Department of Plant Pathology, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa

Background and objectives
Monitoring changes in the frequency of resistance conducted in 10 commercial vineyards under high-, medium- and low-schedule dicarboximide (DC) programmes showed that the resistance incidence fluctuated from low (average 12.7%) over the winter period to high (average 55.8%) during the growing season [1]. In vineyards subjected to a high-schedule DC programme, the frequency of resistant strains increased drastically after bloom. However, in some of these vineyards DC resistance increased early in the season prior to the application of DCs. Folpet is usually applied at the pre-blossom stage in local vineyards to control Phomopsis viticola. In French vineyards, the increase in DC resistance frequencies was reported after the application of folpet [2]. The aim of this study was to determine the sensitivity of B. cinerea isolates, sampled from the high DC schedule vineyards, to folpet.

Materials and methods
B. cinerea isolates were obtained during the post-harvest stage from Dan-ben-Hannah and Waltham Cross vineyards in Simondium and in Northern Paarl. All isolates were screened against discriminatory concentrations of 3 g iprodione/ml or 5 g carbendazim/ml to characterise them for DC or benzimidazole (MBC) resistance, respectively. Fourteen sensitive, 10 ultra-low- and 5 low-level DC resistant strains, characterised in a previous study [1], were also tested against folpet to determine possible cross-resistance. Standard mycelial growth tests of all the isolates were done on a range of folpet-amended potato dextrose agar. Percentage inhibition was calculated and ED50-values determined.

Results and conclusions
Sixty-one percent of the 23 B. cinerea isolates sampled from the Dan-ben-Hannah vineyard in Simondium was resistant to iprodione; 93% of the iprodione resistant isolates were also resistant to carbendazim. Folpet ED50-values of the iprodione sensitive isolates ranged from 4.9 to 29.1 g/ml, whereas the folpet ED50-values of the iprodione resistant isolates ranged from 19.7 to above 100 g/ml. Five isolates were obtained from the Waltham Cross vineyard in Simondium; 20% were resistant to iprodione and carbendazim. The iprodione/carbendazim resistant isolate had a folpet ED50-value of 19.8 g/ml, while the iprodione sensitive isolates had folpet ED50-values ranging from 15.0 to 43.5 g/ml. Ninety-five percent of 22 and 21 isolates from the Northern Paarl Dan-ben-Hannah and Waltham Cross vineyard, respectively, were iprodione resistant. The iprodione sensitive isolate from the Waltham Cross vineyard was resistant to carbendazim, and had a folpet ED50-value of 15.6 g/ml. Iprodione resistant isolates had folpet ED50-values ranging from 21.5 to above 100 g/ml. The iprodione sensitive isolate from the Dan-ben-Hannah vineyard had a folpet ED50-value of 11.9 g/ml, and the iprodione resistant isolates had folpet ED50-values ranging from 21.6 to above 100 g/ml. The previously characterised DC sensitive isolates had folpet ED50-values ranging from 3.9 to 19.7 g/ml, the ultra-low-level DC resistant isolates from 5.5 to 62.1 g/ml, and the low-level DC resistant isolates from 11.3 to above 100 g/ml. The early increase of DC resistance frequencies in vineyards under the high DC schedule can thus be attributed to the dual resistance in B. cinerea sub-populations to DCs and folpet. This phenomenon was not observed in the low and medium DC schedule vineyards, despite the use of folpet during the pre-blossom stage. This may be explained by the significantly lower DC resistance balance value reported in these vineyards [1]. Reduced sensitivity to folpet in B. cinerea populations affects the management of DC resistance, and the repetitive use of this broad-spectrum fungicide in high DC schedule vineyards, or vineyards with a high DC resistance balance value, should be avoided.

1. Fourie PH, 1996. MSc.Thesis, University of Stellenbosch, South Africa.
2. Leroux P, Clerjeau, M, 1985. Crop Protection 4, 137-160.