of sporulated leaf area, according to Mersha et al. (2012). All assays were repeated at least twice. Among the 13 tested field isolates, 11 were resistant to mefenoxam (with an average sporulated leaf surface varying from 20 to 60%). Only two field isolates (collected in 2012 and 2014, both from two different areas in Emilia-Romagna) proved sensitive to mefenoxam while all isolates were sensitive to mandipropamid and azoxystrobin. Similar to a report from Israel (Cohen et al. 2013), our results confirm that poor control of P. belbahrii in areas of Italy with intensive use of mefenoxam (2 to 3 per cycle of production) is due to resistance development.
Collina, Marina; Merighi, Martina; Turan, Ceren; Pirondi, Alessandro; Minuto, G.; Brunelli, Agostino
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/600397
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