However, in garden centres, other areas with public access and home gardens the range of permissible fungicides is limited.
Rose downy mildew outbreaks can make plants unsellable. An increasing number of biostimulant and natural products are being marketed that claim to enhance a plant's resistance to disease and could have potential to help manage mildew and other rose diseases, but there is little scientific evidence on product efficacy.
HDC project HNS 135 carried out by ADAS pathologist Tim O'Neill evaluated a number of products for prevention and control of downy mildew and assessed their effect on the shelf-life of container-grown roses.
After three years of trials, advice to growers can be summarised as a combination of better understanding of the disease and correct use of the best treatments found. Prolonged wetness markedly increases the risks of downy mildew outbreaks so avoid locating container plants in hollows or near windbreaks. Increase plant spacing and time irrigation so that the leaves dry quickly. Remove fallen leaves and trimmings before restocking an area to prevent disease carry-over.
Of all the natural products and biostimulants, the foliar fertiliser potassium phosphate (eg Farm-Fos-44) provided partial control of mildew and improved marketability. Efficacy was improved significantly by the addition of a silicon-based wetter (Silwet-L77) but not with Epsom salts. Do not apply phosphate fertiliser above the recommended rate as this can cause leaf yellowing and even premature leaf fall in some cultivars.