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Answer: Peach leaf curl, caused by the fungal organism Taphrina deformans, is a common disorder in peach and nectarine orchards, especially during wet springs. Infected leaves become misshapen, deformed, and necrotic, resulting in premature defoliation with subsequent re-sprouting of new leaves. This kind of stress reduces fruit yield and predisposes the tree to pest attack.
The infection period for leaf curl is when new leaves start emerging from buds in the spring. Spraying after the buds have opened is ineffective because infection takes place as the young leaves emerge, and the fungus develops inside the leaf. Accordingly, sprays must be applied during the trees' dormant period—after the leaves have fallen and before the first budswell in the spring. Many orchardists spray just prior to budswell during February and March. Orchards with a history of severe peach leaf curl benefit from a double application: in the autumn at leaf fall and again in late winter or early spring just before budswell.
Fortunately for the organic grower, lime sulfur—one of the most effective fungicides for control of peach leaf curl—is allowed in certified organic production. Bordeaux and copper fungicides—also approved for certified organic programs—are effective as well, but not as effective as lime-sulfur.
University trials comparing Kocide™ (copper hydroxide), lime-sulfur, several synthetic fungicides, and Maxi-Crop™ seaweed for leaf curl control indicated that lime-sulfur and one of the synthetics (ziram) were best, roughly twice as effective as Kocide. Seaweed sprays, despite positive anecdotal reports, were completely ineffective.
Severe leaf curl infection can cause the tree to shed many of its leaves and to replace them with a second flush of growth. At this time, the tree will benefit from a soil application of a quickly-available soluble fertilizer such as compost tea or fish emulsion to help it recover.
There are various levels of resistance to leaf curl among varieties; however, because of the relative ease of controlling the disease, breeding for resistance has not been a priority. Redhaven, Candor, Clayton, and Frost are some of the cultivars with resistance to leaf curl, though none is immune.
For more information, see the ATTRA publication Peaches: Organic and Low-Spray Production, available at https://attra.ncat.org/attra-pub/summaries/summary.php?pub=6. This publication describes the major diseases and insect pests of peaches and discusses organic or least-toxic control options for each. It emphasizes the considerable climatic differences between the arid West, which is relatively amenable to organic peach production, and the humid East, where it is more difficult to grow peaches without synthetic fungicides and insecticides.