First Report of Resistance to Metalaxyl in Downy Mildew of Sunflower Caused by Plasmopara halstedii in Spain.

Fifty-two isolates of Plasmopara halstedii Farl. Berl. & de Toni (causal agent of sunflower downy mildew) collected from sunflower (Helianthus annuus L.) in Spain from 1994 to 2000 were evaluated for metalaxyl resistance. The pathogen was identified on the basis of the morphology of the sporangiophores and zoosporangia recovered on the underside of the leaves (2). Metalaxyl (Apron 20% LS) at 2.0 g a.i./kg of seed (labeled European rate) was applied as seed dressing to the susceptible sunflower 'Peredovik'. There were two replications of 40 plants, and the test was repeated three times. Inoculum (sporangia bearing zoospores) was produced on artificially inoculated plants. Seed were germinated in a humidity chamber at 28°C for 24 to 48 h. When the radicle was 0.5 to 1.0 cm long, untreated and treated seedlings were inoculated by dipping the entire plant in an aqueous suspension of 6.0 × 104 sporangia per ml for 4 h, planted in a sand/perlite mixture (2:3 vol/vol), and grown at 16 to 21°C with a 12-h photoperiod. Plants were incubated for 24 to 48 h at 100% relative humidity and 15°C in the dark to enhance sporulation. After 12 days, disease incidence (DI) of inoculated plants was determined as a percentage of plants displaying sporulation of the fungus on the cotyledons and/or true leaves (3). DI was 95 to 100% for the untreated seedlings, but mildew did not develop on seedlings treated with metalaxyl for 51 of the isolates. The remaining isolate caused symptoms on 67% of the treated plants. This isolate was tested in another experiment in which 'Peredovik' seed was treated with metalaxyl at 0, 0.5, 2.0, 3.5, and 5 g a.i./kg of seed. There were four replications of 12 seedlings per treatment, and seedlings were inoculated as described previously. DI in the untreated control was 77%, which was not significantly different from the DI for seed treated with metalaxyl at 0.5, 2.0, and 3.5 g a.i./kg of seed (97, 73, and 96%, respectively). DI for seed treated with metalaxyl at 5.0 g a.i./kg of seed was 37%, which was significantly lower than the other treatments. Although resistance of P. halstedii to metalaxyl has been reported in France (1), to our knowledge, this is the first report of resistance of sunflower downy mildew to metalaxyl in Spain. References: (1) J. M. Albourie et al. Eur. J. Plant Pathol. 104:235, 1998. (2) G. Hall, Mycopathologia 106:205, 1989. (3) M.L. Molinero-Ruiz et al. Plant Disease 86:736, 2002.