The walnut trees native to the nation have diseases that can affect them adversely. The two walnut species with the widest distribution in the U.S. are the black walnut and the butternut, found across much of the East. Others like the California walnut and Arizona walnut tree grow in parts of the West. These trees are at risk from a pair of diseases that are specific to walnuts as well as some that can attack many types of plants.
Walnut blight is a bacterial disease brought about by either of two bacteria that harm only walnut trees. The disease mostly affects the new growth on a walnut tree as well as the actual nuts. It wreaks the most havoc when it occurs during damp and cool weather patterns when the trees are in the flowering stage. The bacteria has the ability to survive the winter inside what looks like a robust and healthy bud, renewing its assault on the tree in the spring. Walnut blight brings about blackened spots on the tree's leaves and can create holes and spots on the fruit, readily apparent on the husks of the walnuts. In some instances, the nut crop will undergo severe damage from this disease.
Walnut Leaf Blotch
The fungus that is the agent for causing walnut leaf blotch can spread extensively over a tree when the weather is damp, carried from one leaf to another by rain droplets dripping off infected parts of the tree. May and June are prime months for this ailment, also called walnut anthracnose, to emerge. Walnut leaf blotch makes the walnuts change from their customary green to black, causing them to fall off before they ripen. It can also form brownish spots and blotches on leaves, precipitating leaves to fall off from different sections of the tree. The disease is typically not fatal to a tree but can destroy the nuts and blight the appearance of the tree.
Other diseases that also may impinge on other trees besides the walnut include crown gall, downy spot, nectria, botrytis, blackline and honey fungus. The last two are the most dangerous for a walnut tree. Blackline is viral in nature and makes leaves turn yellow, droop, fall off early and can sometimes kill an entire tree. Honey fungus goes by other names such as oak root fungus. The fungus responsible will get into a walnut tree through the roots and rot them out. Capable of spreading to nearby walnuts via the ground, this fungus can result, in the worst cases, in death for the tree as it prevents nutrients from making it into the upper parts of the plant.