Common Walnut Tree Diseases

Plant diseases affecting walnut trees can be caused by various pathogens – fungi, bacteria, or viruses. These diseases often spread through wind, water, soil, or insects, finding their way into the tree through natural openings or wounds. In this article, we explore some common diseases that can affect walnut trees and their impact. Read on to learn more.

Five Most Common Diseases In Walnut Trees

Most Common Diseases In Walnut Trees

Crown Gall (Agrobacterium tumefaciens)

One prevalent soil-borne disease, that can spread from one tree to another, affecting walnut production is crown gall. It is caused by the bacterium Agrobacterium tumefaciens.

This disease shows up as irregular growths at the trees’ base, reducing water and nutrient flow. You may or may not be able to see the galls at the ground level. Sometimes they are below the surface of the soil on the roots of the tree.

Crown and Root Rot (Phytophthora and Armillaria mellea)

Phytophthora and Armillaria mellea, also known as oak root fungus, are soil-borne fungi that cause crown and root rot in walnut trees. These diseases can severely impact tree health.

Root rot causes slow growth during the growing season, early yellowing of leaves in autumn and eventual death. Crown rot can cause sudden death early in the growing season.

Walnut Blight (Xanthomonas juglandis)

Walnut blight, caused by the bacterium Xanthomonas juglandis, is a common disease affecting both walnut nuts and foliage. This bacterium affects all green growth, so it impacts the leaves and the developing nuts while they are still encased in green husks.

It thrives in wet and humid spring conditions and spreads easily in windy, rainy weather. While some walnut cultivars with early leaf dates are more susceptible, fungicide treatments can be effective in managing the disease in most years.

This interesting and informative video presents narration in Hindi; however, the text, charts and illustration captions are in English.

Bark Canker and Branch Wilt (Brennaria rubrifaciens, Brennaria nigrifluens, and Hendersonula Toruloidea)

Deep bark canker, shallow bark canker, and branch wilt are diseases that can affect the trunk, branches, and scaffolds of walnut trees. These conditions can weaken the tree’s structure and pose potential hazards.

Bark canker causes deep cracks to appear vertically in the trunks of walnut trees early in the springtime. By late spring, you will see dark brown goo oozing out of the cracks.

As this happens on the outside, dark brown or black streaks develop inside the tree. Eventually, they appear just below the bark as dark streaks or spots.

The pathogen that causes this malady can be spread by grafts that contain the pathogen but show no symptoms of the disease.

It is also possible for trees that have been damaged by woodpeckers or inclement weather or poorly pruned to simply pick up the pathogen.

Branch wilt also affects the trunks and scaffolds of walnut trees, but the symptoms of this disease differ from those of bark canker.

Trees impacted by branch wilt display withered, yellowed leaves at the tips of branches on the sunniest exposure (i.e.: southwest) from midsummer into the autumn months.

The walnuts can also be blighted by the pathogens that cause this malady. As the disease spreads, more leaves lose their luster, wilt, dry out, turn brown but do not fall from the tree.

Further progression of the disease leads to peeling bark under which masses of black, dusty fungal spores can be found. When the wood of these disease limbs is cut, dark, wedge-shaped discolorations are exposed.

Like bark canker, branch wilt is spread through injury to the tree, including high winds, sunburn, frost damage and other splits and wounds.

Walnut Trees and Thousand Cankers Disease

Blackline Disease (Cherry Leaf roll Virus)

Blackline disease, caused by the cherry leaf roll virus (aka: cherry leaf roll nepovirus or walnut ringspot), is prevalent in California coastal regions and San Francisco Bay area counties. This viral disease is transmitted through pollen.

Trees impacted by this virus display small, brown or black circular leaf spots. With the passage of time, the spots spread to become concentric rings presenting a target-like pattern. You may also see small, round fruiting structures within the circles.

Integrated Pest Management in the Home Landscape for trees and shrubs:

10 Best Practices In Integrated Pest Management To Help Prevent Walnut Tree Diseases

The good news about all of these walnut tree diseases is that all can be kept under control with best practices and responsible sourcing, care and maintenance.

Protecting walnut trees from the various threats they face is essential. With knowledge and proactive measures, you can safeguard your valuable trees from the perils lurking in the environment.

Given the diversity of walnut diseases, implementing a consistent Integrated Pest Management program is crucial for control.

Keep in mind that integrated pest management (IPM) focuses on sustainable and environmentally friendly approaches to pest control. Follow these ten steps to keep your trees happy, healthy and disease-free.

  1. Identification and Monitoring: Regularly inspect your walnut trees for signs of diseases such as leaf discoloration, wilting, or unusual growth patterns. Keep a record of any issues you observe.
  2. Cultural Practices: Maintain tree health by providing adequate water, nutrients, and proper pruning. Healthy trees are less susceptible to diseases.
  3. Sanitation: Remove fallen leaves, branches, and any infected plant material from the vicinity of your walnut trees. This reduces the presence of disease sources.
  4. Biological Control: Encourage beneficial insects and organisms that naturally control pests, like ladybugs or parasitic wasps. Avoid the use of broad-spectrum pesticides that harm these beneficial fauna.
  5. Chemical Control as a Last Resort: If a disease outbreak is severe and threatens the trees’ health, consider targeted chemical treatments. Consult with a local agricultural extension service for guidance on safe and effective options.
  6. Timing: Apply any necessary treatments at the right time in the diseases’ life cycle for maximum effectiveness. Timing is crucial in IPM.
  7. Rotation: If you have a lot of walnut trees, practice crop rotation to reduce the buildup of soil-borne diseases.
  8. Disease-Resistant Varieties: Whenever possible, choose disease-resistant walnut tree varieties for planting.
  9. Educate Yourself: Stay informed about common walnut tree diseases in your region and keep learning about updated IPM strategies.
  10. Record Keeping: Maintain detailed records of your pest management efforts, including when and what treatments were applied and their effectiveness.

By following these steps, you can develop a complete and effective integrated pest management plan to protect your walnut trees from common diseases while minimizing environmental impact.

For comprehensive guidance on managing walnut diseases, refer to this guide.

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