In the Bay Area, fruit tree diseases such as peach leaf curl, brown rot, shot hole, fire blight, and apple or pear scab are common problems due to the region’s cool and wet winters. Experts say that winter is the best time for home gardeners to take preventive steps before symptoms appear in spring.
Many pathogens overwinter on twigs, buds, bark, and mummified fruit. They infect new growth when buds swell and leaves emerge during late winter and spring. To reduce disease risk, gardeners are advised to prune trees to improve air circulation, remove diseased plant material and fallen fruit, and apply dormant fungicide sprays if needed. Guidance on timing and application can be found through University of California Integrated Pest Management (UC IPM) resources on fruit tree care.
Peach leaf curl is a frequent concern. The disease is caused by the fungus Taphrina deformans and affects peaches, nectarines, and sometimes apricots. Typical symptoms in spring include thickened, puckered leaves with red or yellow areas that may drop early. “Once you see these symptoms in spring, it is too late to treat them for that year; any sprays at that point will not cure infected leaves.” The fungus survives summer as spores on bark and twigs but becomes active with cool wet weather as buds swell. According to UC IPM guidance: “Treat after leaves have fallen in late fall or early winter. A second application is recommended in late winter just before buds swell; this is important especially if it has been a wet winter.” More details can be found at UC Integrated Pest Management – Peach Leaf Curl.
Other diseases also benefit from preventive action during winter:
– Brown rot blossom and twig blight affects stone fruits like peaches, plums, cherries, almonds, and apricots. Symptoms include blighted blossoms with tan spore masses and gummy discharge at flower bases. Gardeners should remove mummified fruit after harvest and again in winter; prune out blighted twigs well below visible dead tissue; open canopies for better air flow; and refer to UC Integrated Pest Management – Brown Rot Blossom and Twig Blight for more information.
– Shot hole disease (Coryneum blight), caused by Wilsonomyces carpophilus, creates small holes in leaves after spots fall out along with rough spots on fruit. Spores survive on twigs through winter before spreading via rain splash. Removal of infected twigs and avoidance of overhead irrigation are key strategies; dormant fungicide sprays may be needed where shot hole has been severe (UC Integrated Pest Management – Shot Hole Disease).
– Fire blight targets apples, pears, crabapples, quince, Pyracantha species—and related plants—causing shoots to look scorched with bent tips known as shepherd’s crooks. There is no cure but pruning 8–12 inches below affected areas during dormancy helps manage spread (“During dormant pruning (on a dry day), remove blighted twigs…”). Pruning tools should be disinfected between cuts (UC Integrated Pest Management – Fire blight). Choosing resistant varieties also helps long-term management.
– Apple and pear scab fungi overwinter in fallen leaves under trees then infect new growth in spring causing olive-green scabby spots on leaves or fruit which may lead to premature leaf drop (UC Integrated Pest Management – Apple and Pear Scab). Removing fallen leaves—composting them hot enough (at least 131°F) or disposing through green waste—is an effective step.
Experts emphasize that “winter is the most important—and most overlooked—season for preventing fruit tree diseases.” Sanitation practices such as removing debris combined with dormant-season pruning improve outcomes significantly.
The advice comes from UC Master Gardeners of Alameda County who offer further support via their Help Desk page for those seeking assistance with gardening questions.
