The walnut family has approximately 59 species that range in habitat across the globe. Two of the most prominent are the black and English walnuts. The trees have attractively pinnate leaves that grow in feathery rows on thick gnarled branches. The bark is grooved and has many fissures in its greyish-brown surface. Walnut trees are long lived and can get nearly 100 feet tall. These giants are prone to numerous pests and diseases in cultivation and the wild.
Bacterial infections abound in tree species and the walnut is no exception. Walnut blight is a bacterial disease that is the result of two different bacteria. The disease makes black holes or spots in the leaves, fruit and flowers of the walnut. Flowering time is when the tree is most affected and when temperatures are cool but precipitation is high. The bacteria overwinter in new growth and surface when conditions are prime. Trees need to be situated where pH is at least 6.0, and any infected plants should be cut out to prevent the spread of the disease.
Fungal diseases are spread via spores that hitch rides on air streams, water drops and even animals. Walnut leaf blotch is a fungal disease caused by anthracnose, a common problem in ornamental and shade plants. The greatest damage is to the fruits as walnuts turn black and fall off the plant. The fungus can also cause severe defoliation, which ruins the ability of the plant to produce plant sugars via photosynthesis.
Winter Scorch and Dieback
Walnut is susceptible to frost damage, and young trees can experience winter scorch, which causes twig dieback and affects the budding leaves. Walnut trees that are planted in narrow valleys, hillsides with northern exposure and places with low airflow can experience cold damage. Most of the damage will not injure mature trees, but saplings can be killed. Lumber trees that receive damage lose value, and in plantations the economic losses can be severe.
Walnut trees secrete a chemical that is toxic to other plants. It is a defence mechanism that ensures there are no competitors nearby that might take water and nutrients that the walnut needs for its growth. The chemical is called juglone and it inhibits growth and prevents germination of competitor species. All parts of the walnut tree excrete the substance, and it causes wilting, yellowing and death of some plants that come in contact with it. Juglone can persist in soil even after the removal of a walnut tree.
Walnuts and other deciduous trees are especially affected by pests and insects. Two of the most common are walnut gall mite and codling moth. Gall mites create irregular growths in leaves where the larvae overwinter. Mites won't kill a walnut tree, but the galls are unsightly and can damage young growth. Codling moths are extremely damaging to soft-fruited trees like apples, but in the walnut they are less able to destroy the nuts. They can cause discolouration and some kernel damage.
- University of Minnesota; Growing Black Walnut; Melvin B. Baughman, Carl Vogt; 2002
- Walnuts Web; Walnut Tree Diseases and Pests
- Virginia Cooperative Extension; Trees for Problem Landscape Sites- The Walnut Tree; Bonnie Appleton, Roger Berrier, Roger Harris, Dawn Alleman, Lynette Swanson; May 1, 2009
- Botanical Online; Walnut Tree