By: Jolene Adams, jolene_adams[at]ix[dot]netcom[dot]com, Hayward, CA
Downy Mildew (fungus – Peronospora sparsa) is a highly destructive disease on roses, causing severe and rapid defoliation. The fungus is active only under cool, damp conditions. Any garden that enjoys mild spring and fall temperatures with a high level of rainfall is subject to this scourge. Leaf symptoms vary from angular blotches (yellow, purple to brown) to a scorch like burn. Downy mildew has been a recent problem in the rose growing fields in California. It has also developed in commercial operations where roses are grown temporarily in containers. Regular overhead watering and thick growth from close spacing create ideal conditions for this disease which can spread quickly under cool, humid conditions. Preventive fungicides that are legally sold in your state should be applied just before the weather becomes ideal for disease development.
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