Bacterial Twig Blight (bacterium – Pseudomonas saliciperda): Leaves turn brown and wilt and blighted branches die back for several inches. Brown streaks can be seen in sections of the wood. Bacterium overwinters in the cankers, so young leaves are infected as soon as they unfold. The damage can be confused with frost injury. Prune out infected twigs and spray in early spring with an approved fixed copper fungicide.
Crown Gall (bacterium – Agrobacterium tumefaciens): Mainly a nursery disease. Large, rough, woody swellings or galls on the lower part of the stem and crown of the plant. Infected plants may be deformed, stunted or even killed. Weeping willow is susceptible. No practical control is known for this disease.
Cytospora Canker (fungus – Cytospora sp.): Affects willows the same as poplars. Discolored, sunken, often sharply defined areas develop on twigs, branches or trunk. Cankers enlarge and gradually girdle infected parts causing death to portions beyond. Weeping willow is susceptible, but rarely occurs on black willow. Prune out and destroy dead and cankered parts. Spray with an approved fixed copper fungicide.
Leaf Spots (fungi – Cercospora sp., Gloeosporium sp.): Small to large, round to irregular spots of various colors on leaves. Leaves may wither and drop early. Begin spraying when buds begin to swell in the spring.
Rust (fungus – Melampsora sp.): Lemon-yellow spots on lower leaf surface. Later in the season the pustules are dark colored. The disease may be severe enough to cause leaf drop. Although rust infections are not considered serious, they may result in heavy defoliation of young trees.
Powdery Mildew (fungi – Phyllactinia guttata and Uncinula salicis): White powdery growth on leaf surfaces may become heavy late in the season, especially on tender leaves of sprouts.
Tarspot (fungus – Rhytisma salicinum): Spots are very thick, jet black, discrete and about one-fourth inch in diameter. It looks like a drop of tar on the leaf. Rake up an burn dead leaves, as the fungus overwinters on them. Spray early in April with a fungicide.
Cotton Root Rot (fungus – Phymatotrichum omnivorum): Most willow species are highly susceptible. Plants suddenly wilt and die, leaves usually hanging on the plant. Roots will be decayed with the bark peeling off very easily. (See section on Cotton Root Rot)