Horticulture Week
29 May 2009

Apple growers whose scab control is inadequate should soon know whether it is due to the development of fungicide-resistant strains of the pathogen or simply poor spraying.

The answer may be found by a two-year Horticultural Development Company (HDC)-funded project being undertaken at East Malling Research by Dr Xiangming Xu.

Although there is anecdotal evidence that some scab strains from orchards regularly sprayed with a triazole fungicide appear to have reduced sensitivity to the product, he believes that most scab control failures are due to poor spray timing. Nevertheless, in the US and Canada the overuse of triazole (or DMI) fungicides has led to the emergence of scab strains that are less sensitive to these products. In the long term this may result in poor control of the disease.

The project’s main aim is to generate information on the baseline sensitivity of the scab population to the widest-used fungicides. This will enable growers to develop more sustainable anti-resistance strategies to control the disease.

Xu warned that if reduced scab sensitivity to one fungicide exists, care would be needed to select alternatives without jeopardising disease control and resistance management. The possibility of loss of fungicides (such as the triazoles under the new EU pesticide legislation) makes anti-resistance management even more important, he maintained.

HDC communications manager Andrew Tinsley said that in Canada a number of fungal diseases have quickly developed resistance to kresoxim-methyl, a strobilurin widely used on apples, vines (and arable crops) but so far there is no evidence of that happening in fruit here, although it has in cereals.

“We’re trying (in the scab project) to discover exactly what’s happening in orchards rather than what growers think is happening,” Tinsley explained