Issue 12, July 20, 2015

Willow Blight

Scab and black canker are two diseases of willow (Salix spp.) associated with rapid blighting of leaves and shoots as well as dark brown to black stem cankers. Willow blight is the term used to describe a plant simultaneously infected with both diseases. Willow blight can cause extensive defoliation and dieback to susceptible hosts.

Willow scab is caused by the fungal pathogen, Venturia saliciperda. Initial infections occur as willow leaves are expanding in early spring (March-April). The fungus spreads from leaf tissues, through the petiole and into young twigs where dark cankers form, eventually girdling the twigs. Twigs girdled by cankers develop shoot blight symptoms above the point of infection. Distinctive, olive-brown spore masses form on the undersides of diseased leaves. These spores lead to infections of developing tissues as long as wet weather persists.

Willow blight.

Black canker is caused by the fungal pathogen, Glomerella miyabeana. Symptoms of black canker first appear late spring to early summer (April-June). Infected leaves develop irregular, brown to black lesions. Leaves shrivel and die as infections spread to petioles and into twigs. Small, sunken cankers form near the twig-petiole junctions. Depending on the host's susceptibility, cankers may remain small and elliptical in shape, or encircle the twig. Fruiting bodies, exuding pink masses of spores (conidia) form in the sunken cankers. Conidia produced in the cankers cause new and secondary infections throughout the summer and fall.

Several key differences are available when comparing the two pathogens. Willow scab occurs earlier in the season. The undersides of infected leaves are often covered in velvety, olive-brown spore masses. Cankers are common on smaller willow twigs and shoots. Black canker occurs later in the season. Cankers are commonly observed on larger twigs and shoots and are often covered in distinctive pinkish spore masses.

Control options are limited. Raking and destroying leaf debris will have little value as both the pathogens are capable of overwintering in cankered, infected twigs. Pruning diseased branches from small trees may provide some benefit. Preventative applications of fungicides may provide some control. However, fungicides may not be feasible on large trees, especially those located near water that could potentially be contaminated by the application. (Travis Cleveland)

Travis Cleveland

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