NC State Extension Publications

Introduction

Powdery mildew is a disease of apple that affects apples leaves, young shoots, flowers, and fruit. Apple powdery mildew is present in all apple growing regions of the United States, but tends to be most severe in the Pacific Northwest, where netlike russeting on fruit can severely impact the value of the crop.

Pathogen

Apple powdery mildew is caused by the fungal pathogen Podosphaera leucotricha.

Symptoms and Signs

Primary Powdery Mildew: Primary mildew infection occurs as green tissue emerges from infected terminal buds in the spring. Primary infections appear as silver to white colored stunted or malformed shoots covered in white spores of the powery mildew fungus. As the season progresses, shoot dieback, severe leaf curling, or dried/cracked lesions may be apparent on shoots infected with primary mildew.

Secondary Powdery Mildew Infection: Secondary powery mildew appears as distinct white lesions on the front (or occasionally backside) of young, developing leaves. The white, felt-like appearance of the lesion is due to conidia (spores) of the powdery mildew on the surface of the leaf. As the season progresses, the mildew may appear "burnt out" or red in color, particularly along back edges of leaves.

Fruit Infection: When powdery mildew infection is severe, fruit may become infected. Net- or webbed-like russeting on the fruit surface is the most obvious symptom of fruit infection.

Emerging mildew

Primary powdery mildew on emerging shoot of 'Rome Beauty'

Sara Villani, NCSU

silver shoot

'Rome Beauty' shoot infected with spores of the the powdery mildew fungus

Sara Villani, NCSU

Leaf mildew curl

Curling along the longitudinal axis of a leaf infected by the powdery mildew fungus

Sara Villani, NCSU

secondary mildew

Sporulating powdery mildew (secondary) lesion on 'Rome Beauty' leaf

Sara Villani, NCSU

mildew on fruit

Net-like russeting on fruit caused by the powdery mildew fungus

Keith Yoder, Virginia Tech

Disease Cycle

The powdery mildew pathogen, Podosphaera leucotricha, overwinters as mycelium in buds of terminal shoots that were infected during the previous season. As early as tight cluster, conidia are produdced from the overwintering mycelium and infect emerging shoot and leaf tissue (primary infection). Conidia from shoots with primary powdery mildew infection are dispersed by wind and can infect flowers/fruit and young leaves (secondary infection). Although older leaves are resistant to infection (ontogenic resistance), a resurgence in terminal growth during the mid to late summer can result in renewed mildew activity.

Unlike many other fungal pathogens, spores of P. leucotricha do not germinate in free water. Rather, germination and subsequent infection is favored during conditions of high relative humidity and temperatures between 66-72oF.

Management

Effective management of apple powdery mildew should involve consideration of host susceptibility, tolerance for fruit infection (usually fresh market vs. processing), and fungicide efficacy.

Host Resistance

There is a wide range in suscepticibility of apple cultivars to powdery mildew. Cultvars with greater susceptibility to the disease include Cortland, Rome Beauty, Granny Smith, Honeycrisp, Ginger Gold, Jonagold, and GoldRush. Red Delicious, Golden Delicious, Stayman, Fuji, and Gala tend to be more resistant (i.e. less suscptible) to powdery mildew.

Fungicides

Seconday powdery mildew infections can be managed by foliar fungicide applications. In North Carolina, applications targeting powdery mildew should begin at tight cluster and continue until susceptible leaf tissue is not longer available (end of terminal shoot growth). Depending on the weather conditions, cultivar susceptibility, and physical mode of action of the applied fungicdie, applications targeting powdery mildew should be maintained on a 7 to 14 day interval in the Southeastern US, using a tighter interval during periods of rapid shoot elongation/growth. In general, highly susceptible leaves that emerged after a fungicide application will not be protected. Several S.I. fungicides (FRAC 3) and strobilurin fungicides (FRAC 11) have demonstrated excellent control against powdery mildew. Conversely, mancozeb and captan do not provide adequate control against the disease.

Example Commercial Fungicides for Apple Powdery Mildew
Active Ingredient Example Formulated Product Pre-harvest Interval (days) FRAC Code
myclobutanil Rally 40WSP 14 3
triflumizole Procure 480SC 14 3
flutriafol Topguard Fungicide Specialty Crops 14 3
trifloxystrobin Flint Extra 14 11
kresoxim-methyl Sovran 30 11
pyraclostrobin + fluxapyroxad Merivon 0 11 + 7
trifloxystrobin + fluopyram Luna Sensation 14 11 + 7
sulfur Microthiol Disperss 1 M2

Author:

Extension Specialist (Apple and Ornamental Plant Pathology)
Entomology & Plant Pathology

Publication date: July 3, 2018

North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation.

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.

North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation.