scienceExpert DiagnosisUpdated Feb 20, 2026

Boxwood Blight

Boxwood blight is a fast-moving fungal disease of boxwood shrubs caused by Calonectria pseudonaviculata (syn. Cylindrocladium buxicola). It produces ringed leaf spots, rapid leaf drop, and twig dieback, and spreads easily when conditions are warm and wet. This page helps you recognize symptoms, stop spread with targeted sanitation and cultural changes, and use fungicides and quarantine practices when necessary.

Boxwood Shrubs for Hedges and Formal Edges showing blight symptoms

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Quick Diagnosis

Most Likely Cause: Boxwood blight (Calonectria pseudonaviculata).

Look for tan-brown circular leaf spots often with dark margins, rapid defoliation from the bottom up, and black fruiting structures on lesions after moist weather; these are classic symptoms on boxwood hedges and shrubs that point strongly to boxwood blight. Spread occurs by splash, tools, and infected plants, so check lower leaves and the soil surface near the base of plants for fallen symptomatic debris.

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Boxwood blight strikes most often in Zones 5-9, favored by warm temperatures and high humidity that keep leaves wet for extended periods; the heightened risk in Zone 5 landscapes comes from longer periods of spring and fall leaf wetness. In landscapes with dense plantings, poor airflow, or frequent overhead irrigation, infections can move through a hedge within a single season.

The pathogen is the fungus Calonectria pseudonaviculata (also listed historically as Cylindrocladium buxicola). It survives on infected leaves and twigs and can be carried on nursery stock, contaminated tools, clothing, and mulch. Early detection and prompt action are essential to prevent heavy losses in formal hedges or specimen plantings.

If you need to adjust care habits, start with irrigation timing and canopy thinning. Growers who water in the morning so foliage dries quickly reduce the window for infection, while pruning for better airflow lowers disease pressure around each shrub.

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Plant Problem — See AlsoBoxwood Root Rot

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How boxwood blight behaves in the landscape

Boxwood blight needs leaf wetness to infect-spores land on wet leaves and penetrate quickly in warm, humid weather. Once established, the fungus produces abundant spores on fallen leaves, which restart infections with each rain or irrigation event.

Cultivar susceptibility varies: some boxwood types show slower disease progression but few are fully immune. Planting resistant selections and spacing for airflow reduces overall risk, but sanitation and careful plant movement remain the most reliable defenses.

Fungicides are protectant, not curative: they reduce new infections when applied regularly during high-risk periods and always according to label directions and extension recommendations. Rely on cultural controls first and use chemicals as part of an integrated approach.

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Environmental Baseline

Before diagnosing specific failures, confirm your Boxwood Shrubs for Hedges and Formal Edges's environment matches its core care requirements.

forestBoxwood Shrubs for Hedges and Formal Edges Care Needs

Light: Full sun to partial shade
Water: Moderate, consistent moisture, hates soggy soil
Temp: Hardy to about -20°F once established

homeTypical Indoor Home

Humidity: 30-50% (Low)
Temp: 65-72°F variable
Light: Often too dim or direct

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Possible Causes

Sorted by likelihood

1. Calonectria pseudonaviculata infection

Likelihood: High

The fungus directly infects leaves and young twigs. It produces characteristic lesions and fruiting bodies when conditions are wet, and it can kill foliage quickly under favorable weather.

Identification

remove_circle_outlineCircular tan to brown leaf spots with darker margins; spots often develop a bleached center.
remove_circle_outlineRapid defoliation starting on older leaves and progressing upward through the plant.
remove_circle_outlineBlack pinpoint fruiting bodies visible on wet lesions with a hand lens or after rain.
remove_circle_outlineTwig cankers and dieback where infected leaves fall but twigs remain symptomatic.

The Fix

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Plant Problem — See AlsoBoxwood Winter Bronzing

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Root Health Examination

A direct inspection of the root system distinguishes root rot from drought stress — saving weeks of guesswork.

check_circleHealthy Roots

Firm to the touch
White or light tan color
Earthy, pleasant smell

cancelCompromised Roots

Mushy or slimy texture
Dark brown or black color
Sour, rotting odor

Inspection Step: Gently slide the pot off while supporting the base of the stems. The outer root ball gives sufficient clues without disturbing all the soil.

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When to Worry

A few yellow leaves are normal. If more than 20% of foliage turns yellow within a week, or new growth is affected, act immediately — check the roots first.

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Recovery Protocols

Recovery takes time. Once the root cause is corrected, implement a 30-day stabilization window.

Immediate (0-2 weeks)Take action and contain spread

Prune out isolated, lightly infected shoots and bag material. Sanitize tools between cuts. Stop overhead watering and inspect the rest of the planting. Missing this window allows spores to build up on the soil and debris.

Short (2-8 weeks)Monitor and apply protectant fungicides

If disease pressure continues, apply a labeled protectant fungicide on a **7-14 day** schedule during wet weather, following local extension guidance. New growth may be protected but already-infected foliage will not recover.

Medium (8-24 weeks)Assess plant vigor and remove heavy infections

Healthy shrubs with light infection often recover leaf area over months if you stop new infections and avoid drought stress. Severely defoliated or repeatedly infected plants may require removal to protect nearby boxwood.

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Comparison — See AlsoBoxwood vs Privet

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Preventing Future Issues

Remove and dispose of heavily infected plants or severe sections-bag and burn where allowed, or send to municipal green-waste if permitted-and sanitize pruners and tools between cuts with 10% bleach or 70% alcohol to avoid moving spores. Quarantine new boxwood purchases and inspect for leaf spots for at least 2-4 weeks before planting them near existing shrubs. What boxwood blight is and what causes it; how to confirm infection and assess severity in your landscape.

Boxwood Blight

Quick Diagnosis

How boxwood blight behaves in the landscape

Environmental Baseline

forestBoxwood Shrubs for Hedges and Formal Edges Care Needs

homeTypical Indoor Home

Possible Causes

1. Calonectria pseudonaviculata infection

Identification

The Fix

Root Health Examination

check_circleHealthy Roots

cancelCompromised Roots

When to Worry

Recovery Protocols

Preventing Future Issues

2. Water splash and high humidity spread

Identification

The Fix

3. Contaminated plant material and tools

Identification

The Fix

Boxwood Blight

Quick Diagnosis

How boxwood blight behaves in the landscape

Environmental Baseline

forestBoxwood Shrubs for Hedges and Formal Edges Care Needs

homeTypical Indoor Home

Possible Causes

1. Calonectria pseudonaviculata infection

Identification

The Fix

Root Health Examination

check_circleHealthy Roots

cancelCompromised Roots

When to Worry

Recovery Protocols

Preventing Future Issues

Related Reads

2. Water splash and high humidity spread

Identification

The Fix

3. Contaminated plant material and tools

Identification

The Fix