Fruit trees in the Sacramento Valley are susceptible to several fungal diseases due to the region’s Mediterranean climate, which features cool and wet winters. These conditions allow many fungal pathogens to thrive during rainy periods and infect new growth in spring.
According to Yolo County Master Gardeners, peach leaf curl (Taphrina deformans) is a frequent issue for peaches and nectarines. The disease causes leaves to become distorted, thickened, and red in early spring, leading to premature leaf drop and reduced fruit set. “A dormant-season fungicide is essential. Apply copper-based fungicides between leaf drop in fall and before bud swell in late winter. Avoid applying once buds begin to swell, treatments after that point are ineffective,” the Master Gardeners advise. While there are resistant varieties of peach such as ‘Frost’, no nectarine varieties offer resistance.
Brown rot (Monilinia fructicola) targets stone fruits like peaches, plums, nectarines, apricots, and cherries. It can kill blossoms in spring and cause fruit to rot or “mummify” on the tree. To manage brown rot, the Master Gardeners recommend pruning for better airflow, avoiding overhead watering, removing infected fruit including any remaining “mummies,” and picking fruit promptly—even if not fully ripe—to reduce infection risk. Copper fungicides should be sprayed when blossom buds swell and possibly again as green fruit ripens.
Powdery mildew (Podosphaera spp.) affects apples, peaches, plums, and grapes by forming a white powdery coating on leaves or fruit. The advice includes pruning for air circulation and using sulfur or potassium bicarbonate preventatively before symptoms appear. Oils such as horticultural oil or neem may also help control infestations.
Apple scab (Venturia inaequalis) impacts apples and crabapples with green-to-black lesions on leaves and fruit leading to early leaf drop. Management strategies include removing fallen leaves during winter to decrease overwintering spores; using copper-based sprays or oils up until full bloom; applying sulfur sprays after wet periods; and choosing resistant varieties like ‘Liberty’ or ‘Enterprise.’
Shot hole disease (Coryneum spp.) infects apricots, peaches, and plums by causing small purple spots that drop out of leaves leaving holes behind; it can also result in gumming lesions on twigs or fruit. Infected twigs should be removed while irrigation methods that splash water onto leaves should be avoided. Treatment involves applying copper fungicide after fall leaf drop and before spring rains.
General recommendations from the Master Gardeners include pruning trees in winter for increased airflow; removing plant debris that could harbor fungi; avoiding wetting foliage during irrigation; maintaining tree vigor through proper care; and following label instructions when using approved anti-fungal treatments.
Gardeners with questions are encouraged to contact jmbaumbach@ucanr.edu with detailed descriptions—and photos if available—using “Ask MGs” as the subject line.
