Powdery Mildew Management Review for 2025

Anna Wallis, Kerik Cox, Mike Basedow

Incidence of apple powdery mildew has been increasing over the past few years across NY. Most likely, this is related to environmental conditions more conducive to disease development, especially warmer, more humid conditions early in the season, with greater intervals between rain events. This article includes a brief review of the pathogen, disease, and management options.

The Cornell fact sheet on the pathogen is now available on the NYSIPM Fact Sheet Website: https://cals.cornell.edu/integrated-pest-management/outreach-education/fact-sheets/apple-powdery-mildew-podosphaera-leucotricha

Host and Biology

Powdery mildew (PM) of apple is caused by the fungus Podosphaeria leucotrica. There are many species of PM. Each is host-specific (meaning powdery mildew of grapes or squash are caused by different fungal species that will NOT affect apples and vice versa).

The pathogen overwinters on the plant in protected structures (chasmothecia) within infected buds from last season. Ascospores are produced in the spring which cause primary infections on developing shoots adjacent to infected buds under the right conditions. Another type of spore (conidia) cause secondary infections on new leaf tissue. These spores can also land on developing fruitlets, leading to a failed infection producing russeting on the surface of the fruit. There are many generations of conidia produced throughout the summer.

Factors promoting PM infections

Mild winter: may allow greater survival of pathogen through winter; at 0-5°F some suppression of PM occurs, at -11°F 95% of infected buds may be killed; dependent on duration and timing of cold periods.
Pre-bloom weather (warm and humid, little rain): extended pre-bloom spray intervals for apple scab, more conducive conditions for PM pathogen growth and infection
Post-bloom weather (warm and humid, little rain): rapid secondary spread of pathogen
Vigorous trees: may “outgrow” fungicide coverage more quickly, new growth is most susceptible to infection; pay attention to trees with a light crop, higher fertility

Symptoms to look for

Twigs: buds slow to develop, or do not leaf out; covered in powdery white mycelium
New shoots & flower clusters: distorted or twisted leaves and flowers, covered in powdery white mycelium, tending to curl upward (rather than downward like leafhopper damage); newest shoots are the most susceptible to infection
Leaf tissue later in season: later in season, infections may dry out and turn brown; older leaves are less susceptible, so after terminal bud set risk of infection is low
Fruit – russeting, netted-like patterning on surface of fruit, caused by failed infections; may be easily confused with similar symptoms caused by chemical frost damage, but these abiotic causes will have more regular pattern/location (e.g. chemical damage may be on the lower side of the fruit where chemical/spray might collect and on all fruit in orchard; whereas PM symptoms may be patchy throughout the canopy)

Environmental Conditions

Powdery mildew could be considered the ‘fair weather pathogen’ of apple. Disease development occurs when conditions are very humid and warm, but when there is little rain. Spores cannot survive in freestanding water, so rain inhibits growth and prevents infection. This is very different than many other pathogens which rely on wet conditions and ample rainfall. Often ‘powdery mildew seasons’ are years where scab is of low risk.

Specific conditions for infections. Conidial growth occurs from 50-77ºF (10-25ºC), with optimal conditions 66-72ºF (10-22ºC). Slow or minimal growth occurs below 50ºF or above 86ºF. Infections can occur rapidly, with new symptoms visible as soon as 48 hours after infection and new spores being produced in as little as 5 days.

an illustrated thermometer with print in blue, black, and red.

Management considerations

Cultural Management. There are considerable differences in susceptibility between apple varieties. Where possible, choose PM-resistant varieties for new plantings. Concentrate scouting efforts and focus management efforts on more susceptible cultivars. Lists of cultivar susceptibility are available in the Database of Apple Diseases maintained by Dr. Awais Khan’s lab at Cornell AgriTech and in a Fruit Quarterly article authored by Dr. Dave Rosenberger.

Mechanical Control. Prune to create an open canopy. Improving airflow will reduce relative humidity. Greater light penetration will also decrease disease incidence by exposing the pathogen to more UV light. Sanitation during the dormant season will remove dead, distorted, stunted terminal shoots. But we don’t recommend any extra than standard pruning, it is hard or impossible to detect during the dormant season and not an effective use of time.

Chemical Control. The focus of chemical management for apple powdery mildew should be on secondary infections, beginning at bloom to second cover. Keep new foliage covered through terminal bud set when trees stop producing new leaves (typically in July).

Effective Modes of Action include the single site fungicides: DMI (group 3), SDHI (group 7), and QoI (group 11), with QoI’s being superior in recent efficacy trials. Make sure to rotate to prevent development of resistance to these critical fungicides. Sulfur is very effective, but there is a risk of phytotoxicity at temperatures over 85ºF. It is important to note that common multi-site fungicides in groups M3, M4, M7 are NOT EFFECTIVE (i.e. Captan, Mancozeb, and Syllit). A new product labeled for PM control is Axios (group 52), which is a novel mode of action. There is little public data available on efficacy to date.

Efficacy of fungicides for apple powdery mildew, by FRAC group*

$A table of fungicide products listing name, frac code, and efficacy against common diseases, and PHI's.$

*Compiled from 2024 Cornell Tree Fruit Guidelines, Table 6.1.1 and pesticide labels.

References:

Rosenberger, D.. 2003. Susceptibility of New Apple Cultivars to Common Apple Diseases. Fruit Quarterly, 11(2): 17-22. Online at https://bpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/d/3767/files/2013/11/Susceptibility-of-New-Apple-Cultivars-to-Common-Apple-Diseases-24quvu4.pdf

Rosenberger, D. 2012. Controlling apple mildew. Scaffolds 21(4), 2 April 2012. On-line at http://www.scaffolds.entomology.cornell.edu/2012/SCAFFOLDS%204-2-12.pdf.

Strickland D.A., Hodge, K., and Cox, K.D. 2021. An Examination of Apple Powdery Mildew and the Biology of Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon from Past to Present. Plant Health Prog. https://doi.org/10.1094/PHP-03-21-0064-RV

Strickland, D.A., Villani, S.M., and Cox, K.D. 2022. Optimizing use of DMI fungicides for management of apple powdery mildew caused by Podosphaera leucotricha in New York State. Plant Disease 106(4): 1226-1237.

Strickland, D., Ayer, K., Olmstead, D., and Cox, K. 2023. Refining Management of Apple Powdery Mildew in New York State with Weather-Based Fungicide Application Timing Programs. Plant Disease 107:1425-1432. Online at https://apsjournals.apsnet.org/doi/full/10.1094/PDIS-08-22-1825-RE