Young shoots are particularly affected but even with slight infections the white fungal growth on leaves, stems and flowers and associated leaf yellowing make plants unsightly and mean that growers have to use regular treatment with fungicides. Weekly sprays may be necessary.
HDC project HNS 156 was set up to help growers by providing data on the efficacy and crop safety of new fungicides as the extension of use arrangements are being replaced with the SOLA approval system for ornamentals. It also aimed to help growers avoid resistance problems by developing a sustainable fungicide regime.
Resistance can develop when the same fungicide or products from the same fungicide group are used repeatedly. There is a relatively high risk of resistance developing in powdery mildew fungi because of their short life cycles and abundant spores.
Based on results for field and protected crops, three eight-spray programmes with reduced resistance risk that complied with the requirements for sustainable powdery mildew control were developed. Full detailed advice on the products suitable for each species are in the grower summary report and an HDC fact sheet will be published shortly.
Applying fungicides at 14-day intervals at 250 litres per hectare may be insufficient to provide season-long control on highly susceptible species. Growers are advised to use a shorter spray interval and higher volume.