The fungus disease, leaf curl, can infect peaches and nectarines when new leaves emerge in spring. Trees must be sprayed with an effective fungicide while they are dormant to kill spores that overwinter on bark surfaces. Trees can usually be sprayed in December and again in January if weather permits. Trees should not be sprayed after buds begin to open since the spray can damage the flowers.
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, August 2012
For years, gardeners sprayed their trees with lime sulfur (calcium polysulfide) or a powdered copper fungicide (tribasic copper sulfate) to prevent leaf curl but these products were discontinued in 2010 and are no longer available for home use.
For information on current fungicide products to prevent leaf curl, see UC Pest Note 7426 www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7426.html .