First report in England of changes in the susceptibility of Puccinia horiana , the cause of chrysanthemum white rust, to triazole and strobilurin fungicides

This is a first report of Puccinia horiana strains from England tolerant either to triazole or to both triazole and strobilurin fungicides.Elsewhere in Europe there is tolerance to carboxamide fungicides (Dirkse et al., 1982).Increasing tolerance to fungicides can undermine phytosanitary control measures against this EC II/AII listed organism.For two decades, propiconazole controlled chrysanthemum white rust (CWR) caused by Puccinia horiana on Dendranthemum morifolium (Dickens, 1991).However, from March 1999 reports began to accumulate of chrysanthemum growers in the Midlands failing to control the disease on various cultivars, using propiconazole and another triazole, myclobutanil.In September 1999, plants of cv.White Fresco bearing pustules suspected of being fungicide-tolerant were collected from a grower in Manchester.A method to assess fungicides against CWR (Dickens, 1991) was adapted to confirm tolerance.This involved placing the suspect plants above rooted cuttings of susceptible cv.Margaret for 3 days in a damp chamber, minimum temperature 208C, to allow natural infection via sporidia.An isolate of CWR from cv.Wendy, collected from a nursery in Clwyd, Wales, not exposed to fungicides, was used as a sensitive control.Curative sprays of propiconazole, myclobutanil or the strobilurin azoxystrobin, at 0, 100, 200 or 500 mg a.i.L 21 , were applied to run-off with a hand sprayer to the latently infected plants (at least three plants per cultivar per treatment).Both triazoles failed to prevent the suspect tolerant isolate from developing pustules even when applied at 500 mg a.i.L 21 (five times the maximum permitted concentration).This contrasted with azoxystrobin, where suppression was partial at 100 mg a.i.L 21 and complete at 500 mg a.i.L 21 (twice maximum permitted concentration).Both triazoles and azoxystrobin, however, were still effective against the Welsh sensitive strain.Here suppression was either partial or total at 100 mg a.i.