Diseases and pests of the flowering dogwood

While the flowering dogwood is a tree of exceptional beauty, it is not without its vulnerabilities. A number of diseases and pests can affect its health, sometimes with serious consequences. Proactive management, which includes selecting resistant varieties, maintaining tree vigor through proper cultural practices, and early detection of problems, is the cornerstone of keeping a dogwood healthy. An informed gardener who can recognize the common signs of trouble is well-equipped to intervene effectively, protecting their investment and ensuring the tree’s longevity and seasonal splendor in the garden landscape.

Major fungal diseases

Among the most significant threats to the flowering dogwood are several fungal diseases, with dogwood anthracnose (Discula destructiva) being one of the most serious. This disease typically thrives in cool, wet conditions and can cause tan-colored spots with purple borders on the leaves and flower bracts. As the disease progresses, it can move into the twigs, causing cankers and dieback, and can eventually infect the main trunk, which can be fatal to the tree. Symptoms often appear first on the lower branches and progress upwards through the canopy.

Powdery mildew is another extremely common fungal issue affecting dogwoods, appearing as a white, dusty or felt-like coating on the surfaces of the leaves. While it may not be as lethal as anthracnose, a severe infection can be quite damaging. It can cause leaves to twist, distort, and drop prematurely, which reduces the tree’s photosynthetic capacity and overall vigor. Powdery mildew is most prevalent in shady, humid locations with poor air circulation, conditions that allow the fungal spores to germinate and spread easily.

Spot anthracnose (Elsinoe corni) is a different disease from the more destructive dogwood anthracnose and is generally less of a threat to the tree’s life. It primarily affects the flower bracts, causing small, circular spots with reddish-purple borders. While it can mar the beauty of the spring floral display, it rarely causes significant long-term harm to the tree. This disease is also favored by wet spring weather, and its severity can vary greatly from year to year depending on the environmental conditions during the flowering period.

Several other fungal leaf spot diseases, caused by pathogens such as Cercospora and Septoria, can also affect dogwoods. These typically manifest as small, distinct spots on the leaves during the wet months of summer. While a heavy infestation can lead to some premature leaf drop, these diseases are generally considered cosmetic and do not pose a serious risk to the tree’s health. Management for all these fungal issues hinges on prevention, including improving air circulation through pruning, avoiding overhead irrigation that wets the foliage, and cleaning up fallen leaves in the autumn to reduce the amount of overwintering fungal inoculum.

Destructive insect pests

The most damaging insect pest of the flowering dogwood is unquestionably the dogwood borer (Synanthedon scitula). This pest is the larval stage of a clearwing moth that resembles a wasp. The adult moth lays its eggs on the bark, and upon hatching, the small larvae tunnel into the cambium layer just beneath the bark. This tunneling activity disrupts the transport of water and nutrients, which can girdle branches or even the main trunk, leading to canopy dieback and, in severe cases, the death of the tree. The borer is particularly attracted to trees that are stressed by drought, sunscald, or mechanical injury.

Another pest that can cause noticeable damage is the dogwood clubgall midge (Resseliella clavula). The adult midge lays its eggs in the terminal buds of new shoots. The feeding of the larvae inside the shoot causes the formation of a spindle-shaped swelling or “clubgall” at the tip of the twig. This gall prevents the twig from elongating further, leading to stunted and distorted growth. While a light infestation is mostly a cosmetic issue, a heavy presence of these galls can disfigure the tree and reduce its overall vitality.

Scale insects are another group of pests that can infest dogwoods. These small, immobile insects attach themselves to the bark of twigs and branches, where they feed on the tree’s sap using their piercing-sucking mouthparts. A heavy scale infestation can cause yellowing of leaves, branch dieback, and a general decline in tree health. Some scale species also excrete a sticky substance called honeydew, which can lead to the growth of black, sooty mold on the branches and leaves below.

Various other insects may occasionally feed on dogwoods, such as aphids, which feed on the sap of new growth, and the dogwood sawfly, whose larvae can skeletonize leaves. However, these are typically less damaging than borers and heavy scale infestations. The primary defense against all insect pests is to maintain a healthy, vigorous tree, as stressed plants are always more attractive targets. Avoiding trunk wounds from lawnmowers and string trimmers is especially critical in preventing dogwood borer attacks.

Prevention through cultural practices

The most effective strategy for managing diseases and pests on flowering dogwoods is prevention through sound cultural practices. This approach focuses on creating an environment where the tree is healthy and resilient, making it inherently less susceptible to problems. It all begins with planting the right tree in the right place. Siting the dogwood where it receives morning sun and afternoon shade, in well-drained, acidic soil, sets the stage for a vigorous life by minimizing environmental stress from the very beginning.

Maintaining proper air circulation is crucial for preventing fungal diseases. This can be achieved by giving the tree adequate space to grow without being crowded by other plants or structures. Strategic pruning to thin out a dense canopy can also significantly improve airflow, allowing leaves to dry more quickly after rain and reducing the humidity that fungi need to thrive. When pruning, it is important to remove any dead, damaged, or crossing branches, as these can be entry points for both diseases and pests.

Watering practices play a significant role in disease management. Using soaker hoses or drip irrigation to apply water directly to the soil, rather than using overhead sprinklers, helps to keep the foliage dry and less hospitable to fungal spores. If overhead watering is unavoidable, it should be done in the early morning so that the leaves have ample time to dry in the sun. Furthermore, maintaining consistent soil moisture through proper irrigation and mulching helps prevent drought stress, which is a major predisposing factor for attack by the dogwood borer.

Sanitation is another key component of a preventative program. In the autumn, after the leaves have dropped, it is important to rake up and remove all fallen leaves and debris from around the base of the tree. This material can harbor overwintering fungal spores and insect eggs, which would otherwise emerge the following spring to re-infect the tree. A clean garden bed reduces the primary source of inoculum for many common diseases, giving the tree a much healthier start to the new growing season.

Choosing disease-resistant cultivars

One of the most powerful tools a gardener has for avoiding disease problems is the selection of genetically resistant cultivars at the time of planting. In response to the devastating impact of diseases like dogwood anthracnose and powdery mildew, plant breeders have developed a number of improved varieties of Cornus that show excellent resistance. Opting for one of these cultivars can dramatically reduce the need for fungicide applications and other intensive management practices down the line.

The Stellar series of dogwoods, which are hybrids between the native Cornus florida and the Asian Kousa dogwood (Cornus kousa), are renowned for their high resistance to both dogwood anthracnose and powdery mildew. Cultivars in this series include ‘Aurora’, ‘Celestial’, ‘Stellar Pink’, and ‘Constellation’. These hybrids combine the disease resistance of the Kousa dogwood with the large, showy bracts characteristic of the flowering dogwood, making them an excellent choice for modern landscapes.

Researchers have also developed improved cultivars directly from the native Cornus florida species. The ‘Appalachian’ series, including ‘Appalachian Spring’, ‘Appalachian Joy’, and ‘Appalachian Mist’, were specifically selected for their strong resistance to dogwood anthracnose. ‘Appalachian Spring’, in particular, has shown a very high level of resistance and is a superb choice for areas where this disease is prevalent. These cultivars allow gardeners to enjoy the classic beauty of the native flowering dogwood with much less worry about its most serious disease.

While Kousa dogwoods (Cornus kousa) are a different species, they are often recommended as an alternative to Cornus florida in areas with high disease pressure. They are naturally highly resistant to dogwood anthracnose and tend to be more tolerant of drought and sun than their American counterparts. They bloom about two to three weeks later than Cornus florida, extending the flowering season in the garden, and also offer attractive exfoliating bark and edible raspberry-like fruit. For those struggling with diseases on native dogwoods, the Kousa dogwood and its many cultivars offer a beautiful and resilient substitute.

Treatment options for active problems

When preventative measures are not enough and an active disease or pest infestation occurs, various treatment options are available. For fungal diseases like powdery mildew and anthracnose, fungicide applications can be effective if started at the first sign of disease. It is critical to choose a fungicide that is labeled for use on dogwoods and is effective against the specific pathogen. For best results, these sprays often need to be applied preventatively, beginning at bud break in the spring, and may require multiple applications throughout the cool, wet periods when the disease is most active.

For controlling insect pests, the approach depends on the specific insect. For dogwood borers, prevention is paramount, but if an infestation is present, control is difficult because the larvae are protected beneath the bark. Insecticide sprays containing permethrin or other appropriate active ingredients can be applied to the trunk and main branches to prevent new larvae from boring into the tree. The timing of these sprays is crucial and must coincide with the period when the adult moths are flying and laying eggs, typically from late spring through mid-summer.

In the case of pests like scale insects and aphids, horticultural oils and insecticidal soaps can be very effective and are generally less toxic to beneficial insects than broad-spectrum insecticides. Dormant oil can be applied in the late winter or very early spring before new growth begins to smother overwintering scale insects. During the growing season, summer-weight horticultural oils or insecticidal soaps can be used to control the crawler stage of scales and active aphid populations. Thorough coverage of the infested areas is essential for these contact pesticides to work.

Ultimately, any treatment plan should be part of an Integrated Pest Management (IPM) approach. This means relying on chemical controls as a last resort and instead prioritizing cultural practices, resistant varieties, and the conservation of beneficial insects that can act as natural predators. When a treatment is deemed necessary, it is important to correctly identify the problem, select the least toxic and most effective control method, and apply it at the proper time and according to all label directions to maximize effectiveness and minimize any negative impact on the environment.

_{📷 Flickr / Szerző: David Illig / Licence: CC BY-NC-SA 2.0}