Ask Your PCA: What are options for controlling walnut blight?

By Justin Nay
Integral Ag Services, Durham

Walnut blight can take 25% to 30% of developing walnuts. The Chandler variety blooms later and has less risk of blight, but they are not immune from the disease. 

Walnut varieties such as Vina, Tulare, Hartley and Howard are more susceptible to blight, but there are far fewer acres of these varieties. The main damage from blight is a reduction in yield. 

Blight starts around bloom and causes the developing nutlets to become infected, turn black and shrivel, then abort from the tree when they are golf-ball size or a little smaller. Some later infections also occur and can cause downgrades such as staining. 

If rains continue through March, getting into walnut orchards may not be possible, so aerial applications may be the only option. Some walnut trees are very tall, and aerial spraying is the only way to treat the tops of the trees, but it doesn’t provide good coverage on the lower canopy. If walnut orchards are flooded, aerial spraying may be the best scenario, especially for large-scale growers with a lot of ground to cover.

In 2024, aerial sprays were on par with ground spraying in terms of costs. Another benefit to aerial applications in March is greater availability because walnut growers aren’t competing with almond growers for aerial applications. 

Large operations have to use more preventive options and can’t wait on weather forecasts to determine when to make a blight application to cover their orchards. Small-scale farmers have more flexibility and can watch the weather and make an application before incoming rain.

Because of economics, most walnut growers continue to use manzate/copper for blight control. A smaller percentage of growers use kasumin, a newer chemical that has proven effective, but it’s more expensive. The key is to make timely applications for the best results.