Red Leaf Blotch

Symptoms of red leaf blotch first appear in late April to early May as small, pale, yellowish spots or blotches that affect both sides of leaves. As the disease progresses, the blotches grow larger (0.4–0.8 in or 1–2 cm) and turn yellow-orange with a reddish-brown center. At advanced stages of disease development, leaves become necrotic, curl, and drop prematurely, thus decreasing the photosynthetic capacity of the tree and leading to a general decrease in yield. Only the leaves are affected. The primary inoculum are ascospores that form in perithecia (sexual fruiting bodies) on fallen infected leaves from the previous growing season. During spring and summer, leaves contain the pycnidia (asexual fruiting bodies) of the fungus, which produce filiform (thread-like) conidia. These asexual spores do not cause infections on leaves but may have other roles in the disease cycle.

The disease is monocyclic, with only one primary infection cycle per year. The main infection period occurs after petal fall, when young leaves emerge and spring rains occur, and may continue until May. Rain contributes to but is not essential for the release and dispersal of ascospores from the perithecia. The disease may not be noticed before early to mid-May, as infection remains latent for approximately 35 to 40 days. The infection of leaves decreases drastically after May and with high summer temperatures. Rain combined with mild temperatures in spring generally leads to higher disease incidence.

Cultural practices that focus on eliminating the primary inoculum on infected fallen leaves can help mitigate red leaf blotch. These practices consist of either removing leaf litter or applying urea to accelerate leaf litter decomposition. However, such strategies are only effective when applied over a wide area.

One preventive fungicide application at petal fall and two additional applications at 2 to 3 weeks and 5 to 6 weeks after petal fall are effective at managing the disease. These timings overlap with management timings for shot hole, scab, and rust and should be implemented as part of the orchard integrated pest management program. Fungicide mixtures with Fungicide Resistance Action Committee (FRAC) mode-of-action group numbers 3 and 7, 3 and 11, 7 and 11, and 7 and 12, and fungicides with mode-of-action group number 3 (triazoles) are most effective at controlling red leaf blotch. The multi-site fungicide chlorothalonil (M5) applied during petal fall is also highly effective. Fungicides applied during bloom and after symptoms are visible are not effective.