Volutella blight, or Pseudonectria canker—Pseudonectria rousseliana =Volutella buxi
Volutella blight is a common disease of boxwood. It kills shoots and the leaves terminal (outward and upward) from the site of stem cankers caused by the fungus.
Identification
Symptoms become apparent in late winter or spring when shoots infected the previous growing season fail to grow and flush new leaves. Leaves turn red then brown and finally tan or yellowish. The dead leaves bend upwards and lay abnormally close to their stems. The dead leaves remain attached for a significant amount of time after they are killed.
Examining stems where shoots with green leaves meet those with dead leaves reveals that bark has discolored, sunken areas (cankers). As cankers age, the bark around them becomes loose.
During periods of high humidity, such as fog, overhead irrigation, or rain, the fungus develops pinkish orange fruiting structures (sporodochia) that appear to have a waxy surface. The sporodochia occur on discolored leaves and cankered stems.
Lookalike. Symptoms of Volutella blight can be confused with those of boxwood blight, Cylindrocladium buxicola (PDF). Unlike with Volutella blight where dead leaves remain attached for a prolonged period, after stems are killed by boxwood blight the leaves drop rapidly. Boxwood blight can be distinguished by the black streaks that develop on infected green stems. Boxwood blight symptoms develop later in the growing season, during summer. The same plant can be affected by both boxwood blight and Volutella blight.
Life cycle
The fungal pathogen infects leaves and stems of boxwood plants that have been stressed, such as by drought, and damaged by other causes such as pruning wounds or sunburn. All species and cultivars of boxwood are susceptible to Volutella blight.
The pathogen overwinters in boxwood branches, leaves, and plant debris that were infected the previous growing season. Germinating spores penetrate plants at the base of small dead shoots, branch crotches where dropped leaves accumulate, pruning wounds, and weather-damaged areas.
Spores of P. buxi have a wet, sticky texture and travel a relatively short distance with air movement. Volutella blight is commonly spread by the movement of infected plants (e.g., when purchased from a nursery) and on hands, gloves, and tools that contact infected tissue and become contaminated with fungal spores.
Solution
Provide boxwoods with a good growing environment and appropriate cultural care. Vigorous plants are less likely to be affected by Volutella blight. For example, providing adequate irrigation during the dry season reduces plant stress and helps to prevent sunburned tissue that is susceptible to infection by the fungus. Other important practices include:
- Avoid excess watering (e.g., soggy soil) and excess fertilization.
- Avoid overhead irrigation. Use drip or flood irrigation and direct any hand watering to the soil beneath plants while keeping foliage dry.
- Improve air circulation, such as by removing dense, inner stems and removing nearby plants that are crowding boxwood.
- Minimize wounding of plants. When pruning, do so with a sharp tool and make each cut just outside the branch collar and branch bark ridge.
- Prune off dead and diseased branches. To avoid spreading the spore masses, prune only when conditions and foliage are dry. During a single year, do not remove more than one-third of the healthy branches from a boxwood when pruning. For a video of how to correctly prune boxwood see Volutella Blight Disease of Boxwood from the University of Maryland.
- Sanitize pruning tools after working on a diseased plant and before moving to the next plant. Wipe blades clean then dip or spray them with a 10% bleach solution, 70% isopropyl rubbing alcohol, or a commercial disinfectant as directed on the product label.
- Shake plants periodically to knock down dead leaves that have lodged in branch crotches. Rake up leaves dropped from diseased plants and dispose of them away from boxwoods.
Where Volutella blight has been a problem, foliage and stems can be preventively sprayed in the late winter or early spring with Bordeaux mixture, another copper fungicide, or chlorothalonil to reduce new infections. To be effective, the aboveground parts must be thoroughly covered with the spray. Reapplication may be warranted after prolonged wet weather.
Adapted from Volutella Blight of Boxwood (PDF), University of Kentucky and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
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Boxwood shoots killed by Volutella blight.
Volutella blight stem canker with fruiting bodies of the fungus.
Salmon-colored fruiting bodies of Pseudonectria rousseliana on the underside of a leaf.
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