Powdery Mildew on Rosebushes
By Stan V. Griep – Consulting Rosarian – Rocky Mountain District
Member: The American Rose Society, The Loveland Rose Society &
The Denver Rose Society
Powdery mildew on roses will form what looks like a white powder over the surface area of the rose leaves/foliage, and it may also spread to the stems and new rose buds. It can disfigure the leaves and they will not come back to their normal shape even after the powdery mildew has long since been killed. Powdery mildew loves to attack the fresh new foliage of rose bushes but will also attack all of the rosebushes foliage, it can also stunt the bud growth causing disfigured blooms and left unchecked will prevent the bud from opening. Warm dry days followed by cool humid nights are perfect conditions for an outbreak of powdery mildew.
Powdery mildew can be a very tough customer in the rose bed, spreading from one infected rosebush to other rosebushes quickly if left unchecked. Some varieties of rose bushes are very resistant to this fungus while others seem to attract it. I have seen Mister Lincoln hybrid tea roses with severe cases of powdery mildew that were planted right next to a Honey Bouquet floribunda rose bush that had no signs of the powdery mildew on it. In today’s rosebush market, there are several rose bushes that are listed to be disease resistant. This means just that, they are “resistant” to disease but that does not mean they will not contract a disease in the right conditions. However, if one wants to have less disease problems with their roses, seeking out the disease resistant varieties may well be worth the effort, this holds true when looking at powdery mildew. Some rosebushes are specifically listed to be highly resistant to powdery mildew, such as Honey Bouquet, Just Joey, Electron, Voodoo, Tournament of Roses, Europeana, Pope John Paul II, and Scentimental to name just a few.
Using a good earth friendly product like Green Cure sprayed upon our rosebushes at the Prevention Rate every 10 to 14 days is a very good idea. If your rosebushes do happen to get some powdery mildew started, spray them ASAP with some Green Cure at the Cure Rate. The instructions for both application rates are on the product label. Some garden websites speak of what they call the “Cornell Formula” for use as a homemade cure for powdery mildew on rose bushes. The ingredients given on these websites, for the supposed “Cornell Formula”, calls for baking soda and dish soap along with another ingredient or two. I have even seen cooking oil listed for use in the “Cornell Formula” mix! While baking soda (sodium bicarbonate) does have some ability to work against the fungal infections, problems that arise with the “homemade” mixes can be due to the dish soap and the cooking oil in these mixes. Some dish soaps will burn the leaves of the rose bush and can even act against the effects of the baking soda. Cooking oil easily separates itself from the mix enough that it is sprayed onto the foliage in a concentrated form, thus clogging leaf pores and leading to severe leaf burn or leaf scorch. For that reason I do not recommend using any such homemade mixes for fungus treatments on rosebushes. Here is a link to some great information on the Green Cure website for you as well: http://greencure.net/why_is_greencure_fungicide_better.asp
The website also lists places where the product can be purchased on-line. It is easy to buy Green Cure at Amazon.com or at Planet Natural online as well.
Keep an eye on your wonderful rosebushes as any disease or pest attack upon them that is caught early on, is far easier to gain control of. I also highly recommend that you water your rosebushes well the night or day before the application of any fungicide, miticide or insecticide. A well hydrated rosebush is far less likely to have any sort of negative reaction to a given spray application.
Photos of Powdery Mildew on Rosebush Leaves: