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Severe red leaf blotch infections in almond trees can lead to defoliation and increased stress on trees. Leaf loss during the summer depletes carbohydrate reserves and reduces the number of fruiting buds available for the following growing season.
During a UCCE Experts webinar, UC fruit and nut pathology specialist Dr. Florent Trouillas discussed how the invasive fungal disease is affecting almond orchards and yields.
Since the 2024 outbreak of red leaf blotch (RLB) in the northern San Joaquin Valley, the disease has spread rapidly, with some orchards experiencing 100% infection on 100% of trees. The disease has now been reported in all almond-growing regions of California and has been found in most almond cultivars.
Trouillas said environmental conditions have contributed to the spread, but it is likely the disease was introduced into California through the movement of infected plant material.
Red leaf blotch is caused by the fungal pathogen Polystigma amygdalinum. The infection cycle begins after petal fall as young leaves emerge from vegetative spurs. Those leaves remain susceptible to infection for several months. After the initial infection, the pathogen survives on infected leaf litter until spores are dispersed during the following growing season.
As infected trees begin to defoliate, leaves develop fungal fruiting structures. Trouillas said pycnidia, a noninfectious fungal fruiting body, develop during the summer and play a role in reproduction of the pathogen.
Symptoms Appear Too Late
One challenge with red leaf blotch is that symptoms do not appear until 35 to 45 days after the initial infection.
Growers may wait until they see the characteristic red leaf blotch symptoms before applying a fungicide, but by that point the infection has already been developing for weeks.
“The period where we have high numbers of spores in the air is where we need to think and consider fungicide application and intervention,” Trouillas said.
With funding from the Almond Board of California, researchers are working to better determine the timing of RLB sporulation with the goal of developing a decision-support tool to help growers better time fungicide applications.
Studies have shown rainfall is required to trigger sporulation of Polystigma within an orchard. Trouillas noted microsprinklers can have a similar effect, triggering sporulation under some conditions.
Timing Is Critical for Control
Trouillas cited a table developed by UC Riverside plant pathologist Jim Adaskaveg showing fungicide timings for red leaf blotch and other almond diseases.
He noted fungicide applications are not effective during bloom or after disease symptoms are visible. Instead, applications should be made at petal fall as young leaflets emerge, two weeks after petal fall and, if wet weather persists, five weeks after petal fall. Those timings also provide control of anthracnose, bacterial spot, brown rot and green fruit rot.
“That timing is absolutely critical for fungicide intervention,” he said.
To integrate RLB management with control of scab, shot hole and rust, Trouillas recommended fungicide mixtures from FRAC groups 3+7, 3+11 and 7+11, along with FRAC 3 triazoles. He advised rotating active ingredients to reduce the risk of fungicide resistance.
Several large field trials conducted from Stockton to Merced evaluated fungicide performance against red leaf blotch. Trouillas said some products provided excellent control while others offered only moderate efficacy.
Among the most effective products were Quadris Top, Merivon, Adament, Quash, Miravis Duo, Luna Sensation, Miravis Prime, Rhyme and Pristine.
He noted that even with the most effective fungicides, at least two applications are needed for successful disease management.
Sanitation Helps Reduce Disease Pressure
Cultural practices aimed at reducing the primary inoculum include applying zinc sulfate to hasten leaf drop, removing infected leaf litter, applying urea or lime sulfur to accelerate leaf decomposition and using beneficial microbes for biosanitation.
Trouillas noted these sanitation practices are most effective when implemented across a wide area rather than in individual orchards.
Cecilia Parsons
Cecilia Parsons has lived in the Central Valley community of Ducor since 1976, covering agriculture for numerous agricultural publications over the years. She has found and nurtured many wonderful and helpful contacts in the ag community, including the UCCE advisors, allowing for news coverage that focuses on the basics of food production.
She is always on the search for new ag topics that can help growers and processors in the San Joaquin Valley improve their bottom line.
In her free time, Cecilia rides her horse, Holly in ranch versatility shows and raises registered Shetland sheep which she exhibits at county and state fairs during the summer.