Management Strategies for Red Leaf Blotch of Almonds

C.T. Feng, Ph.D. Plant Pathologist, JH Biotech, Inc.

G. Hollenbeck, Pest Control Advisor (PCA), JH Biotech, Inc.

Red Leaf Blotch (RLB) of almond, caused by Polystigma amygdalinum, is a disease currently observed in the Mediterranean region, Spain, the Middle East, and California, USA (1, 3, 5). In recent years, the incidence of this disease has been expanding, posing a significant impact on the almond industry.

As RLB is spread by the sexual generation of an ascomycete fungus (1), it is a monocyclic disease. If the humidity is high in autumn and winter, and the temperature remains above 20°C for an extended period, it will result in an increased spore load the following year (6). The initial infection period mainly occurs in spring and continues into summer, with airborne spores detectable from February to September (3). The ascospores of P. amygdalinum are released on the surface of fallen leaves under moist and warm conditions, especially after continuous rainfall (5). These spores are spread by the wind and infect the young leaves on the tree. Higher temperatures lead to an increase in the number of inoculum produced in the region (6). Symptoms of the disease first appear as yellow spots on the leaves, which gradually turn orange and eventually reddish-brown as the disease progresses. The number of spots on the leaves indicates the quantity of inoculum. Severe RLB can lead to leaf necrosis, curling, and defoliation, resulting in reduced photosynthesis and nutrient accumulation, which in turn leads to a decrease in yield (5).

Current research on management includes chemical and biological control. Chemical control studies have shown that fluopyram and trifloxystrobin, as well as their mixtures or a mixture of pyraclostrobin and boscalid, can achieve over 90% control efficacy (4). When applied 15 days after rainfall exceeding 15mm and when temperatures are between 10-15°C, the efficacy is comparable to that of periodic applications (every two weeks, three weeks, or monthly after petal fall), but with fewer applications (2-4 applications after rain, compared to 5-9 periodic applica-tions). Biological control studies have found that applying two microorganisms, Myrothecium inundatum and Fusarium oxysporum FO12, in combination with urea to RLB-affected almond fallen leaves showed that M. inundatum, especially when incorporated into the soil, significantly reduced inoculum, while F. oxysporum FO12 showed no effect (2).

JHB’s Perspective:

From the research on Red Leaf Blotch of almond, we understand that since RLB is a monocyclic disease, its initial development rate is slower than polycyclic diseases, allowing early detection and prevention. As the inoculum originates from almond leaves, the management of fallen leaves becomes crucial. In Spain, urea treatment of leaves is used to adjust the carbon-to-nitrogen ratio of the fallen leaves, promoting the rapid growth of decomposing microorganisms, which in turn quickly decomposes the fallen leaves and pathogens. Due to the dry conditions on the ground, which limit the growth rate of decomposing microorganisms, incorporating the leaves into the soil can maintain moisture, accelerate decomposition, and reduce the release of inoculum into the air.

Using a plant residue decomposer can effectively break down almond fallen leaves. The carbon-to-nitrogen ratio of almond leaves is about 40, so 1-2 pounds of urea are needed for every 400 pounds of leaves. Our plant residue decomposer, BioAct® SD, has the ability to rapidly decompose fallen leaves and twigs. It contains cellulase, protease, and Bacillus subtilis and Pseudomonas putida, which can produce cellulase, hemicellulase, and protease. It can quickly decompose leaves while releasing nutrients, including various minerals, vitamins, and amino acids, back into the soil for absorption by the almond roots. During the decomposition of the leaves, the pathogens on the leaves are also decomposed. BioAct® SD can be applied at 1.5 pounds per acre along with 5-10 pounds of urea, dissolved in water, sprayed on the fallen leaves, and then incorporated into the soil. This treatment only needs to be applied once annually during leaf management.

Phosphite may induce resistance and enhance the immunity of almond trees is another key preventative measure. Although there are no reports that Phosphite-based products control RLB, almond trees in the same environmental conditions are also vulnerable to other fungal diseases, such as anthracnose, downy mildew, powdery mildew, and rust. Phosphite stimulates the production of salicylic acid, jasmonic acid, and ethylene in plants, further promoting the generation of PR proteins. These proteins are systemically produced in various parts of the plant to resist pathogen attacks. Phosphite type of products may be applied at 1-3 quarts per acre and sprayed on the leaves of almond trees.

BioAct® SD and phosphite type of products may be used as preventative measures to protect almond trees from various fungal diseases. These measures reduce the incidence of fungal diseases and protect almond trees from disease-related yield reductions, forming an integral part of integrated crop management.

Figure 1. Early symptoms of red leaf blotch include small, pale yellowish spots or blotches that
affect both sides of the leaves.

Figure 2. Advanced symptoms of red leaf blotch include larger, yellow -orange blotches (1-2 cm) that turn reddish brown in their center.

(Photo Credits) Alejandro Hernandez and Florent Trouillas

References

  1. Cannon, P. F. 1996. Systematics and diversity of the Phyllachoraceae associated with Rosaceae, with a monograph of Polystigma. Mycol. Res. 100(12): 1409-1427.
  2. Lόpez-Moral, A., Agustí-Brisach, C., Ruiz-Prados, M. D., Lovera, M., Luque, F., Arquero, O., and Trapero, A., 2023. Biological and urea treatment reduce the primary lnoculum of red leaf blotch of almond caused by Polystigma amygdalinum. Plant Dis. 107: 2088-2095.
  3. Pons-Solé, G., Miarnau, X., Torguet, L., Lázaro, E., Vicent, A., and Luque, Jordi. 2023. Airbone inoculum dynamics of Polystigma amygdalinum and progression of almond red leaf blotch disease in Catalonia, NE Spain. Ann. Appl. Biol. 183: 33-42.
  4. Torguet, L., Zazurca, L., Martinez, G., Pons-Solé, G., and Luque, J. 2022. Evaluation of fungicides and application strategies for the management of the red leaf blotch disease of almond. Horticulturae 2022, 8: 501.
  5. Trouillas, F., Hernandez-Rosas, A., Frias, R., Maguvu, T., Zuber,  C., and Gordon, P. 2024. First detection of red leaf blotch: a new disease of almind in California. California almonds website: https://www.almonds.com/almond-industry-news/new-threat-california-almonds-red-leaf-blotch
  6. Zúñiga, E., Romero, J., Ollero-Lara, A., Lovera, M., Arquero, O., Miarnau, X., Torguet, L., Trapero, A., and Luque, J. 2020. Inoculum and infection dynamics of Polystigma amygdalinum in almond orchards in Spain. Plant Dis. 104: 1239-1246.