Red leaf blotch

Red leaf blotch, caused by the fungus Polystigma ochraceum, is a notable fungal disease that primarily targets stone fruit trees, with a specific focus on almond plantations. This pathogen is a member of the Ascomycota phylum and is known for causing localized tissue death and significant physiological disruption within the infected leaves.

The primary host for this pathogen is the almond tree, although the disease can affect other related species depending on environmental conditions. The infection targets the foliage directly, causing symptoms that can interfere with the tree's photosynthetic ability, eventually leading to weakened growth and reduced harvest potential if left untreated.

Symptoms of the infection typically appear as bright red or orange-red blotches on the leaf surface. As the fungus matures, these spots thicken, and the tissue becomes leathery. On the underside of these spots, the fungus produces its fruiting bodies (apothecia), which are responsible for the dispersal of spores to healthy plant tissues.

The development and spread of Polystigma ochraceum are heavily dependent on wet spring weather and moderate temperatures. The fungus overwinters in fallen leaves on the orchard floor. When moisture is sufficient, the spores are released into the air or carried by water splashes, landing on fresh spring foliage to initiate a new infection cycle.

Effective management strategies require an integrated approach. The most critical control measure is sanitation, specifically the removal and destruction of fallen leaves where the fungus persists during winter. Additionally, chemical control via fungicides should be applied during the early vegetative stage, specifically during bud burst and leaf emergence, to protect the new growth from infection.