Willow plants (Salix spp.) range from towering Pacific willows (Salix lucida) to smaller pussy willows (Salix discolor). Desirable for their form and showy catkins, willows are a favorite with many home gardeners. Regardless of the particular species, all willows can suffer from insect pests, diseases and even environmental conditions that cause early leaf drop.
Diseases can cause willow trees to drop leaves early. Fungal diseases cause galls to form, which are growths that encircle branches, eventually cutting off the supply of nutrients to the leaves. Willow scab causes cankers to form, which are areas of cracked, dead wood. The cankers have the same effect as galls. Galls usually form near the base of the tree, while cankers infect twigs and stems. Other fungi that travel on water infect the leaves themselves, especially during unusually wet spring weather. Affected leaves turn yellow, then brown and often develop unsightly spots. They may also curl up before dropping from the tree.
Insects also plague willow trees, and severe infestations can cause premature leaf drop. Aphids, scale and other sap-sucking insects severely damage leaves when they suck the juices from them. Caterpillars and larvae eat young leaves, sometimes causing them to drop from the tree. Giant bark and willow aphids are the most common insect pests of willows, followed by Greedy scale, the Carpenterworm, several species of caterpillars (including leafrollers), and several species of boring insects, which not only weaken the wood but carry fungal diseases into the wood on their bodies.
Prevention & Treatment
It's difficult to protect willow trees from diseases and insect pests that cause early leaf drop, especially if they are very large. Small trees can be sprayed with a fungicide each spring. Clearing the area around your tree helps air circulate and exposes the tree to sunlight, which keeps the foliage dry. This can prevent fungal spores from developing on the leaves. Spray your tree with an insecticide as soon as you notice insect activity. Prune away any branches infected with galls, cankers or insect pests, especially if other, nearby branches do not yet show any symptoms. Remove dropped leaves from under the tree, as some fungi can overwinter in the soil and infect the tree again the next spring.
One cause of mid-summer leaf drop cannot be prevented or treated. Extreme changes in temperature, especially in mid-summer, can cause the leaves of a willow tree to shed early. A heat wave, for example, can stress the tree, causing leaves to drop, according to Bill Blackledge, a master gardener with BBC broadcasting. In such cases, the tree will not usually be severely damaged, but will recover when temperatures stabilize, although new leaf growth will likely not occur until the next spring.