Disease New to California Threatens Almond Crops
July 18, 2024 
Advanced symptoms of red leaf blotch include larger, yellow-orange blotches (1 to 2 cm) that turn reddish-brown in their center. Photos by Alejandro Hernandez and Florent Trouillas
Symptoms of red leaf blotch (RLB), a plant disease caused by the fungus Polystigma amygdalinum, have been observed for the first time in California across the Northern San Joaquin Valley.
Molecular DNA testing by the laboratory of Florent Trouillas, University of California Cooperative Extension fruit and nut crop pathology specialist, has detected P. amygdalinum. Pest identification was confirmed by the California Department of Food and Agriculture and U.S. Department of Agriculture.
The disease, named for the characteristic orange-to-dark red blotches that appear on infected leaves, is typically nonlethal for trees but has been a longstanding problem for almond-growing regions across the Mediterranean. Causing trees to lose their leaves prematurely, the fungal pathogen can significantly diminish crop yields in the current year and the next.
“It is one of the most severe diseases of almonds for Spain and the Middle East, Trouillas, who co-authored an explanatory article on the UCCE San Joaquin Valley Trees and Vines blog, said.
With symptomatic trees seen in multiple orchards across Madera, Merced, San Joaquin, and Stanislaus counties, Trouillas said RLB is already “somewhat widespread.”
“From the first observations so far, it seems like it affects some of the most-planted cultivars, like ‘Nonpareil’ and ‘Monterey’,” he added. “We’ve observed it in a diversity of cultivars already.”
Preventive Measures Urged
According to Trouillas, RLB caused by P. amygdalinum is “highly specific” to almond trees, and generally only affects their leaves. Infection typically happens at petal fall, when small leaflets are first emerging and most susceptible to disease. After the pathogen’s latent period of about 35 to 40 days, the first symptoms appear — small, pale-yellow spots on both sides of the leaves.
Those blotches become yellow-orange and then reddish-brown in the advanced stages of the disease during June and July. Now, with RLB symptoms becoming more prominent, Trouillas and UC Cooperative Extension advisors across the Central Valley have seen an uptick in calls.
“PCAs [pest control advisers] have been confused because they’ve never seen anything like this,” Trouillas, noting that the yellow-orange-red blotches are symptoms unique to RLB and cannot be confused with other known almond diseases, says.
Applying fungicides after RLB symptoms appear is ineffective, Trouillas says. The best thing growers can do at this point is to report symptomatic trees to researchers so they can track the prevalence and distribution of the disease.
Growers who see signs of this new disease in their orchard should contact their local UC Cooperative Extension farm advisor.
Preventive measures are the best way to manage RLB, Trouillas adds. He urges concerned growers to think ahead to next winter/spring and plan for fungicide applications at petal fall and — if rains persist — also at two weeks and five weeks after petal fall. Fortunately, those are the same three key timings for managing other diseases, like shot hole and almond anthracnose.
“Because RLB is something that is introduced and potentially aggressive, it will be important for growers to keep that in mind next year and be on schedule for next year’s spraying program,” Trouillas says.
Additional information on RLB can be found at https://www.sjvtandv.com/blog/first-detection-of-red-leaf-blotch-a-new-disease-of-almond-in-california.
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