Nectarines, peaches and other fruit trees that have a low winter chilling requirement may occasionally bloom earlier than normal. Early fall rain and cool winter weather can satisfy the trees' chilling requirement, and this would allow the dormant buds to grow. Flower buds usually open first because they require less cold than leaf buds. Deciduous fruit trees may also bloom prematurely in fall after early rain if the trees were not watered enough during a period of hot weather in late summer. The heat and water stress breaks down chemicals in the buds that keep them dormant.
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, August 2012
It's too late now to spray your tree for leaf curl since it is already blooming and starting to set fruit. You can still prune your trees but the new leaves may be attacked by leaf curl especially if we have rainy weather.
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.
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 .