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Walnut Blight: Detection and Prevention

April 28, 2022

By The Certis Team

Walnut blight can be detrimental to crop production if the conditions are ideal for the spread of disease. Unfortunately, blight varies a great deal from year to year, so it’s difficult for growers to know when it may strike.

The disease is caused by the bacterium Xanthomonas juglandis and although it is highly unpredictable, three main factors affect its spread: weather conditions, pathogen population, and walnut variety (or the disease host).

Because walnut blight has so many variables, it’s important that growers can detect its signs and symptoms in order to understand proper prevention and management to protect walnut crops every growing season.

Walnut Blight Types and Conditions

The two types of walnut blight epidemics can proceed in either a monocyclic or polycyclic manner, which determines how the disease progresses.
• For monocyclic disease, infections that happened during the spring do not cause inoculum and secondary infections.
• For polycyclic spread, infections during the spring lead to productions of inoculum that do spread and infect new tissue.

Typically, the cycles of walnut blight bacteria are dependent upon weather conditions and rainfall during the season. Infrequent rainfall during the spring may lead to monocyclic progress, while frequent spring rainfall would tend to favor polycyclic disease epidemics.

Raindrops can spread disease because if bacteria are present, the water can spread the disease by splashing bacteria onto new green tissue, thus infecting, and potentially destroying, new growth.

Walnut Blight Signs and Symptoms

The most common sign of walnut blight involves the appearance of black lesions on catkins. You may notice dark, sunken lesions:
• on the ends of flowers (end blight)
• on the sides of the nut as it matures (side blight)
• on the shoots
• on the leaf blades (which can also appear as reddish spots at first)

All green tissue (specifically early-leafing varieties) is susceptible to walnut blight, so early detection is imperative because significant damage occurs when the disease spreads to the developing nuts.

As the bacteria spread inside the walnut, it grows towards the center of the nut easily in the season (especially with ideal weather conditions), which destroys the kernel. Since rain spreads bacteria and infection, check regularly after rainfall. Continue to check throughout the season as nuts develop because blight may occur randomly at any time, even later in the season.

Walnut Blight Prevention and Management

Managing walnut blight is usually possible with early detection and keeping prevention in mind since the disease has so many variables out of your control.

For disease prevention, follow these steps:
• Monitor weather conditions and check for dew and rainfall to understand the potential for more severe infection.
• Apply bactericides to trees every 7-10 days (following label instructions and weather patterns) in the “prayer stage” of opening until the last flower cycle.
• Assess walnut blight damage levels in June to understand disease pressure in the orchard and the timing of the first spray for the next growing season.

It may be ideal to estimate inoculum levels by collecting 50-100 buds per orchard block to be evaluated in a lab or by using disease levels of the previous season to compare. From there, you can assess whether you have a low, moderate, or high risk for disease pressure and treat accordingly.

Take these measures to deter the spread of disease:
• The first spray timing is important – don’t wait too long as new growth develops on the tree
• Blight population increases in dormant buds, resulting in high disease pressure, so check regularly
• Implement spray coverage by air and ground so that disease doesn’t sneak in (ensure thorough coverage especially if tree canopies are dense)

Although there’s no “set it and forget it” way to stop walnut blight, using preventative measures and effective management programs will allow you to achieve higher yield without loss due to disease pressure.

Learn more about how to fight walnut blight with the Certis Biologicals leading biopesticide solutions for blight.

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