Red leaf blotch of almond

Red leaf blotch of almond, caused by the fungus Polystigma amygdalinum, is a specific and highly detrimental disease affecting almond trees. The fungus acts as a parasite that primarily attacks the foliage, leading to serious physiological disruption. It completes its life cycle by over-wintering in fallen leaves on the orchard floor, where it prepares to release infectious spores in the following spring.

The visual symptoms of the disease are quite characteristic: during the growing season, bright red or orange spots emerge on the leaves. These spots gradually thicken and become leathery, eventually displaying small black dots on the leaf surface, which represent the fruiting structures of the pathogen. In severe infection cases, leaves curl, turn necrotic, and fall off prematurely.

Environmental conditions play a critical role in the development and dissemination of the disease. High humidity and rainfall during the spring period, specifically coinciding with bud break and early leaf development, are the primary triggers for ascospore release. Wind and water splashes facilitate the spread of the pathogen, allowing the infection to jump from ground debris to the succulent young canopy.

The economic impact of this disease is significant, as it severely compromises the photosynthetic capacity of the almond tree. The premature defoliation induced by the fungus weakens the tree's health, prevents proper nutrient storage, and drastically reduces the tree's winter hardiness. Over time, recurring infections lead to a decline in orchard vigor and a measurable drop in annual nut yields.

Integrated pest management strategies are necessary for effective control. The cornerstone of prevention is orchard sanitation: removing and destroying fallen leaf litter to eliminate the source of primary inoculum. Chemical control should be timed carefully, focusing on protective fungicide applications during the early spring before infection takes hold. Regular pruning also improves air circulation within the canopy, reducing the leaf wetness duration required for fungal spore germination.