Why Your Dogwood Looks Sick: Diagnosing Anthracnose, Borers, and Powdery Mildew Before They Cause Serious Damage

Flowering dogwoods are spring showstoppers — but Cornus florida is also one of the most disease-prone ornamental trees in the eastern United States. Three problems account for the vast majority of serious decline: Discula anthracnose, dogwood borer, and powdery mildew. They look different, work differently, and demand different responses. Treating for fungus when a moth larva is eating the cambium wastes a season and leaves the real problem untouched.

This guide covers each problem in detail: what it looks like, why it happens, and what the timing is for effective treatment. For the full growing picture, including siting, soil, and watering, see the dogwood growing guide.

Quick Diagnosis: What Are You Looking At?

Use this table before you act. Getting the diagnosis right is the most important step — the three problems overlap in some surface symptoms but require completely different responses.

Dogwood Anthracnose

Discula destructiva is a fungal pathogen introduced to North America that spread rapidly through Cornus florida populations during the 1970s and 1980s. Left unchecked in favorable conditions, it can kill a flowering dogwood in two to three years. No other common dogwood problem carries that timeline.

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How It Spreads

The fungus overwinters as tiny black pin-point fruiting structures — called conidiomata — on dead twigs and fallen leaves. When wet spring weather arrives, these structures release masses of orange spores that rain and wind disperse to young, developing foliage. The infection pathway is predictable: leaves first, then twigs, then branches, then the main trunk. Once cankers reach the trunk and begin to girdle it, the tree cannot be saved.

Two factors dramatically increase susceptibility. The first is shade: Discula thrives in cool, moist conditions, which is why anthracnose is most severe at elevations above 2,000 feet, where trees spend more time in fog and cool air. The second is excessive nitrogen fertilization — high-nitrogen fertilizers push lush, succulent new growth that the fungus infects far more easily than hardened mature tissue.

What It Looks Like

First symptoms appear in mid-to-late May as spots up to a quarter inch across with tan or brown centers and reddish-purple borders. In wet conditions, spots merge and kill entire leaves. As the season progresses, infected twigs die back from the tips. Look for dead gray leaves that remain attached to branch tips through winter — this is one of the most distinctive signs of Discula anthracnose, rarely seen with other dogwood diseases. On heavily infected trees, water sprouts (epicormic shoots) on the lower trunk signal that the tree is redirecting energy away from dying upper branches.

Treatment

Start with cultural controls before reaching for fungicides:

• Prune dead or dying branches during dry weather — wet conditions spread spores on pruning tools
• Remove fallen leaves and any dead leaves still clinging to branches in autumn
• Eliminate water sprouts from the trunk and main branches
• Water during drought to reduce stress, using drip irrigation rather than overhead sprinklers

If the disease is limited to leaves and small twigs, fungicide applications made at bud break and repeated every 10 to 14 days until leaves are fully expanded can arrest its progress. For fungicide resistance management, rotate between FRAC groups: thiophanate-methyl (Group 1) alternated with propiconazole or tebuconazole (Group 3), with chlorothalonil (Group M5) as an option for alternation. A late-summer application helps if wet conditions persist into August.

Trees where cankers have reached the main trunk and only a few active branches remain cannot be recovered. Removing them is the right call — the dead wood overwintering nearby is a spore source for every other dogwood in the area.

Dogwood Borer

The dogwood borer (Synanthedon scitula) is a clearwing moth native to eastern North America. Adults have clear wings with a black-and-yellow body pattern that closely mimics a yellowjacket wasp — most gardeners who see one dismiss it as a wasp and move on. The damage happens out of sight, inside the bark, and by the time the external signs are obvious, the infestation is well established.

The Wound-Entry Mechanism

This is the most important fact about the dogwood borer, and the one most management guides understate: the larva cannot chew through intact, healthy bark. It requires an existing wound to gain entry. A lawn mower nick, a string trimmer scar, a pruning stub that wasn’t cut cleanly, a crack from winter freeze — any of these becomes an invitation. The female moth actively seeks out damaged bark to lay her eggs. Once the eggs hatch (within 8 to 9 days), larvae tunnel directly into the cambium layer, the thin living tissue just beneath the bark that carries water and nutrients throughout the tree.

This mechanism makes the dogwood borer, unlike anthracnose, almost entirely preventable through cultural practice rather than chemistry.

Life Cycle and Timing

Adult moths emerge starting in mid-May and remain active through September, with peak egg-laying activity in late June and July. There is one generation per year. Larvae overwinter in their feeding galleries inside the tree, resume activity in spring, and pupate before emerging as adults. The life cycle repeats on an annual schedule — which is why consistent spring treatment, rather than reactive treatment, is the effective approach.

What It Looks Like

The first external sign is reddish-brown, crumbly frass accumulating at wound sites on the trunk. This material looks like fine sawdust mixed with excrement and is a reliable indicator of active larval feeding. As the infestation progresses, the tree walls off the damaged tissue with callus, creating the swollen, knotty, or gall-like areas on the trunk that become more visible over time. Red leaves dropping early in the season, particularly on lower branches near the trunk, combined with trunk frass, is a strong indicator of advanced infestation.

Young trees are acutely vulnerable. A single season of heavy feeding can girdle a small tree. Larger, older trees typically show progressive branch dieback over multiple years of infestation before decline becomes obvious.

Treatment and Prevention

Once larvae are established inside the wood, insecticide spray cannot reach them effectively. Treatment must happen before larvae enter — which means spring timing is everything. Apply a permethrin-based trunk spray starting in early May and repeat four times at three-week intervals, ensuring thorough coverage of the lower trunk and any wound sites. Pheromone traps deployed in April can confirm whether borers are active in your yard and help time the first application.

For an active infestation where frass is visible, inserting a stiff wire into entry holes can mechanically kill larvae. This is practical only for limited, accessible infestations.

Prevention comes down to three practices. First, keep mowers and string trimmers away from the trunk — a 3- to 4-inch mulch ring extending 3 feet from the base eliminates the need for close equipment operation and keeps the root zone cool and moist. Second, when pruning, cut just outside the branch collar and remove stubs completely; stubs are open entry points. Third, stressed and drought-weakened trees attract more egg-laying females — consistent watering during dry periods reduces the tree’s vulnerability.

Powdery Mildew

Powdery mildew on dogwood is caused by Erysiphe pulchra, with Phyllactinia guttata as a secondary pathogen. It is the most common of the three problems on flowering dogwood, visible on many established trees each summer — but also the most manageable. Unlike anthracnose or borers, it rarely kills trees outright. Its significance is indirect: repeated infections weaken trees and reduce their resistance to secondary problems.

The Humidity Paradox

Here’s what makes Erysiphe pulchra behave differently from most fungal pathogens: high rainfall actually suppresses it. Water on the leaf surface prevents spore germination and physically washes spores away before they can establish. What the fungus needs is high relative humidity combined with warm days and cool, damp nights — ambient moisture that settles on leaves overnight, then evaporates as temperatures rise through the day. Dense plantings, shaded spots with poor air circulation, and sheltered sites under a tree canopy create these exact conditions.

This explains a pattern many gardeners find puzzling: powdery mildew often flares during dry stretches in midsummer. The mechanism is humidity without rainfall — and under optimal conditions, the spore-to-spore infection cycle runs as fast as seven days.

What It Looks Like on Dogwood

The classic white powdery coating is present — but on Cornus florida, the visible fungal growth is often minimal while the leaf damage is pronounced. Red and brown lesions, leaf scorch along the margins, and leaves that curl and distort are frequently the most visible signs. This leads to misdiagnosis as drought stress or sunscald, particularly in mid-to-late summer when the connection to a fungal pathogen isn’t obvious. If you see the white coating on any leaves, even lightly, combined with browning and distortion, powdery mildew is the likely driver.

Treatment

OSU Extension advises against routine fungicide application for powdery mildew in established landscape trees — the disease weakens trees but rarely justifies the cost and effort of regular chemical treatment. For most situations, cultural management is sufficient:

• Prune to open the canopy and improve air circulation
• Mulch beneath the tree to maintain consistent soil moisture — drought stress amplifies the leaf scorch that mildew triggers
• Avoid overhead watering in late afternoon, which maintains the leaf surface moisture that favors nighttime infection

For nursery stock, show gardens, or trees that are mildew-affected every season without improvement, myclobutanil or propiconazole give the best control. Apply at the first sign of symptoms and repeat every 7 to 14 days while warm, humid conditions persist.

The most cost-effective long-term response is variety selection — replacing a chronically affected tree with a resistant cultivar rather than committing to annual treatments.

When Not to Treat

Knowing when to stop is as important as knowing when to start.

Anthracnose — late-stage disease: If a tree has lost more than half its branching structure with only a few active limbs remaining, fungicide and pruning won’t reverse the damage. Removing the tree prevents overwintering spores on dead wood from spreading to healthy dogwoods nearby.

Powdery mildew — late summer appearance: Applying fungicide to heavily damaged leaves in August or September gives no benefit to the current season’s growth. It’s more productive to improve cultural conditions now and plan for a resistant replacement if the same tree struggles every year.

Borers — after July: Trunk spray insecticides are ineffective once larvae are established deep in feeding galleries. Missing the May-to-July treatment window means redirecting effort to wound prevention for next season rather than applying ineffective late-season sprays.

Resistant Varieties Worth Planting

If your flowering dogwood has struggled with disease for two or more seasons, replacement with a resistant cultivar is the most reliable long-term fix. Dogwoods planted in thoughtfully designed companion planting arrangements — where neighboring shrubs provide partial shade without crowding — can further reduce disease pressure by improving airflow while moderating light intensity.

Cornus kousa and the Rutgers-bred hybrids offer resistance to both fungal diseases and are significantly less vulnerable to dogwood borer than Cornus florida — the trifecta of protection that makes them the practical default for new plantings in disease-prone regions.

Frequently Asked Questions

Can my dogwood recover from anthracnose?

Yes, if it’s caught early. When damage is limited to leaves and small twigs, pruning infected material during dry weather and applying fungicide from bud break the following spring can stop the disease’s progress. Trees where cankers have reached the main trunk with most branches already dead rarely recover — removing them protects neighboring trees more than treating them will.

Is the dogwood borer contagious between trees?

Not in the way a fungal disease is — it’s a moth that chooses individual trees to lay eggs on. However, a heavily infested tree produces large numbers of adults that will target other dogwoods with wounds nearby. If you have multiple dogwoods in your yard and one is infested, treat all of them during the May-to-July spray window rather than just the affected tree.

My dogwood has white powder but otherwise looks fine — should I treat?

Probably not this season. OSU Extension recommends against routine fungicide application for powdery mildew in landscape dogwoods. Improve air circulation around the tree, mulch to maintain consistent soil moisture, and monitor. If the same tree shows heavy mildew every summer, replacing it with a resistant cultivar is a better long-term investment than annual sprays.

What’s the difference between spot anthracnose and Discula anthracnose?

Spot anthracnose, caused by Elsinoe corni, produces small spots on flower bracts in spring. It’s widespread, generally harmless, and rarely needs treatment. Discula anthracnose, caused by Discula destructiva, is the dangerous one — it progresses through the tree’s branching structure season by season and can kill a Cornus florida within two to three years. Spot anthracnose affects the bracts; Discula anthracnose kills the tree.

Sources

• Dogwood Anthracnose — University of Maryland Extension
• Key to Common Problems of Dogwoods — University of Maryland Extension
• Discula Dogwood Anthracnose Fact Sheet — UNH Extension
• Dogwood Diseases and Insect Pests — Clemson HGIC
• The Dogwood Borer — UConn IPM
• Dogwood Borer — NC State Extension
• Dogwood Anthracnose — PNW Pest Management Handbooks
• Dogwood Powdery Mildew: Signs and Symptoms — Ohio State University BYGL