By Stan V. Griep
American Rose Society Consulting Master Rosarian – Rocky Mountain District
The powdery mildew that attacks roses is known as Sphaerotheca pannosa var. rosae. It can be frustrating to gardeners when having to deal with roses with powdery mildew. Keep reading to learn more about the causes of powdery mildew on roses and how to get rid of it.
Identifying Powdery Mildew on Roses
Powdery mildew on roses will form what looks like a white powder over the surface area of the rose leaves, 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 been killed.
Powdery mildew on rose bushes loves to attack the fresh new foliage of the plants and will also stunt the bud growth, causing disfigured blooms and, if left unchecked, will prevent the buds from opening. Warm, dry days followed by cool, humid nights are perfect conditions for an outbreak of powdery mildew.
The only product I have found that works extremely well on this fungal disease is Green Cure. Potassium bicarbonate is one its main ingredients. Apply this fungicide as directed to treat powdery mildew.
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 calls for baking soda and dish soap along with another ingredient or two. I have even seen cooking oil listed for use in the mix! While baking soda (sodium bicarbonate) does have some ability to work against the fungal infections, problems with the “homemade” mixes can be the dish soap and cooking oil.
Some dish soaps will burn the leaves and can even act against the effects of the baking soda. Cooking oil could separate itself from the mix enough that it would be sprayed onto the foliage in a concentrated form, thus clogging leaf pores and leading to severe leaf burn or leaf scorch.
Roses Resistant to Powdery Mildew
Powdery mildew on roses can be a very tough customer in the rose bed, spreading to other roses quickly if left unchecked. Some varieties of rose bushes are 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 that had no signs of the powdery mildew.
In today’s rose bush 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 it. However, if one wants to have less disease problems with their roses, seeking out the disease resistant varieties may well be worth the effort.