Most dogwood “fungus” is one of four diseases, and three of them are mainly cosmetic. Clemson Cooperative Extension lists powdery mildew, spot anthracnose, Cercospora and Septoria leaf spots, and dogwood (Discula) anthracnose as the main diseases of flowering and kousa dogwoods. Only Discula anthracnose regularly kills trees. Getting the diagnosis right tells you whether to spray, prune or simply rake up the leaves.

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The short answer: white, powdery growth on the leaves is powdery mildew, now the most common dogwood disease (University of Maryland Extension). Tiny tan spots with reddish-purple borders on the flower bracts are spot anthracnose. Angular brown spots, ⅛ to ¼ inch and bordered by veins, in wet summers are Cercospora or Septoria leaf spot (Clemson). Large tan blotches with purple borders, spreading into twig dieback and trunk cankers, are Discula anthracnose, which can kill a tree. Prevention is the same for all four: a healthy tree, good air flow, no overhead watering, prompt cleanup of fallen leaves and resistant varieties.

Identify the disease

Disease	Main sign	When	Serious?
Powdery mildew (Erysiphe pulchra)	White, powdery coating on upper leaf surfaces and new shoots	Mostly late summer	Usually minor; severe if early
Spot anthracnose (Elsinoe corni)	Tiny (⅛ in) tan spots with reddish-purple borders, first on flower bracts	Wet springs	Minor unless repeated
Cercospora and Septoria leaf spot	Angular to irregular ⅛–¼ in spots bordered by veins	Wet summers	Minor
Discula anthracnose	Tan or purple-bordered leaf blotches, twig dieback, trunk cankers	Cool, wet weather	Serious, can kill

Powdery mildew

Clemson describes powdery mildew as one of the most common fungal diseases of flowering dogwood. It is caused by Erysiphe pulchra and infects leaves and tender new shoots, and wind spreads its spores. The University of Maryland Extension notes that it has now overtaken anthracnose as the most common dogwood problem.

Treatment: Clemson notes that most infections develop late in summer and cause little long-term damage. Infections that start early in the season can be severe and may justify fungicides. They work best when started as soon as symptoms appear. Clemson lists copper-based products and the systemic fungicides myclobutanil, propiconazole, triadimefon and thiophanate-methyl, and advises alternating products with different modes of action. For new plantings, both Clemson and Maryland recommend mildew-resistant cultivars.

Spot anthracnose

Clemson describes spot anthracnose, caused by Elsinoe corni, as a common disease of flowering dogwood in wet springs. It infects the flower bracts first, then the leaves, young shoots and fruit. Spots are about ⅛ inch, tan with reddish-purple borders, and badly affected bracts and leaves wrinkle. It often causes little lasting damage, but severe or repeated infections can weaken a tree.

Treatment: if spotting is severe, Clemson suggests fungicides beginning at bud break and repeated every 10 to 14 days until the leaves are fully expanded. Kousa dogwood is resistant.

Cercospora and Septoria leaf spots

Clemson notes that both diseases are most common in wet summer weather and that the fungi overwinter in infected leaves. Both cause small spots, about ⅛ to ¼ inch, that are angular to irregular and usually bordered by leaf veins. Cercospora spots are tan-brown with diffuse borders.

Treatment: rake up and dispose of fallen leaves in autumn, water at the base rather than over the leaves, and prune to improve air flow. Fungicides are rarely needed.

Discula anthracnose

This is the dogwood disease to take seriously. Clemson calls Discula anthracnose a serious disease that can cause branch dieback and tree death. Maryland Extension notes that early symptoms appear in mid to late May as leaf spots with tan or purple borders. Young green stems and water sprouts are especially susceptible, and dark cankers girdle stems and cause dieback. To tell it from other leaf spots, Maryland suggests looking at the underside of the leaf with a hand lens for numerous tiny tan to brown dots, about the size of a printed period, scattered across the blighted area.

Clemson notes that multiple cankers can girdle the main trunk and kill the tree, and that it occurs mostly at higher elevations, in cool, wet conditions.

Treatment:

Prune out and dispose of dead and cankered twigs and branches, and remove water sprouts.
Rake and dispose of fallen leaves.
Improve air flow and keep the tree well watered in dry spells.
Fungicides labeled for spot anthracnose are also labeled for Discula anthracnose (Clemson). Apply them in spring as leaves emerge on high-value trees.
Replant with resistant kousa dogwood or a resistant hybrid if a tree dies.

Prevention for all four

Clemson’s most important prevention advice is to keep trees vigorous through good planting, sanitation, pruning, irrigation and resistant cultivars. Flowering dogwood is shallow-rooted and prefers moist, well-drained, acidic soil with afternoon shade. Water deeply in dry periods, mulch the root zone, avoid wetting the leaves and give the tree room for air to move. For more on site and care, see why a dogwood is dying and is my dogwood tree dead?.

Sources

Dogwood Diseases and Insect Pests, Clemson Cooperative Extension HGIC
Dogwood Anthracnose, University of Maryland Extension
Dogwood (HGIC 1009), Clemson Cooperative Extension, Home & Garden Information Center
Growing Flowering Dogwood Trees, University of Maryland Extension

Top photo: Jerzy Opioła, CC BY-SA 4.0, via Wikimedia Commons

Dogwood Fungus: Identify and Treat the 4 Common Diseases