Peach leaf curl (fungal pathogen: Taphrina deformans) is a fungus disease that can cause severe early defoliation and crop loss on nearly all peach and nectarine cultivars.
The most common and striking symptom of leaf curl occurs on the leaves (foliage). Infected leaves are severely deformed and often display a variety of colors (light green and yellow to shades of red and purple). The fungus causes the meristematic cells at leaf margins to proliferate quickly and randomly, which results in the leaves becoming variously wrinkled, puckered, and curled (photo 2). As these infected leaves mature, naked asci containing ascospores of the pathogen are produced on the surface giving them a dusty appearance, after which the leaves turn brown, shrivel, and drop from the tree.
Many infected fruits drop early and go unnoticed; those that remain may become crooked at the stem end like a small yellow squash, while others develop reddish to purple and have "wart-like" deformities on the surface.
The pathogen occurs commonly almost wherever peaches are grown. The fungal pathogen overwinters as conidia (blastospores, "hyphal like" spores) in protected sites in the bark and around the buds. Primary infections occur during the early spring. Starting when the buds swell until the first leaves appear from the buds. Infections on young peach leaves occur at temperatures of 10°C to 21°C. Few infection occurs below 7°C. Infections appear mainly when rain wash the overwintered spores into the buds and cold temperatures lengthen the development time of the leaves (they are exposed for a long time to the pathogen before they are fully expanded and are able to resist the penetration of the fungus). If temperatures after bud swelling are warm and leaves develop quickly, infections rarely become established, even when spring rains occur. Wetness from rain (or other factors) for over 12,5 hours are needed for leaf infection but only when the temperature is below 61°F (is 16°C) during the wet period. Maximum infection occurs when trees are wet for 2 days or more, a frequent occurrence west of the Cascades. Although infected, symptoms may not appear if temperatures remain above 69°F (21°C). Fruit are susceptibel after petal fall until air temperature remains above 19°C. Rainfall of 0,5 inch and wetness of 24hours are needed for fruit infection.
FieldClimate.com shows the Risk of a Taphrina defomans infection by calculation of temperature.