For the past week, questions of “what’s going on with the dogwoods?” have inundated county Extension offices, but until yesterday only a few of these samples had been submitted to the UK Plant Disease Diagnostic Laboratory (PDDL) for diagnosis. Concerned homeowners reported symptoms ranging in severity from mild leaf scorch to sudden, alarming defoliation. Other symptoms included leaf distortion, marginal burn, brown blotches, and a white, powdery coating on leaves. The dozen or so dogwood samples I examined in the PDDL yesterday—and another dozen I “surveyed” during a neighborhood walk yesterday evening—had two common foliar diseases: powdery mildew and anthracnose. Both of these diseases are caused by fungi that are quite host-specific, meaning the powdery mildew fungus that infects dogwood can spread to nearby dogwoods, but does not infect other, unrelated plants. Likewise, the anthracnose fungus on dogwoods is different from the anthracnose of other shade trees, even though symptoms are similar.
Dogwood Powdery Mildew
On dogwood, young expanding leaves infected with powdery mildew are often crinkled and distorted and fail to reach a normal size. In addition to white, powdery fungal growth easily visible on leaves, foliage often becomes scorched, either along the leaf margins or in patches on the leaf blade, sometimes even before the powdery growth is obvious. Later in summer, heavily infected trees may appear drought-stressed due to excessive moisture loss through the leaves. Powdery mildew was clearly visible on all of yesterday’s dogwood samples as well as all the neighborhood trees I surveyed (Figure 1).
High humidity favors powdery mildew, therefore, cycles of wet, followed by hot, drier weather this spring promoted exceptional early development of powdery mildew. Dramatic scorching occurred in some trees when infected leaves were exposed to sudden, intense heat at the end of May.
Some of the browning and leaf drop reported recently, however, did not fit the typical symptoms associated with powdery mildew. Close examination of samples confirmed what I suspected: dogwood anthracnose disease, caused by the fungus Discula destructiva, was present, too, on about half of the samples in the “mini-survey” (Figures 2 and 3).
Symptoms of anthracnose include blighting/blackening and collapse of leaves, and/or large, irregular brown patches, often (but not always) with a dark purple border. These symptoms tend to be worse in the lower canopy. Other shade trees such as ash or maple readily drop anthracnose-infected leaves, but such extensive leaf drop on dogwood is less common. (Only a few trees in my survey were defoliating, but homeowners have reported dramatic leaf drop in some landscapes). Cool, rainy weather during leaf expansion favors anthracnose, thus, weather patterns this spring also promoted anthracnose infections.
Management and Prognosis
Returning to the current dogwood drama, at this point the anthracnose disease must “run its course.” The window of treatment with fungicides is past once symptoms are apparent, and treating large trees can be difficult and expensive. Raking up and removing fallen leaves will reduce disease risk next year. However, small twig tips may harbor the anthracnose fungus, increasing the risk of disease on a given tree in subsequent years, providing weather conditions are cool and wet during leaf expansion.
Since humidity favors powdery mildew, we are likely facing severe dogwood powdery mildew situations all season. Leaves may become more scorched as the summer progresses, and although powdery mildew is not considered a “lethal” disease, reduced leaf function may weaken trees over time, leaving them vulnerable to borer infestation and opportunistic diseases. Mulching and irrigation directed at the root zone will reduce moisture stress to infected trees.
For more information on dogwood anthracnose and powdery mildew, refer to UK Extension Plant Pathology factsheets:
By Julie Beale, Plant Disease Diagnostician