The willow tree (Salix sp.) is fast growing, deciduous, water loving tree and is commonly found along lakes and streams. There are over a hundred different species of willow. There are two subgroups of willow; the weepers have long, pendulum-like branches and the contorted willow has angled and crooked branches. Willows are susceptible to a number of diseases including bacterial blight, crown gall, powdery mildew and rust.
Crown gall on willow trees is caused by the bacterium Agrobacterium tumefaciens that leads to the formation of galls on the stems and roots of the tree. If the number of galls increases, it leads to stunted tree growth, discolouration and eventual death. The presence of crown gall also makes the tree susceptible to a number of other secondary tree diseases. The young galls are spongy and smooth and get rough and hard as they mature. The bacteria commonly invade the tree through wounds. Best prevention method is to minimise tree wounding and to use sterilised tools when trimming and cutting. Using a chemical fungicide to paint the galls is also an control method.
Willow scab is caused by the fungus Venturia saliciperda and causes scablike, black lesions to form on the shoots. The young shoots and leaves are most susceptible to the fungus and, once afflicted, can die within a very short time. Early signs of willow scab include masses of olive green spores along the veins of the underside of leaves. Willow scab often occurs with black canker on willow trees. The fungus continues to reside on the destroyed twigs over the winter and become the main reason for infection on new growth in the spring. Willow scab can be controlled with a combination of chemical and cultural treatments. Chemical control includes the use of fungicides to reduce infection and pruning of infected twigs and stems is recommended to reduce the severity of the disease.
The willow trees that have willow scab often have black canker as well. Black canker results from the fungus Glomerella miyabeana and afflicts willow trees later in the season. The disease causes cankers to appear on the larger stems of the tree and leaves. Tree foliage starts to show dark brown to black lesions or spots. As the disease spreads towards the stem it causes leaves to shrivel and die. Gradually the fungus colonises the stem tissue and starts to create dark brown and black cankers. Black canker can be controlled with the timely application of recommended fungicides and pruning of the infected limbs.