Sensitivity of downy mildew to phenylamide fungicides in vineyards of the Western Cape
Vermeulen, A K
A total of 242 downy mildew samples from different farms in the Western Cape were obtained throughout the growing season and tested for resistance. EC50 values for metalaxyl were determined for 128 isolates, which ranged from 0 to above 200 µg/ml. EC50 values were clustered by means of a Ward’s minimum variance cluster analysis. Cluster 1 was subdivided into 3 clusters, the most sensitive cluster of which was regarded as the sensitive reference cluster. The mean EC50 value from this group was used to determine the resistance factors (RF) of the other clusters. Isolates in Cluster 2 (RF = 48.8) were characterised as ‘low level resistant’, those in Cluster 3 (RF = 96.2) as ‘medium resistant’, Cluster 4 (RF = 143.9) and 5 (RF = 248.1) as ‘resistant’, and those in Cluster 6 (RF = 513.4) as ‘highly resistant’. Given the uncertainty regarding the practical implications of the resistance factors, the isolates in Clusters 3 to 6 were regarded as ‘resistant’ when the resistance frequencies for the various sites were determined. Resistance frequencies varied between farms depending on the history of phenylamide use. With the exception of Farm BB, of which only two isolates could be tested, vineyards with no history of phenylamide use had low metalaxyl resistance frequencies (10 and 13.3%). Markedly higher resistance frequencies were, however, observed in vineyards with a history of phenylamide use (25 to 90.9%). Metalaxyl was used in only two of these vineyards (Farm DK) during the 2000/2001 season, and high resistance frequencies were observed in both cases: 81.3% for the Victoria vineyard, and 60.0% for the Festival Seedless vineyard.
Given the presence of phenylamide resistance in all the vineyards, especially the vineyards where these compounds have been applied, anti-resistance strategies should be implemented to manage fungicide resistance and avoid resistance build-up against existing and new fungicides.
Fourie, P H. 2003. Metalaxyl resistance of grapevine downy mildew, Plasmopara viticola, in South African vineyards. Poster presented at the 8th International Congress of Plant Pathology. 2-7 February, Christchurch, New Zealand.
Fourie, P H and Vermeulen A K. 2003. Fungicide resistance in downy mildew populations in Western Cape vineyards. Poster presented at the 27th National Congress of the South African Society for Enology and Viticulture. 5-7 November, Somerset West, South Africa.
Fourie, P H. 2000. Beheer van swamsiektes. Presentation at the Eksteenskuil Subsistence Farmers’ Day. 6 April, Eksteenskuil, South Africa.
Fourie, P H. 2000. Epidemiology and control of downy mildew on grapevine. Presentation at the South African Society for Enology and Viticulture Table and Raisin Grape Short Course. 1 August, Upington, South Africa.
Fourie, P H. 2003. Survey for metalaxyl resistance in Plasmopara viticola populations in Western Cape vineyards. Paper presented at the 41st Congress of the Southern African Society for Plant Pathology. 19-22 January 2003, Bloemfontein, South Africa.
Vermeulen, A K. 2004. Identifisering en bestryding van wingerd siektes. Presenation at the Eksteenskuil Subsistence Farmers’ Day. 13 November, Eksteenskuil, South Africa.
Fourie, P H, Vermeulen, A K. 2002. Nabetragting van die 2001 – 2002 donsige skimmel epidemie, WineLand, Mnth Sep (p. 97-101)
Fourie, P H. 2004. Metalaxyl sensitivity status of downy mildew populations in Western Cape vineyards, South African Journal of Enology and Viticulture, v. 25 (1) (p. 19-22)