Dogwood anthracnose, caused by the fungal pathogen Discula destructiva, is a serious disease that affects various dogwood species. All varieties of the native flowering dogwood (Cornus florida) and Pacific dogwood (C. nuttallii) are susceptible to this disease. It was inadvertently introduced into eastern North America from Asia in the late 1970s and quickly spread throughout the region, decimating natural populations of flowering dogwood in southern New England. Because D. destructiva is non-native in North America, flowering dogwood has no natural defense against the pathogen, allowing the fungus to spread unchecked within the tree. While North American species, Cornus florida (flowering dogwood) and C. nuttallii (Pacific dogwood), are highly susceptible, Cornus kousa, C. alternifolia and C. amomum are relatively resistant.
Tan spots with irregular purple margins appear on the leaves in lower crown in the spring and progress up the tree if cool and wet spring weather persists. Spots on the leaves may remain small or gradually enlarge, manifesting as big, brown, irregularly shaped blotches. These blotches can cause scorch of the leaf margin or blight the whole leaf, spreading into the petiole and infecting the shoot (Figure 1). Dead gray leaves often remain on the tips of these infected twigs all winter and spring. At flowering, bracts also are infected. More spots and blotchy areas often show up at or near the leaf tips, centered approximately on the mid vein. Blotchy areas usually spread down the mid vein, giving a wedge-shaped appearance to the diseased area. If weather remains dry and unfavorable for disease development, leaves show only the brown spots with dark brown to purple margins until the end of the growing season, instead of developing into blotches.
Due to the disease spread on the leaves, fungal growth will spread to the twigs, especially on leaf nodes, causing brown elliptical cankers. Affected twigs may show sunken tan to brown spots with purple borders, which eventually enlarge and girdle the twig, resulting in twig dieback. Spores produced by the fungus are spread typically by rain, insects or birds in cool, moist weather. Blighting of terminal leaves is common on C. nuttallii during fall after years of extensive spring or fall leaf blighting, resulting in death of terminal buds. This reduces spring budbreak and causes C. nuttallii to refoliate via axial buds or epicormic growth during the mid-summer. These branches are very prone to infections, which may progress into the main stem (Figure 2). Multiple cankers can girdle individual branches or kill the entire tree as trees become severely weakened by infections.
D. destructiva persists as cankers on the trunks and branches of its hosts or in twigs and dead leaves as blister-like fruiting structures known as conidiomata. Early in the growing season, fruiting structures erupt through the bark of infected twigs and on the underside of spotted leaves. Conidia formed in conidiomata on cankers are splashed by rain onto newly expanded leaves, flowers and succulent tissues to initiate new infections in the spring under abundant moisture, humidity and shade. Thus, infection establishes on lower canopy branches first. Once established, the pathogen produces large amounts of conidia that spread upward within the canopy by rain splash. Conidia may infect the current season's shoots directly, forming small cankers, which are usually rapidly surrounded by callus tissue. However, shoots infected from blighted leaves become more severely cankered and die back. While short-range dispersal can occur with rain splash, long-range dispersal is usually facilitated by insects, birds or contact with infected plants.
Control of the disease is difficult, particularly in forest settings. In the landscapes, parks and gardens, a combination of cultural and chemical controls can be used. Because the pathogen overwinters in the cankers and dead leaves on twigs, removal of the infection source by pruning plays an important role. In addition, emphasis is given to watering during periods of drought or yearly fertilization, ensuring adequate sunlight, mulching and use of resistant cultivars. Mulching trees can help to reduce watering needs as well as protect trunks from mechanical injury.
- Host resistance: Tartarian dogwood (Cornus alba), redosier dogwood (C. sericea), kousa dogwood (Cornus kousa), cornelian cherry (C. mas) and hybrid dogwood (C. florida x C. kousa) have reasonable resistance to the disease.
- Healthy dogwoods are able to withstand disease infection much better than stressed trees. The key to maintaining their health is to plant in a suitable location and take proper care to minimize stress, including watering during drought, avoiding too much mulch against the trunk, fertilizing and pruning.
- Avoid digging native trees from the woods and transplanting them into landscapes. This practice can introduce the disease into a neighborhood that was previously disease-free.
- Avoid over application of fertilizer, which can result in succulent new growth with greater susceptibility to the disease.
- Prune out all dead or dying twigs and limbs during dry weather. All water sprouts or suckers on trunks and branches should also be removed.
- In the fall, rake and remove fallen leaves. Remove any dead leaves still attached to the branches.
- During extended dry periods, provide trees with a soaking irrigation of the root zone and maintain a 2- to 3-inch layer of composted mulch over the root zone.
- Fungicide treatments will reduce new infections, but they will not cure existing ones. Treatments should be applied three to four times at 14-day intervals starting at bud break.
- Registered fungicides can be used on trees in landscapes in the spring at bud break, followed by additional sprays every 10 to 14 days until leaves are fully expanded. Trees also should be sprayed once in the fall after the leaves have changed color, but before leaf drop.
- Tank-mix and/or alternate products from different groups with different modes of action can be used to prevent the build-up of resistant fungi. Limit the use of any one group during the growing season.
- Bayer Advanced Disease Control at 0.75 fluid ounces per gallon water
- Bonide Fung-onil Multi-Purpose Fungicide at 2.25 teaspoons per gallon water
- Mancozeb-based products – Group M3 fungicides (24-hour re-entry)
- Fore 80 WP at 1.5 pounds per 100 gallons water, plus a spreader-sticker
- Protect DF at 1 to 2 pounds per 100 gallons water, plus 2 to 4 ounces spreader-sticker
- Propiconazole-based products – Group 3 fungicides
- Banner MAXX at 2 to 4 fluid ounces per 100 gallons water (12-hour re-entry)
- Infuse Systemic Disease Control at 1 tablespoon per gallon water
- ProCon-Z at 8 to 12 ounces per 100 gallons water (24-hour re-entry)
- ProPensity 1.3 ME at 2 to 8 fluid ounces per 100 gallons water (24-hour re-entry)
- Liquid systemic Fungicide (ferti-lome) at 0.5 fluid ounces per gallon water
Figure 1. Leaf symptom of dogwood anthracnose (Photo credit: Robert L. Anderson, USDA Forest Service, Bugwood.org)
Figure 2: Spread of infection from twig to the main stem (Photo credit: Joseph OBrien, USDA Forest Service, Bugwood.org)
Author: Mahfuz Rahman, WVU Extension Specialist – Plant Pathology