Preventative Fungicide Treatments Key to Managing Red Leaf Blotch in Almond Orchards

A newly identified disease called red leaf blotch has been confirmed in California almond orchards. First detected in Merced County in May 2024, the disease has now been found in orchards from all almond-producing areas of the state.

Caused by the fungus Polystigma amygdalinum, red leaf blotch was previously known to affect almonds in Mediterranean and Middle Eastern regions, and researchers are urging California growers to be proactive in managing the disease.

Recognizing the Symptoms

As the name suggests, red leaf blotch primarily affects the leaves of the tree. According to UCCE Plant Pathology Specialist Florent Trouillas, the disease first shows up in late April through mid-May, usually on young leaves. Early symptoms can be subtle — small, pale-yellow spots may appear on either side of the leaf. As the disease progresses, these spots grow larger, turning orange or reddish-brown and reaching up to one to two centimeters in size. In more advanced cases, the blotches can merge, causing the leaves to curl, scorch, and fall off the tree prematurely. This defoliation weakens the tree and reduces photosynthesis, which can impact both current-year yields and the development of flower buds for the following season.

Trouillas has confirmed the disease in several varieties including Aldrich, Butte, Carmel, Fritz, Independence, Monterey, Nonpareil, Padre, Shasta, and Wood Colony. Red leaf blotch is not lethal to trees, but its effect on canopy health and productivity can be significant if not addressed early.

Understanding How It Spreads

Red leaf blotch is considered a monocyclic disease, meaning it only has one infection cycle per season. The fungus survives the winter in fallen infected leaves. In spring, spores are released from this leaf litter and spread by rain or wind, typically infecting young leaves just after petal fall. Trouillas notes that the disease has a long latent period of 35 to 40 days, so by the time symptoms are visible, the infection window has long passed. This makes early, preventive management the key to controlling the disease.

Preventive Fungicide Program

Because symptoms appear weeks after infection, preventive fungicide applications are critical. Based on findings from UC research and international experience, a three-spray program is recommended for orchards at risk. The first spray should be applied at petal fall, followed by a second application two to three weeks later, and a third spray five to six weeks after petal fall if wet weather continues. These timings coincide with disease pressure and can also serve to manage other spring diseases like shot hole, rust and anthracnose.

The most effective products come from FRAC groups 3+7; 3+11; 7+11 and FRAC 3-triazoles, including mixtures like fluopyram and trifloxystrobin, or pyraclostrobin and boscalid. Growers are advised to rotate modes of action and follow all resistance management and product label guidelines to prevent product failure over time.

Orchard Sanitation

Sanitation plays a moderate role in controlling red leaf blotch by reducing the amount of fungal spores available to infect next year’s crop. UC Cooperative Extension recommends removing or breaking down infected leaf litter before the next season. This can be done by mowing or shredding leaves in the fall or winter. Applying urea or other nitrogen-rich materials to leaf litter can speed decomposition, helping to suppress the fungal population. If conditions allow, incorporating the leaves into the soil through light tillage can also be beneficial.

It’s important that growers avoid transporting infected debris or equipment from one orchard to another. Tools, trailers or bins that have come in contact with infected plant material should be cleaned before moving to new sites. The more growers in a region who follow these sanitation practices, the more effective they will be — spores can easily move on wind or rain between neighboring blocks.

Monitoring and Research

Growers should begin scouting for symptoms in early May and report any suspected cases to their local UCCE farm advisor. Early reporting helps researchers better understand the disease’s spread and behavior in California growing conditions.

Research led by Trouillas and his team at UC Davis is ongoing. They are currently studying fungicide efficacy, timing of spore release and host susceptibility, environmental triggers, and how different almond varieties respond to infection. They are also developing improved diagnostic tools to help detect the disease earlier in the season.

Red leaf blotch is a new disease for California almond growers, but with early action and coordinated management, it can be kept under control. Preventive fungicide sprays, diligent orchard sanitation and regional cooperation are the best tools currently available. Stay in touch with your local UCCE advisor and watch for further updates as more research results become available.