Fungus causing the disease: Podosphaera leucotricha (Ell. & Evherh.) Salm.

Introduction - Powdery mildew is a fungal disease that can reduce the vigor and productivity of apple trees. The cultivars Baldwin, Cortland, Crispin, Gala, Gingen Gold, Idared, Jonathan, Monroe, Paulared, and Rome are particularly susceptible to powdery mildew. Other cultivars may also become infected when inoculum is present and conditions are favorable for infection. The powdery mildew fungus may kill vegetative shoots and flower buds. Furthermore, infected fruits may become russetted.

 

Powdery mildew infection on an upper leaf surface (fig. 1), and a more advanced powdery mildew infection causing leaves to curl (fig. 2).

Symptoms - Powdery mildew may infect all young green tissue. Detection of the disease is usually evident by the visible presense of the fungus (see Signs below). Symptoms can vary greatly between growing seasons, cultivars, and different levels of infection.

Leaves: Initial infections on the underside of the leaf may cause chlorotic (yellow) patches or spots to occur on the upperside of the leaf. This symptom is not unique to powdery mildew, so inspection of the underside of the leaf is necessary to confirm that powdery mildew is the cause. Lesions on upper leaf surfaces are white and powdery in appearance (fig.1). Infected leaves tend to crinkle, curl, or roll upwards along the edges giving them a narrow appearance (fig. 2). Severely infected leaves will usually drop prematurely during the summer. Under conducive conditions, the disease will spread over the entirety of the leaf and progress down the petiole onto young, green shoots.

Shoots: Shoot infections are the typical result of overwintering infections of the fungus in buds. When the terminal buds push in early spring, the fungus advances with the new succulent growth and infects this tissue immediately (fig. 3). Infected terminal shoots often appear stunted. Furthermore, young shoots may be killed outright in the spring (fig. 4), or may survive throughout the season and die in late fall or winter.

 

Powdery mildew infection stunting the growth of a shoot (fig. 3), and shoot death caused by a powdery mildew infected bud (fig. 4).

Flowers and Fruits: On the blossoms, the petals, sepals, receptacles, and peduncles may become infected and covered with the fungus (fig. 5). Blossom infections are less common but are important because infected blossoms will either fail to set fruit or produce small, stunted and/or russeted fruit (fig. 6). These fruit are unmarketable for fresh market consumption.

 

Healthy verses powdery mildew infected flowers (fig. 5), and russet symptom on apple fruit infected with powdery mildew (fig. 6).

Signs (visible presence of the pathogen) -Podosphaera leucotricha is easily seen as white, web-like lesions on any young green tissue. Close inspection of lesions will reveal thin hair-like mycelia with chains of spores. The initial growth of the fungus on twigs appears powdery white but eventually turns a darker brown. Small black fruiting bodies, called cleistothecia, form late in the season. The cleistothecia are visible to the naked eye and are the source of the overwintering spores.

Disease Cycle - Powdery mildew overwinters as fungal strands (mycelium) in vegetative or fruit buds which were infected the previous season. Infected terminals may have a silvery gray color, stunted growth, and a misshapen appearance and are more susceptible to winter kill than are noninfected terminals. Temperatures near -28°C (-18°F) kill a majority of infected buds and the fungus within them. Even at lower temperatures, however, some infected buds survive. The powdery mildew fungus also produces masses of small black structures, called cleistothecia, on infected leaves and terminals in the late summer and fall. Although the cleistothecia contain spores, they do not cause infections.

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As buds break dormancy, the surviving powdery mildew infected buds form primary infection points on developing tissue. Primary mildew infections may occur on vegetative shoots and blossoms and thereby cause a reduction in yield. The powdery white appearance on infected shoots consists of many thousands of spores, called conidia, which are responsible for spreading the fungus and causing secondary infections. Secondary infections usually develop on leaves and buds before they harden off and may reduce the vigor of the tree. Secondary infections also produce the overwintering infected buds as the tree developes during the season.

Conditions Favoring Infection - Powdery mildew infections occur when the relative humidity is greater than 90% and the temperature is between 10-25°C (50-77°F). The optimum temperature range for the fungus is between 19 and 22°C (66-72°F). Although high relative humidity is required for infection, conidia will not germinate if immersed in water. Leaf wetting is not conducive to powdery mildew develoment. Under optimum conditions, powdery mildew will be obvious to the naked eye by 48 hours after infection. A new infection can produce spores in about 5 days at optimal conditions. Nongerminated powdery mildew spores can tolerate hot dry conditions and may persist until favorable conditions for germination occur.

Control Measures - A fungicide spray program is necessary to control the disease when powdery mildew susceptible varieties are grown. The major objectives of the spray program are to: 1) reduce the number of spores produced on newly-developing infected tissues in the spring (primary infection), 2) prevent infections of new shoots, buds, and leaves during the growing season (secondary infection), and 3) prevent fruit infections. Normally, spray programs will include a fungicide for powdery mildew around petal fall. On susceptible varieties and when conditions favor the development of the disease, sprays at 5-7 day intervals starting at tight cluster and continuing through petal fall are recommended. Cover sprays should also be applied until terminal buds have set and are no longer susceptible to infection. Since the fungus is infective during dry periods of high relative humidity when redistribution of fungicides by rain does not occur, spray coverage is extremely important. Refer to the Pest Management Guidelines for Commercial Tree-Fruit Production for proper fungicides and spray timings.

Pruning infected buds during the dormant season has not proven to be an effective cultural practice in eradicating overwintering inoculum and is generally not used in commercial operations.

Printable PDF

Powdery Mildew of Apple, CCE/NYS IPM Disease Identification Sheet #4, 1980.

Additional Information

Pest Management Guidelines for Commercial Tree-Fruit Production - Information on control practices and general tree-fruit production in New York.

Tree Fruit and Berry Pathology Pome Fruit Links!