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New Threat to California Almonds: Red Leaf Blotch


According to a report produced by a collaborative effort between UC Davis and UC Cooperative Extension, red leaf blotch (RLB), caused by the fungal pathogen *Polystigma amygdalinum*, was detected in California almond orchards on the border of Merced and Madera counties in late May, 2024. Previously known for its impact on almonds in the Mediterranean and the Middle East, RLB’s presence in California has raised alarms. 

First Detection and Spread 

RLB symptoms were first noticed in Nonpareil, Monterey, and Fritz almond varieties, characterized by yellow spots and orange to dark red-brown blotches. The disease spread to Madera, Merced, San Joaquin, and Stanislaus counties. Laboratory tests confirmed the presence of *P. amygdalinum*, although official confirmation by the California Department of Food and Agriculture (CDFA) is pending. Growers and pest control advisors (PCAs) need to stay vigilant.

Early symptoms of red leaf blotch include small, pale yellowish spots or blotches that affect both sides of the leaves. Courtesy: Alejandro Hernandez and Florent Trouillas.

Disease Symptoms and Biology 

Symptoms begin as small, pale yellowish spots on both sides of the leaves, progressing to larger yellow-orange blotches with reddish-brown centers. Advanced stages cause necrotic leaves that curl and drop prematurely, reducing the tree's photosynthetic capacity and yield. The disease has one primary infection cycle per season. Ascospores form in perithecia on fallen infected leaves, with infection occurring after petal fall when young leaves emerge and spring rains disperse ascospores. Symptoms appear in late April to mid-May, about 35 to 40 days post-infection. 

Advanced symptoms of red leaf blotch include larger, yellow-orange blotches (1 to 2 cm) that turn reddish-brown in their center. Courtesy: Alejandro Hernandez and Florent Trouillas.

Management Strategies 

Effective control, based on experiences from Spain, involves one preventive fungicide application at petal fall, followed by two more at two and five weeks after, especially if rains persist. These treatments often coincide with those for other almond diseases like shot hole or anthracnose. Fungicides from FRAC groups 7, 11, M3, M4, and some FRAC3 chemistries are most effective. Cultural practices, such as removing leaf litter or applying urea to accelerate its decomposition, are crucial but effective only when widely applied. 

Call to Action 

Growers who suspect RLB in their orchard should avoid collecting leaf samples and transporting them to reduce the potential spread of the pathogen to uninfected areas. Growers who suspect the disease in their orchards are urged to contact their local UC Cooperative Extension farm advisor. Early detection and management are vital to protecting California's almond industry from this new threat.