Field Insights

RED LEAF BLOTCH: An Invisible Enemy 🔎

RLB infects almonds early and shows its symptoms too late. See why prevention is your only option against this destructive disease.

May 2026 | By Syngenta Thrive 2 Min Read

May 2026 | BY By Syngenta Thrive 2 Min Read

A Hidden Early-Season Threat

Fungal leaf disease caused by Polystigma amygdalinum
Infects young leaves early in spring Symptoms appear 35–40 days later
No control possible once symptoms are visible

What Does RLB Look Like?

Early Signs: Small yellow spots on leaf surface
Progression: Lesions enlarge and turn orange/reddish brown; blotches merge and cause leaf curling, scorching and early drop
Impact: Weakens tree canopy and reduces photosynthesis
Infected leaves fuel next year’s infections

Why RLB Detection and Treatment Matters

Early Infection Leads to Long-Term Damage

Causes premature leaf drop
Reduces photosynthesis and energy storage
Stresses trees and reduces quality almond yields
Risks future profit potential

Where is RLB?

New to the U.S. and Moving Fast

Seen in Spanish orchards as early as 1927, with a historical presence in the Mediterranean and parts of the Middle East
First detected in California in 2024, likely introduced through infected plant material
Has spread quickly from Merced and Madera counties to all major almond-producing counties in California
If you see RLB pressure this year, expect more next year!

RLB’s Cycle Works Against You

Overwinters in infected leaf litter Spring rain releases new ascospores at bloom
Spores then infect new leaves, continuing its cycle
Long latency period (35–40 days) hides infection until damage is already done

Why Prevention is Crucial

RLB is monocyclic, meaning there’s only one infection cycle per season — if you miss it, you miss your control window. Fungicides can’t cure infected leaves, and protection must be in place before infection.

An Integrated Approach Is Your Best Bet

Cultural Practices

Managing infected leaf litter
Speeding up leaf breakdown with zinc + urea
Cleaning equipment between orchard passes

Critical Fungicide Timing

Petal Fall: Apply Miravis^® Prime fungicide now to dial up your RLB defense. This is a crucial step to help prevent infection!
Post-Petal Fall: Apply Miravis Duo or Quadris Top^® fungicide 2–3 weeks post-petal fall to help maintain RLB protection.
Follow-Up Spray: For high-risk orchards, a third application is recommended (~5 weeks post-petal fall)

Early Action Helps to Protect Yield

Protecting almonds early with a powerful preventive fungicide program and smart management practices is a way to preserve tree canopy, support almond yield and your orchard’s longevity.

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