Almonds under attack

Nut growers are no strangers to pest pressures. But two new threats — Polystigma amygdalinum, the fungal pathogen behind red leaf blotch, and Carpophilus truncatus, an invasive beetle — are demanding urgent attention.

During December’s Almond Conference in Sacramento, California, a panel of researchers and regulatory officials emphasized proactive management and coordinated prevention as the best lines of defense against these challenges.

Red leaf blotch makes its US debut

Florent Trouillas, a UC Davis plant pathologist, described red leaf blotch as a foliar disease that has plagued almond growers in the Mediterranean for decades.

In May 2024, the disease was confirmed for the first time in California’s Merced County. Since then, it has spread to orchards in Madera, Fresno, San Joaquin, Stanislaus and parts of the Sacramento Valley. Cultivars affected include:

• Aldrich
• Butte
• Carmel
• Fritz
• Independence
• Monterey
• Nonpareil
• Padre
• Shasta
• Wood Colony

The fungus behind red leaf blotch only infects almond leaves and has a long incubation period of 35 to 40 days. Symptoms appear in May, with trees potentially defoliating by late August. Fallen, infected leaves serve as the primary inoculum source for the next season. Red leaf blotch presents a challenge because it only goes through one infection cycle per season, meaning that prevention is key.

Progression of red leaf blotch symptoms. Photos courtesy of Cameron Zuber/University of California Cooperative Extension.

Key takeaways for growers

Orchard practices can help reduce disease by targeting infected fallen leaves, which are the main source of infection:

• Apply zinc sulfate to speed up leaf drop, then remove the fallen leaves or treat them with urea to help them decompose faster. However, these methods are only effective when used across large areas.
• Clean harvest and spray equipment between orchards to prevent disease spread.
• Fungicides applied during bloom or after symptoms appear are not effective.

UC Davis researchers are currently studying the pathogen’s population genetics and infection cycle in California to guide future control strategies. They are also improving detection methods, as a new PCR test can quickly and accurately find the disease in infected leaves, even in early stages. In the meantime, coordinated sanitation and monitoring remain the best tools for growers.

Carpophilus beetle targets nuts

Houston Wilson of UC Riverside introduced another unwelcome guest: the Carpophilus beetle. First detected in California’s Madera and Kings counties in fall 2023, the pest has since been confirmed throughout the San Joaquin Valley. Unlike other sap beetles common to California orchards, the Carpophilus beetle bores through almond shells to lay eggs, causing direct kernel damage.

The beetles overwinter in mummy nuts, with adults emerging in spring and infecting new crop nuts at hull split. The adults can chew through almond shells — making distinctive oval-shaped tunnels — and deposit eggs. The larvae then feed on the kernel, leaving behind frass (a mix of almond meal and excreta).

It can be difficult to identify the Carpophilus beetle without magnification, but visual clues include oval-shaped entry holes and powdery frass, especially in the lower tree canopy.

Key takeaways for growers

Growers can reduce Carpophilus beetle pressure with careful orchard sanitation and by monitoring for early signs of activity:

• Remove and destroy mummy nuts before spring to eliminate overwintering sites, which are a key source of infestation.
• Inspect nuts — especially those in the lower canopy — during hull split. Look for oval entry holes in shells and powdery frass near damaged kernels.
• Researchers are still studying chemical control for Carpophilus beetles, as early trials have shown limited effectiveness. Trials in 2024 tested several active ingredients, including spinetoram, bifenthrin and acetamiprid, with mixed results.
• No pheromone traps are commercially available yet, but experimental lures tested in research settings attracted large numbers of beetles.

Ongoing research aims to refine phenology models based on temperature thresholds and development rates. Other efforts include testing new rearing methods, evaluating chemical controls and studying the beetle’s movement during hull split. These findings will help inform future monitoring and management strategies.

Coordinated statewide prevention

The rising threat of new pests and diseases like the Carpophilus beetle and the pathogen behind red leaf blotch underscore the importance of strong statewide systems to prevent the introduction and spread of such challenges.

The California Department of Food and Agriculture (CDFA) leads a pest prevention system aimed at protecting the state’s crops through exclusion, detection, eradication and control. This includes inspections at points of entry, orchard monitoring and quarantine enforcement.

Current federal rules restrict the import of high-risk plant genera like Prunus — which includes almonds — unless they meet strict quarantine or permit requirements. In California, border stations and delivery inspections screen nursery stock and harvested crops for pests and diseases.

The appearance of red leaf blotch and Carpophilus beetles reflects how global plant trade, reduced fungicide use and climate change can increase vulnerability to invasive threats. In response, state and local officials are boosting monitoring efforts and improving diagnostic tools, including PCR tests for early disease detection.

To modernize these efforts, CDFA and County Agricultural Commissioners are supporting the Comprehensive Pest Prevention Program Analysis (C3PA), the first large-scale review of California’s pest and disease prevention systems in nearly three decades. Led by university researchers, the program aims to improve plant introduction protocols, industry practices and early detection strategies.

As new pest and disease challenges emerge, coordinated prevention efforts will be key to preserving the health and productivity of California’s orchards.