As almond orchards wrap up bloom and begin leafing out, a new disease threat has moved to the top of the watchlist for growers across the valley. Red Leaf Blotch (RLB) is an emerging concern that demands attention, but with the right timing and protective measures, it is entirely manageable.

What Is Red Leaf Blotch?

Red Leaf Blotch is a new disease for almonds in California, though it has long been prevalent in almond-growing districts in Spain and Portugal. It was first identified in Merced County in 2023 at a single location, and by 2024 had spread to many locations across the valley. The disease has spread quickly to all almond-growing areas in only two years, which is indicative of how prolific this pathogen is. In 2025, trials to evaluate fungicide control of the disease were conducted by the Trouillas Lab of UC Davis.

RLB is a foliar disease, meaning it exclusively targets the leaves of almond trees. The incubation period is 30 to 45 days after infection, so symptoms are not evident until late April or early May. Small, pale yellow spots appear first, with orange lesions developing by late May. The disease does not affect the nuts directly, but uncontrolled infection compromises the tree in significant ways:

• Infected leaves have reduced photosynthetic capacity, limiting the tree’s ability to create sugars and carbohydrates
• Reduced energy reserves weaken the tree’s ability to withstand stress, fend off pests, and produce fruit buds for next year’s crop
• Yield losses have been documented in infected orchards in Spain

Understanding the Disease Cycle

The lesions do not produce spores during the summer. After leaf drop in the fall, perithecia develop on fallen leaf material and produce ascospores in late winter. Rain and irrigation promote the release of spores, and high numbers of wind-blown spores have been measured in March and April. Because the disease requires leaf tissue to infect, the infection window opens as soon as trees begin pushing new growth in early spring.

Protective Fungicide: Timing Is Everything

Preventative control is the only option. Once symptoms appear in late April or early May, it is too late to control the disease. Fungicides applied at full bloom are not effective. The critical application timings are:

• Petal fall (leaf out) – apply a protective fungicide as leaves emerge
• 3 weeks post petal fall – a follow-up application at this timing has been shown effective at controlling the disease

Fungicide applications must be in place before a rain event – not after. A protective approach is the standard of care.

What About the Current Dry Conditions?

As of March 12, 2026, we are experiencing a historic hot and dry spell. Many growers and PCAs are asking whether fungicides are necessary when no rain is in the forecast.

The Trouillas Lab has informed us that spore traps are still catching RLB spores. It is likely some infections will occur even in dry conditions. To reduce the risk of a disease problem, a fungicide application is still recommended.

We are also asking growers to leave 3 rows of 15 to 20 trees untreated so we can learn what happens in dry springs. This observational data will be invaluable for refining future management recommendations.

Recommended actions for growers this spring:

• Monitor weather forecasts daily during March and early April
• Apply a protective fungicide at petal fall (leaf out), ahead of any rain event
• Apply a second protective fungicide 3 weeks post petal fall
• Work with your Agri-Valley advisor to select the right fungicide product and rate for your operation
• Leave 3 rows of 15-20 trees untreated to contribute to regional disease monitoring data

The Bigger Picture: March Orchard Management

Red Leaf Blotch management doesn’t happen in isolation. March is one of the most consequential months of the almond growing season, and disease protection must be integrated with a comprehensive approach to orchard health.

Root Health: The Foundation of Everything

No disease management program will perform at its best without healthy roots. Almond tree roots require oxygen to function. Compacted or waterlogged soils restrict oxygen availability and impair root respiration, weakening the tree’s ability to take up water, nutrients, and fight off disease pressure.

Practical steps to protect root health:

• Avoid over-irrigation – never saturate the root zone to the point of oxygen deprivation
• Address soil compaction and maintain a flocculated, open soil structure
• Avoid long irrigation sets that seal the soil surface and restrict airflow

Nutrient Management: Small and Frequent Wins

Trees benefit from fertigating on a frequent basis, introducing smaller amounts of nutrients each time irrigation is applied. This improves efficiency and keeps trees in steady, balanced growth. Potassium deserves special attention – it is taken up by tree roots continuously throughout the season. Begin potassium fertigation in March and carry it through spring and into summer, even as late as July.

Foliar Nitrogen: Consider Low-Biuret Urea

When you are already running a spray application for disease or insect management, consider adding low-biuret urea as a foliar nitrogen source. Urea is readily absorbed through leaf tissue and enhances uptake of other foliar materials in the tank, such as zinc. The carbohydrate stimulation from urea can also make trees less susceptible to pest pressure – an integrated benefit from a single application.

Summary: Your March Action Checklist

• Apply protective fungicide at petal fall (leaf out) – do not wait for rain
• Apply a second fungicide 3 weeks post petal fall
• Even in dry conditions, spore traps confirm RLB spores are present – do not skip fungicide applications
• Leave 3 rows of 15-20 trees untreated to support regional disease research
• Start potassium fertigation in March and continue through summer
• Fertigate frequently with smaller nutrient doses rather than large single applications
• Avoid over-irrigation – protect soil oxygen levels and root health

Contact Agri-Valley Consulting to maximize almond production on your farm.

Any questions or concerns about your orchard’s disease management program? Reach out to the Agri-Valley team today by calling (209) 769-5980.