Powdery mildew is a common fungal disease affecting dogwood trees (Cornus species). It is caused primarily by the fungi Erysiphe pulchra and Phyllactinia guttata. While the infection can make a dogwood tree appear unsightly, it rarely results in the death of a mature, otherwise healthy specimen. The disease does, however, reduce the tree’s photosynthetic capability and can weaken it, potentially leading to increased susceptibility to other problems like the dogwood borer. Effective management involves identifying the problem early, modifying the growing environment, and applying appropriate treatments.
Recognizing Powdery Mildew and Susceptibility
The clearest sign of infection is the development of white, talc-like spots or patches on the leaves, stems, buds, and flowers. Unlike many other fungal diseases, powdery mildew does not require standing water on the leaf surface to spread. Instead, it thrives in periods of high humidity combined with dry leaves. On dogwoods, symptoms can also appear as mottled yellowing, reddish-brown blotches, or premature leaf scorch and curling, especially on susceptible varieties.
The flowering dogwood (Cornus florida) is particularly vulnerable, with many red-flowered cultivars showing high susceptibility. The fungi reproduce rapidly, sometimes completing a life cycle in as little as seven days, allowing the infection to quickly cover new growth. Environmental factors that concentrate the pathogen include poor air circulation, heavy shade, and crowded planting conditions. Resistant species like Kousa dogwood (C. kousa) and specific hybrids, such as ‘Stellar Pink’ or ‘Appalachian Joy,’ offer a long-term solution for future planting.
Cultural Management to Prevent Recurrence
Modifying the tree’s environment is the most fundamental step in preventing the fungus from gaining a foothold. Improving air movement is a priority, achieved by selectively pruning crowded interior branches to allow better light penetration. This structural thinning helps reduce the high humidity pockets that the mildew needs to flourish.
Irrigate the soil directly using soaker hoses or microjet sprinklers, rather than wetting the foliage, to avoid creating a favorable humid microclimate. Watering the tree in the morning allows any accidental moisture on the leaves to evaporate quickly throughout the day. Avoid cultural practices that promote excess new growth, such as heavy nitrogen fertilization, as this tender tissue is appealing to the fungus. Collecting and destroying all fallen leaves in the autumn is necessary because the fungus overwinters in protective structures called chasmothecia on this debris.
Immediate Non-Fungicidal Treatments
For immediate control, physical removal of infected material is the first step. Heavily infected leaves and shoots should be pruned off as soon as they appear, but avoid removing too much foliage, which can damage the tree. Dispose of this infected plant material away from the garden, ensuring it is not added to a compost pile where fungal spores could survive.
Refined horticultural oil, such as neem oil, is an effective treatment that works by smothering the fungal spores. These oils must be applied with complete coverage to both the tops and undersides of the leaves. Application should be avoided when temperatures are above 85–90 degrees Fahrenheit or when the tree is moisture-stressed, as this can cause leaf burn. A simple solution of potassium bicarbonate, mixed with liquid soap as a spreader, can also be sprayed on the foliage to alter the leaf surface pH, making it inhospitable to the mildew.
When to Use Chemical Fungicides
Synthetic chemical fungicides are generally reserved for high-value trees or when cultural practices and non-fungicidal treatments have failed to control a severe, persistent infection. These commercial products contain active ingredients from various chemical classes, such as triazoles (like myclobutanil or propiconazole) or strobilurins (like azoxystrobin). Sulfur-based fungicides and copper products are also effective and available to homeowners.
Timing is critical for maximum effect, as most synthetic fungicides work best as a preventive measure before or at the first sign of the disease. Once the mildew is established, fungicides can stop its spread, but they will not repair the damage to already infected leaves. Applications must follow the label’s specified intervals, often every 7 to 21 days, throughout the period of new growth. To prevent the fungi from developing resistance, it is recommended to rotate between products that have different modes of action.