Powdery Mildew
Sphaerotheca pannosa  
Powdery mildew.
Powdery mildew on nectarine fruit.
Powdery mildew lesion on peach fruit.

 

I. Introduction: Powdery mildew of peach, nectarine, and apricot is a disease that rarely is of major importance. When weather conditions are favorable for infection, however, it can cause some economic loss by reducing fruit quality. Powdery mildew is sometimes called rose mildew, for it seriously affects roses and other woody ornamentals. The causal fungus, Sphaerotheca pannosa, attacks young shoots, leaves, and fruit. Another powdery mildew fungus, Podosphaera clandestina, which causes common mildew on cherry, also infects peach.

II. Symptoms: Infected young leaves may drop or fail to elongate and unfold normally, while those of new shoots become narrow, strap-like, and distorted. Infected leaves may become completely coated with the thick, white, powdery mycelium and spores of the fungus, or the infected areas may appear as whitish patches (photo 2-56). Mature leaves are more resistant to the fungus. This white, powdery appearance is the result of large masses of conidia that are produced on the leaf surface. This whitish growth often can be seen on infected fruit as well (photo 2-57). Affected current year's twigs are stunted in growth and the lateral buds which differentiate into blossom buds may be destroyed.

On developing fruit, the disease first appears as white, round spots two to four weeks after shuck fall. These spots increase in size until a large portion of the fruit is covered (photo 2-58). About the time of pit-hardening, the skin of the fruit under the spot turns pinkish and eventually becomes a dark brown, at which time the fungus and its spores disappear. The fruit surface becomes leathery and hard and may crack. As the fruit matures, it becomes more resistant to the fungus.

III. Disease Cycle: The fungus overwinters as mycelium in infected shoots and dormant peach buds. Flower buds of infected shoots often do not survive the winter. As infected shoots begin to grow in the spring, the fungus within the diseased tissue produces spores (conidia). These conidia are disseminated by air currents and rain, causing new infections on expanding leaves, shoots, and young fruit. Young leaves and immature fruit are more susceptible to mildew. Fruit become resistant with pit hardening and the leaves become resistant with age. The disease is favored by dry, warm weather with moisture in the form of fog, dew, or high humidity. Free moisture is not required for spore germination. Germination can occur at relative humidities of 43 to 100 percent. When environmental conditions are favorable, new lesions will develop within ten days.

IV. Monitoring: Monitor ten terminals on each sample tree for the presence of white, mycelial growth on young leaves (photo 2-56). A total of one to ten infections and greater than ten infections represents moderate and high risk, respectively.

Monitor 25 fruit on each sample tree, two to four weeks after shuck fall for the presence of round, whitish, powdery spots on the fruit surface (photos 2-57, 2-58). A total of ten to 20 fruit infections and greater than 20 fruit infections represents moderate and high risk, respectively.

V. Management:  The disease is effectively managed by avoiding peach cultivars susceptible to powdery mildew.  With susceptible cultivars, such as Redskin and Rio Oso Gem, spray treatments may be needed.   Begin fungicide sprays at petal fall and continue at 10- to 14-day intervals until the pit hardening stage is reached.  Fruit of susceptible cultivars become resistant at this stage.  Under severe conditions, additional sprays may be needed to prevent infections of leaves and shoots. 

Chemical control - commercial growers

Chemical control - home orchardists (pdf file - Acrobat Reader required)

Text prepared by J. W. Travis, J. L. Rytter, and K. S. Yoder

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