Almond Board Reports Challenges of Red Leaf Blotch

Red leaf blotch continues to pose a significant and evolving challenge for California almond growers as the 2026 season progresses, with new monitoring data showing the disease remains active even under relatively dry spring conditions.

According to recent updates from the Almond Board of California, the fungal disease—caused by Polystigma amygdalinum—is still being detected across key growing regions as orchards move through petal fall and early leaf development. Field observations and spore-trapping efforts conducted by University of California Cooperative Extension specialists have confirmed ongoing spore presence in areas such as Stockton and Merced, including in orchards using both sprinkler and drip irrigation systems. 

The continued detection of spores despite a lack of rainfall has underscored concerns that traditional assumptions about disease pressure may not fully apply this season. Researchers note that while humidity alone is typically insufficient to drive infection, free moisture on leaves—from sources such as irrigation or morning dew—can still create favorable conditions for the pathogen to spread. 

Red leaf blotch is a relatively new but rapidly expanding issue for California’s almond industry, which produces the vast majority of the U.S. supply and a large share of the global market. First confirmed in the state in 2024, the disease spread quickly across major growing regions by 2025, raising concerns about long-term yield impacts if not managed effectively. 

In 2026, timing of fungicide applications remains a central focus for disease control. Field trials this season have generally targeted early March for initial treatments, followed by additional applications roughly two to three weeks later. Experts emphasize that the period immediately after petal fall is critical for protecting new leaf growth, as infections typically occur early but symptoms may not appear until weeks later. 

Growers are also weighing cost considerations against disease risk. While dry weather may prompt some to delay or reduce spray programs, specialists caution that such decisions should be based on orchard history, irrigation practices and current disease pressure rather than rainfall alone. Later-season treatments—particularly those applied after May—have shown limited effectiveness in controlling the disease. 

Young orchards are considered especially vulnerable this season. Severe infections in first- and second-leaf plantings can slow vegetative growth and delay production timelines, making early intervention a key consideration for growers managing newer acreage. 

To support in-season decision-making, updated UC Integrated Pest Management guidelines have been released, outlining fungicide options with demonstrated efficacy and offering recommendations aimed at balancing disease control with resistance management. 

Despite differences in weather compared to previous years, including warmer and drier conditions so far this spring, researchers say red leaf blotch remains a persistent threat. Ongoing work is focused on better understanding how environmental factors influence disease development, but current guidance stresses that management decisions should rely on real-time field observations rather than assumptions tied solely to weather patterns.