Almond crop yields are threatened by a new plant fungal disease that has been observed for the first time in California.

Symptoms of red leaf blotch have been found in several orchards in the Northern San Joaquin Valley and the disease already is somewhat widespread, according to a news release from University of California-Davis.

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Pest identification was confirmed by the California Department of Food and Agriculture and U.S. Department of Agriculture.

The disease is named for the characteristic orange to dark red blotches that appear on infected leaves.

Red leaf blotch isn’t usually lethal for trees but causes them to lose their leaves prematurely and can significantly diminish crop yields in the current year and the next.

“It’s one of the most severe diseases of almonds for Spain and the Middle East,” said Florent Trouillas, UC Cooperative Extension fruit and nut pathology specialist.

Symptomatic trees have been found in Madera, Merced, San Joaquin and Stanislaus counties.

“From the first observations so far, it seems like it affects some of the most planted cultivars, like Nonpareil and Monterey. We’ve observed it in a diversity of cultivars already,” Trouillas said.

More on the disease

The fungal pathogen is highly specific to almond trees.

Infection typically happens when small leaflets are emerging, but the first symptoms — small pale yellow spots on both sides of leaves — appear after about 35 to 40 days.

Pest control advisers have been confused because they’ve never seen anything like red leaf blotch, Trouillas said.

The yellow, orange and red blotches can’t be confused with other known almond diseases.

Applying fungicides after symptoms appear is ineffective. Trouillas said the best thing growers can do at this point is to report symptomatic trees to their local UC Cooperative Extension farm advisor so researchers can track the disease.

Thinking ahead

Preventive measures are the best way to manage red leaf blotch.

Trouillas said growers should think ahead to next winter and 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 key times 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 in mind next year and be on schedule for next year’s spraying program,” Trouillas said.