I. Introduction: Powdery
mildew can be a persistent disease of susceptible apple cultivars throughout the
mid-Atlantic region. It is the only fungal apple disease that is capable of infecting
without wetting from rain or dew. Mildew severity and the need for control measures are
related to cultivar susceptibility and intended fruit market.
II. Symptoms: Powdery mildew causes whitish lesions on curled or
longitudinally folded leaves (photo 2-1), stunted whitish-gray twig growth evident on
dormant shoots (photo 2-2), and fruit russeting (photo 2-3). Economic damage occurs in the
form of aborted blossoms (photo 2-4), reduced fruit finish quality, reduced vigor, poor
return bloom and yield of bearing trees, and stunted growth and poor form of nonbearing
III. Disease Cycle: The
mildew fungus overwinters mainly as mycelium in dormant blossom and shoot buds produced
and infected the previous growing season. Conidia are produced and released from the
unfolding leaves as they emerge from infected buds at about tight cluster stage. Conidia
germinate in the high relative humidity usually available on the leaf surface at 50 to 77
F (10-25 C) with an optimum of 66 to 72 F (19-22 C). Germination does not occur in free
moisture. Early-season mildew development is affected more by temperature than by relative
humidity. Abundant sporulation from overwintering shoots and secondary lesions on young
foliage leads to a rapid buildup of inoculum. Secondary infection cycles may continue
until susceptible tissue is no longer available. Since leaves are most susceptible soon
after emergence, infection of new leaves may occur as long as shoot growth continues.
Fruit infection occurs from pink to bloom. Overwintering buds are infected soon after bud
initiation. Heavily infected shoots and buds are low in vigor and lack winter hardiness,
resulting in a reduction of primary inoculum at temperatures below -11 F (-24 C). This
phenomenon has been more commonly observed in other areas with lower winter temperatures
than those commonly experienced in the mid-Atlantic region.
IV. Monitoring: While pruning, note whitened terminal
shoots (photo 2-2) as an indicator of potential inoculum pressure. This may not be the
actual inoculum situation, however, because some mildewed buds do not survive the winter
while others which appeared to be healthy emerge with primary infection. Inspect ten
shoots per tree and record the number that are infected.
From prebloom through fruit set, on larger bearing trees,
count the number of emerging primary infections on shoots (photo 2-1) and blossom clusters
(photo 2-4) on ten trees per five acres (2 ha). A total of one to ten and greater than ten
primary infections represent moderate and high levels of risk, respectively, for fruit,
leaf, and shoot infection. Because of potential chronic yield effects, action thresholds
for fresh and processing market blocks are identical. On smaller nonbearing trees, record
the number of primary infections on 25 trees per five acres (2 ha). Moderate and high risk
levels for leaf and shoot infection on nonbearing trees are the same as those cited above.
During mid-season (3 to 9 weeks post bloom), monitor
secondary infection by determining the percent of leaves with infection on ten terminal
shoots on each sample tree. Twenty percent leaf infection indicates a weakness in the
control program, a high level of risk for fruit and bud infection, and a recurrent problem
with chronic yield reductions in subsequent years. If there is late season growth,
determine the percent of leaves with infection on ten terminal shoots on each sample tree.
Twenty percent leaf infection indicates a weakness in the control program, a high level of
risk for fruit and bud infection, and a recurrent problem with chronic yield reductions in
V. Management: Where mildew-susceptible cultivars are grown, include a
mildewcide in the scab program to control both diseases. The DMI fungicides are
effective against both diseases. Begin sprays at tight cluster and continue until
terminal growth stops. Early season sprays (tight cluster to petal fall) are
essential if mildew is to be managed successfully. The interval between sprays is
generally 7 days during the stages of rapid leaf development before petal fall and 12 - 14
days during the post bloom period. During the early season, fungicide applications
at lower rates on a 7-day schedule are more effective for controlling mildew than higher
rates applied on a 10-day schedule (Table of fungicide
Chemical control -
Chemical control - home
Option 1 -
Virginia Home Orchard
Management Guide web site
Option 2 - Virginia Home Orchard
Management Guide pdf file (Acrobat
Ellis, M.A. 1994.
Apple Powdery Mildew. The Ohio State University Extension Factsheet HYG-3001-94.
Grove, G. G. 1997. Apple Powdery Mildew. Washington State
University Tree Fruit Research and Extension Center.
Sutton, T.B., and Sorensen, K.A. 1997. Disease and Insect Management in the Home Orchard.
North Carolina State University Cooperative Extension.
and Gubler, W.D. 1997. Apple Powdery Mildew. UC Pest Management Guidelines,
University of California Statewide Integrated Pest Management Project.
Text prepared by K.S. Yoder,
Virginia Tech Agricultural Research and Extension Center, Winchester, Va., K.D. Hickey,
Pennsylvania State University, Fruit Research and Extension Center, Biglerville, Pa., and A. R. Biggs,
West Virginia University, Kearneysville Tree Fruit Research and Education Center,