While you may not notice symptoms of peach leaf curl until the spring, you should manage it in the fall and winter.
Peach leaf curl is a fungal disease that primarily affects peach and nectarine trees. Fungal spores overwinter on twigs and flower buds and germinate in the spring causing distorted, reddened leaves. Unfortunately, this disease cannot be effectively managed once the symptoms appear in the spring, so experts recommend pesticide treatments before bud break (when new flower buds begin to open).
Timing of the first treatment in the dormant season should be made after leaves have dropped in the fall and winter. This is often in late November or early December. Treat affected trees with a fungicide product containing the active ingredients copper or chlorothalonil. These active ingredients can be alternated to limit copper buildup in the soil. A single treatment is generally sufficient, but a second spray can be done in late January or early February, again before bud break.
Always read the label of the pesticide product and verify that it is labeled to treat peach leaf curl. Use only the rate listed on the label to avoid fungicide resistance and avoid making applications before it rains. For more management information, see the UC IPM Pest Notes: Peach Leaf Curl and Pesticides: Safe and Effective Use in the Home and Landscape.
If left untreated for several years, trees affected by peach leaf curl can decline and may need to be removed. In some cases, the disease can also affect fruit quality.