Willow scab is a prevalent fungal disease that targets the young shoots and leaves of various willow species (*Salix* spp.). Caused primarily by the ascomycete fungus *Marssonina salicicola*, this pathogen creates distinctively dark, scabby lesions that mar the aesthetic appeal of ornamental willows. While the disease rarely kills a mature tree outright, it significantly weakens the plant, reduces photosynthetic capacity, and makes the host more vulnerable to other stressors. Understanding the lifecycle of the fungus and implementing strategic cultural practices are essential for effective management.
Identification and Symptoms
The most recognizable sign of willow scab is the appearance of small, water-soaked spots on emerging shoots and leaves. These spots rapidly expand into dark brown to black, slightly raised lesions that have a velvety texture due to the dense spore production underneath. In severe cases, the lesions coalesce, causing the leaves to curl, distort, and eventually turn yellow before dropping prematurely. Infected shoots often fail to develop fully, leading to a characteristic thinning of the canopy and a general decline in vigorous growth.
Distinguishing from Other Diseases
It is important to differentiate willow scab from other common foliar diseases, such as anthracnose or bacterial blight. While anthracnose lesions often have defined margins and may produce pink spore masses, and bacterial blight presents with angular, water-soaked spots, willow scab is unique in its velvety, scabby black appearance. Observing the specific texture and color of the lesions on the leaves and young stems is the most reliable method for confirming the presence of *Marssonina salicicola* in the landscape.
Disease Cycle and Favorable Conditions
The fungus overwinters as dormant mycelium within infected fallen leaves and twigs on the ground. When temperatures rise in the spring and new buds begin to swell, the fungus produces conidia (asexual spores) that are splashed by rain or wind onto the emerging shoots. Infection occurs during the cool, wet period of active shoot growth, typically between 15°C and 22°C (60°F to 72°F). The disease progresses rapidly through the season, with secondary spores causing further infections until the host tissue hardens off in late summer.
Primary infection occurs via wind and rain-splashed spores from debris.
Secondary spread happens throughout the growing season under humid conditions.
Optimal infection requires sustained leaf wetness and moderate temperatures.
The fungus survives the winter in plant litter, making sanitation critical.
Cultural Management Strategies
Because chemical controls can be difficult to time and may disrupt the surrounding ecosystem, cultural practices form the cornerstone of managing willow scab. The most effective long-term strategy focuses on disrupting the disease cycle by reducing the availability of inoculum. This involves proactive steps taken during the dormant season and adjustments to irrigation practices during the growing season.
Sanitation and Pruning
Thoroughly removing and destroying infected leaves and twigs is the single most effective cultural control. Raking up fallen foliage in the autumn and disposing of it through burning, bagging, or hot composting eliminates the primary source of overwintering spores. During the dormant winter months, pruning out any visibly infected branches helps to reduce the overall spore load and improve air circulation within the canopy, which aids in faster drying of tissues.
Remove and destroy fallen leaves and debris in autumn.
Prune out infected shoots during the dormant season.
Space trees adequately to ensure good air circulation.
Water at the base of the tree to keep foliage dry.
