Resistance to metalaxyl and to metalaxyl-M in populations of Plasmopara halstedii causing downy mildew in sunflower

Abstract

Metalaxyl-M is a phenylamide that is applied extensively as a seed dressing for controlling sunflower (Helianthus annuus) downy mildew [Plasmopara halstedii] in Spain. The identification of isolates of the pathogen in Spain expressing resistance to metalaxyl in recent years has raised concern about the pathogen developing resistance to metalaxyl-M. This study examined the virulence of populations of P. halstedii in Spain and their sensitivity to metalaxyl and metalxyl-M. Sixty-seven populations of P. halstedii were obtained from diseased plants in fields of Spain in 2002, 2004, and 2006 and characterized by virulence (race) and by reaction to metalaxyl-M applied as seed dressing. The levels of resistance to metalaxyl and to metalaxyl-M were compared with four populations in three in vivo experiments. Race 310 was the most frequent in the 3 years in southern Spain (68% of the populations), and races 100 and 330 were identified also. The only two populations collected outside this area had different virulences (races 703 and 710). Thirty-one percent of the populations were not controlled by metalaxyl-M when applied at the recommended dose (2.0 g a.i./kg seed), and subpopulations collected on plants from treated seed were the same race as those from the initial populations. The incidence of downy mildew depended significantly on the dose when doses of metalaxyl and metalaxyl-M from 0 to 8.0 g a.i./kg seed were applied and on the population tested. A dose-response regression analysis showed ED50 values of the populations between 2.4 and 4.4. Resistance to metalaxyl-M in almost one-third of the populations may be attributed to the extended and frequent use of the fungicide in Spain. The identification of resistance to metalaxyl-M in highly virulent populations of P. halstedii in this study should be considered in the management of both genetic and chemical strategies for the control of sunflower downy mildew.