Like many powdery mildew diseases, high relative humidity but not high rainfall is a key to dogwood powdery mildew, caused by the fungus Erysiphe pulchra. There is a good bit of this disease this year in northeast Ohio, which is quite dry, and yesterday I took a look at some flowering dogwoods (Cornus florida) growing in pots in trials. There was a good bit of fungal mycelial growth and spores en masse evident on the foliage (what is called the “sign” of the pathogen), which is something everyone is familiar with for many powdery mildew diseases. Good examples are lilac powdery mildew or zinnia powdery mildew (caused by different specialized pathogens that do not cross-infect) in that the fungus is readily evident and about all you see of the disease.
There are also “symptoms” of the disease on this dogwood, the reaction of the plant to the fungal infection on the plant. These symptoms are red and brown lesions on the leaves. For many powdery mildew diseases, signs are just about all you see, and these symptoms of foliar desiccation do not develop. On dogwood, often you see relatively little signs of the pathogen with little mycelial growth, but the lesions develop as seen and if soil conditions remain dry, often leaves scorch and curl – from powdery mildew! Diagnosis is ever daunting.
Kousa dogwood (Cornus kousa) is quite resistant to dogwood powdery mildew disease, with flowering dogwood x Kousa dogwood hybrids less susceptible. Mulch to moderate soil moisture, prune to develop greater air movement, and consider fungicides for nursery production to enhance annual plant growth. Fungicides are generally not used for this disease in the landscape.