Treating Walnut Bunch Disease: Bunch Disease In Walnut Trees

By Mary H. Dyer, Master Naturalist and Master Gardener

Walnut bunch disease affects not only walnuts, but a number of other trees, including pecan and hickory. The disease is particularly destructive for Japanese heartnuts and butternuts. Experts believe the disease is spread from tree to tree by aphids and other sap-sucking insects, and pathogens can also be transmitted through grafts. Read on for helpful information regarding symptoms of bunch disease and bunch disease treatment.

Bunch Disease in Walnut Trees

Bunch disease in walnut trees is characterized by stunted leaves and deformed stems. Clusters of fast-growing, wiry shoots take on a bushy, “witches’ broom” appearance when lateral buds produce growth instead of remaining dormant.

Symptoms of bunch disease also include growth that appears earlier in spring and extends later into fall; thus, trees lack cold-hardiness and are highly susceptible to damage in winter. Wood is weakened and prone to wind damage.

Walnut production is affected, and the few walnuts that appear have a shriveled appearance. The nuts often fall from the tree prematurely.

Symptoms of bunch disease may be limited to a few branches, or may be more widespread. Although walnut bunch disease is extremely destructive, infection tends to spread slowly.

Bunch Disease Treatment

To control walnut bunch disease, prune out infected growth as soon as it is spotted – usually in spring. Make each cut well below the affected area.

To prevent spread, be sure to sterilize cutting tools before and after use. Rake up debris after pruning, and destroy it properly. Never compost or mulch affected twigs or branches.

If the damage is extensive or located on the base of the tree, remove the entire tree and kill the roots to prevent spread to nearby trees.

Thus far, no chemical control has been recommended for bunch disease in walnut trees. However, healthy, well-maintained trees tend to be more disease-resistant.

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