The willow tree of the genus Salix is a deciduous tree that includes a wide number of varieties and cultivars within the species. Many see their leaves and twigs develop a blackening caused by several different fungi.
A willow tree that is turning black is likely suffering from an attack of anthracnose, Drepanopeziza sphaerioides, a type of fungal disease that is spread through the propagation of spores via wind and water. Willows that are planted close to water are prone to the disease and extremely wet conditions also enhance the likelihood of contraction.
The weeping willow is especially susceptible to anthracnose. The disease results in the formation of black spots across willow leaves and development of raised, black lesions along twigs and stems. Also called leaf, shoot or twig blight, the degree to which a willow may be damaged is largely dependent on the specific variety.
Although some willows suffer extreme defoliation and abundant leaf loss as the leaves, blacken shrivel and die, new leaves will often replenish the tree after an early leaf drop on a willow. The University of California Davis' Integrated Pest Management System reports on its website that the various anthracnose fungi often overwinter in dropped and infected leaves and twigs.