1Department of Plant Pathology, PAU, Ludhiana 141004, India; 2INRA Centre de Bordeaux, France

Background and objectives
Triazole fungicides are being widely used for the control of powdery mildew of grapes caused by Uncinula necator in India. These systemic fungicides are known to be confronted with the development of resistance in the target pathogens. Therefore, the population behaviour in Uncinula necator was studied with regard to sensitivity to triadimefon and other fungicides.

Materials and methods
Sixteen samples of U.necator on leaves were collected from fungicide-treated vineyards in different regions of India, and the sensitivity of conidial populations to triadimefon was studied following conidial germ-tube length technique and sporulation score method on leaf disks of cv. Cinsaut [1] and compared with sensitive reference strain Ane-17.

Results and conclusions
Among sixteen conidial populations of U. necator studied for their sensitivity response to triadimefon (Bayleton-5), thirteen showed sensitive reaction with their germ-tubes mostly measuring far less than 250 u and with deformations at the discriminatory dose of 0.3 ug/ml, and led to negligible or no sporulation. These were comparable in their response with the reference sensitive strain Ane-17. However, three populations had 30-71% conidia growing at this dose with normal sporulation. These had 4-6% conidia showing normal germ-tube growth at 3.0 ug/ml. Two of these populations, i.e. 1a from Bangalore and 7a from Pune, had about one percent conidia producing normal germ-tubes at 10 ug/ml of triadimefon with formation of few conidiophores and conidia thus confirming resistance to this triazole.
Two monoconidial isolates from the populations la and 7a, which showed moderate resistance to triadimefon, were further studied for sensitivity to different concentrations of triadimenol (Baytan-5) and fenarimol (Rubigan-4) by sporulation test. As compared to the sensitive strain Ane-17 which showed 96% efficacy at 0.3 ug/ml of triadimenol, isolates l a and 7a had only 47 and 66% efficacy respectively, at this concentration. At 1 ug/ml of triadimefon, efficacy increased to 91 and 92% in the two isolates compared to 100% in Ane-17. However, when tested for sensitivity to fenarimol, these isolates were as sensitive as the reference strain.
Although development of triadimefon resistance in U. necator is not wide spread in India and there are no apparent disease control failures due to this problem as in some other countries [2], the study shows prevalence of some resistant forms of the pathogen in triadimefon treated vine-yards, and efforts should be made to minimise and rationalise the use of these fungicides. This forms the first report of resistance development in U. necator to trizole fungicides in India.

1. Steva H, Clerjeau M, 1990. Med. Fac. Landbouw. Rijksuniv, Gent 55, 983-88.
2. Thind T, 1993. Pestology 17, 32-43.