Utah Plant Disease Control No. 51
Created November 1999

Powdery Mildew of Stone Fruits

Powdery Mildew is the most common disease of tart cherry and peach trees in Utah. Mildew infections result in a white powdery mass of fungal growth on susceptible tissue. The fungi that cause this disease are Podosphaera clandestina on cherry and Sphaerotheca pannosa on peach. These fungi frequently infect new vegetative growth, causing reduced vigor, leaf malformation, and reduced viability of buds. In addition to Sphaerotheca pannosa, peach can also be infected by the apple powdery mildew fungus, Podosphaera leucotricha, which causes rusty spot disease on peach fruit .

Mildew is more serious on tart than on sweet cherries, but both can be seriously affected under ideal conditions. Mildew causes uneven ripening of tart cherries and makes mechanical harvesting more difficult. In some years, mildew may infect sweet cherry fruit or petioles, causing distortion and poor quality. Under prevailing Utah climatic conditions, peach powdery mildew affects leaves and shoots and less commonly fruit. Mildew may reduce the vigor of all fruit trees and reduce return bloom. Young trees and vigorously growing shoots are the most susceptible.

DISEASE CYCLE

Cherry Powdery Mildew

This fungus overwinters as cleistothecia. The cleistothecia drop to the orchard floor and tree crotches or become trapped in bark crevices. They are about the size of the point of a pin and, therefore, not easy to see. Spores released from the cleistothecia in the spring are spread by rain or irrigation to young leaves. The earliest infections are found on leaves of suckers or succulent terminal growth near the crotches. These infections produce conidia in repeated cycles during the summer, resulting in the powdery appearance of infected leaves. Late in the summer, the fungus produces the cleistothecia.

Powdery mildew is most common when the relative humidity exceeds 90 percent and daytime temperatures are between 50 - 78F although some infections can occur when humidity is quite low. Long periods of rain are not necessary for infections since the spores will not germinate in free water.

Peach Powdery Mildew

This fungus overwinters as mycelia inside the budscales, primary infection occurs as leaves emerge from these infected buds. Secondary infections occur when conidia produced by primary and subsequent secondary infections are blown or splashed by rain onto susceptible tissues. Fruit (before pit hardening) and succulent terminal growth are susceptible to infection.

Powdery mildew is common under similar relative humidity and temperatures as cherry powdery mildew.

Rusty Spot

Little is known about the disease cycle of this fungus as it pertains to peach. Trees that are most likely to be affected by rusty spot are those adjacent to powdery mildew infected apple plantings . Fruit is the only susceptible tissue to rusty spot infection. They become infected when conidia from infected apple trees come into contact with the fruit. Initially a small circular orange-rust colored lesion appears, which gradually enlarges and becomes indeterminant.

CONTROL

Cherry Powdery Mildew

Tart cherries must be treated routinely; control is not usually necessary in sweet cherries. Removal of suckers in the center of the tree may eliminate a site for early infections. Fungicides are not necessary early in the spring because the primary infections arise from the cleistothecia which germinate in late spring.

The first spray is recommended when very subtle mildew lesions develop on leaves. These lesions are difficult to detect at first but ultimately develop into a cottony growth of mycelium. Control will be poor if spraying is delayed until the mildew is obvious. The recommended date to initiate spraying is available from county agents and has varied from May 5 to June 3. Repeat sprays every two weeks with most fungicides except sulfur which requires applications every five to seven days. It is usually necessary to make two to three applications per year. Fungicide sprays are crucial in young orchards, in orchards with vigorous growth, or in orchards with poor air circulation. In older orchards, it may be possible to achieve good control with one to two applications.

Fungicides for Cherry Mildew

Peach Mildew

Removal of infected fruit during and pruning out terminals with infected buds during normal orchard operations will reduce the amount of infection within the orchard. Planting resistant cultivars such as Angelis, Walton, Johnson, Halford, and Stuart will further reduce mildew within the orchard. Chemical sprays for control are suggested to start at petal fall or shuck split and continue every 7-14 days until terminal growth ceases.

Fungicides for Peach Mildew

Rusty Spot

Control of this disease hinges on controlling powdery mildew on adjacent apple trees. Application of wettable sulfur at shuck split and continuing every 8-10 days until terminal growth ceases has also been effective.

Fungicides for Rusty Spot on Peach

Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician

Listing of commercial products implies no endorsement by the authors or the Utah State Cooperative Extension Service. Criticism of products not listed is neither implied nor intended. Persons using such products are responsible for their use according to the current label directions of the manufacturer. Pesticide labels are legal documents, and it is a violation of federal and state laws to use a pesticide inconsistent with its labeling. The pesticide applicator is legally responsible for its prope
use. Always read and follow the label.

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