Sensitivity to the racemic mixture and isomeric forms of metalaxyl in Indian and European homothallic and heterothallic isolates of Peronospora parasitica in Brassica species

The production of oospores in monoculture (homothallism) and sensitivity to the phenylamide fungicide metalaxyl, in its racemic mixture form of (R)- and (S)-enantiomers (Ridomil-25WP) or the latest isomeric form (R)-enantiomer (Metalaxyl-M), was tested using 11 single-spore and 14 field isolates of Peronospora parasitica . Three isolates were from Brassica napus (UK), 13 from B. juncea (India), four from B. rapa (India), one from Eruca sativa (India) and four from B. oleracea (one from the UK, two from France and one from India). All isolates were found to be homothallic, as they produced oospores in monoculture, except P006 (a UK single-spore isolate derived from B. oleracea and previously known to be insensitive to Ridomil-25WP) which was heterothallic. All isolates were found to be sensitive to the two forms of the phenylamide fungicide metalaxyl (Ridomil-25WP and Metalaxyl-M) except P006 and FP13, a French isolate derived from B. oleracea in Brittany. These isolates were insensitive to both fungicides at the three concentrations used in this study (0.5, 5.0 and 50.0 μg/ml). To our knowledge, this is the first reported case of insensitivity to metalaxyl in P. parasitica from France. This study emphasises the importance of deploying sources of host resistance with fungicides in prolonging the effectiveness of both control procedures.