Powdery Mildew - Tree Fruit
There are several species of powdery mildew that attack tree fruit. Apple
and pear are affected by Podosphaera leucotricha, while stone fruits are
affected by Podosphaera clandestina and Sphaerotheca pannosa.
Mildew can infect both the leaves and the fruit, and may render fruit
unmarketable.
Apple Powdery Mildew
(Podosphaera leucotricha)
Symptoms:
The fungus produces a white powdery growth on new terminal growth and
developing fruit. Later in the spring, as the fungus dries and is sloughed off,
a network of russet appears on the infected surface of the fruit.
![](/web/20061229062031im_/http://www.agf.gov.bc.ca/cropprot/tfipm/images/applemildewlf.jpg) |
Powdery mildew on apple leaves |
![powdery mildew russet on apple](/web/20061229062031im_/http://www.agf.gov.bc.ca/cropprot/tfipm/images/applemildewfr.jpg) |
Powdery mildew-induced russet on apple |
Life Cycle:
On apples the fungus overwinters in terminal buds and is most severe in a
season following a series of mild winters. Severe winter temperatures can
reduce mildew pressure by killing infected buds, which are more susceptible to
winter injury than healthy buds. As infected buds open in the spring, powdery
mildew spores (conidia) are released to initiate primary infections on
blossoms, young leaves, and fruit. Infections causing fruit russet can occur
from about 3 weeks before bloom to 3 weeks after bloom. Additional conidia are
produced on infected leaves and fruit which cause secondary infections. There
are multiple generations pear year, with trees susceptible as long as they are
actively growing. Powdery mildew is favoured by moderate temperatures (10-25
°C) and high relative humidity.
Dormant Monitoring:
The number of mildew sprays required on bearing trees prior to blossom can be
predicted by estimating the percentage of one-year old shoots showing white
fungus on the bark surface during the dormant season. If more than 15% of
one-year-old shoots have mildew, two pre-bloom sprays are required. Spray once
prior to bloom for levels between 5 and 15%. No pre-bloom spray is needed if
the mildew level is below 5%. Note that eliminating all pre-bloom sprays may
increase the risk of damage on highly susceptible varieties.
Cultural Control
- Avoid overcrowding of trees and branches.
- Prune out twigs with white fungus growth on the surface.
Chemical Control
Early spring applications of fungicide (beginning no later than tight
cluster) are necessary to prevent secondary spread of powdery mildew in
susceptible apple varieties. Neglecting control early in the year will result
in poor control during the season.
- Fruit - To prevent fruit infection and subsequent russeting, apply
a pink spray of Nova (myclobutanil), Nustar (flusilazole), Sovran
(kresoxim-methyl), Flint (trifloxystrobin) or Kumulus (sulphur). Nova, Nustar,
Sovran or Flint should be used in preference to sulphur if mildew infection
was severe in the previous year. Senator (thiophanate-methyl) and lime sulphur
are also effective for the control of powdery mildew.
- Foliage - A single pink spray will not protect the foliage of
susceptible varieties such as Jonagold, McIntosh, Granny Smith, Gala, Braeburn
and Gingergold. Two pre-bloom sprays are needed for good control of powdery
mildew where disease levels are high. Sprays of Kumulus, Nova,
Nustar, Sovran, Flint or Senator must be applied at 10-day intervals from
tight cluster stage until terminal growth ceases, to keep foliage free of
mildew and to reduce carry-over to the next season. Control of foliage powdery
mildew is particularly important on young trees needing growth stimulation. On
more resistant varieties such as Red Delicious, damage from moderate amounts
of mildew on terminal growth is relatively minor and does not warrant an
extensive spray program.
Senator may harm predatory mites. If several sprays
are applied, follow mite counts closely for indications of spray
interference with predatory mite population. Lime-sulphur also disrupts
integrated mite control.
Resistance management – Rotate between different chemistries to
help prevent the development of fungicide resistance. Use recommended rates
for powdery mildew control. Reduced rates will result in poor control and an
increased risk for resistance development. WARNING - There is apple scab resistance to Senator in most orchards of the central and north Okanagan.
SPRAY INJURY WARNING - Sovran may severely injure certain varieties
of cherries. Do not allow drift onto cherries. Be sure to clean all residues of Sovran out of your spray tank
before spraying cherries.
- Non-bearing trees - Funginex (triforine) may be used for powdery
mildew control on apple nursery stock and non-bearing trees. Apply 2.5 L
Funginex per hectare (75 mL/100 L) at tight cluster, pink and petal fall
stages. Warning: Funginex is not registered for use on bearing trees.
Pear Powdery Mildew
Causal agent: Podosphaera leucotricha
White powdery fungus growth on terminal shoots of pears is less evident than
in apples. However pears are susceptible to fruit infection, which appears as
black marks and russeting on young fruit. Powdery mildew does not overwinter in
pear buds, as it does on apple. Primary inoculum affecting pears originates
from nearby apple orchards.
![Powdery mildew damage on Anjou pear](/web/20061229062031im_/http://www.agf.gov.bc.ca/cropprot/tfipm/images/pearmildew.jpg) |
Powdery mildew damage on pear
Photo courtesy P. Sholberg, Agriculture & Agri-Food Canada |
Cultural Control
Avoid planting Anjou pears near susceptible apple cultivars. Bartlett and
Flemish Beauty are more resistant to powdery mildew.
Chemical Control
Fruit
- Spray at the pink stage with Kumulus (sulphur), Flint (trifloxystrobin),
Sovran (kresoxim-methyl), lime-sulphur, or Senator (thiophanate-methyl).
SPRAY INJURY WARNING - Sovran may severely injure certain varieties of
cherries. Do not allow drift onto cherries. Be sure to clean all residues of Sovran out of your spray tank
before spraying cherries.
Apricot Powdery Mildew
Powdery mildew is not normally a problem on apricots, but some fruit russeting
has been observed in recent years. Primary inoculum is thought to originate
primarily from nearby peaches and infected roses.
There are no fungicides specifically registered for control of powdery
mildew on apricot, now that Benlate is no longer available. Do not apply
sulphur to apricots as it will cause defoliation. If Topas or Indar are used
for brown rot control at, or shortly after husk-fall, these products will also
help to control powdery mildew.
Peach and Nectarine Powdery Mildew
Causal agent: Sphaerotheca pannosa
Powdery mildew appears in late spring or early summer as white mildew spots
on the fruit and foliage. Later the spots on the fruit turn a tan colour. When
severe, it may crack the fruit.
![powdery mildew on nectarine](/web/20061229062031im_/http://www.agf.gov.bc.ca/cropprot/tfipm/images/mildewnectarine.jpg) |
Powdery mildew on nectarine fruit |
Cultural Control
- Provide good air circulation through trees.
- Cling peaches, nectarines and seedling peaches are especially susceptible
and can serve as a source of infection.
Chemical Control
- Apply Kumulus or Nova at husk fall and repeat in two weeks for
sulphur or two to four weeks for Nova.
Sweet Cherry Powdery Mildew
Causal agent: Podosphaera clandestina
Symptoms
On leaves, powdery mildew appears as patches of white, powdery or felt-like
fungal growth. Severely affected leaves and shoots are often puckered or
distorted. Cleistothecia (fungal fruiting bodies) appear in older mildew
colonies as small, black specks. Leaf infections are usually first observed
about 4-6 weeks after bud break, and become increasingly obvious as the season
progresses. Young, expanding leaves are more susceptible than mature leaves.
Fruit infection appears as a white powdery bloom as the fruit ripens, or as
roughly circular, slightly depressed areas on the fruit surface with or without
any obvious growth of powdery mildew spores.
![](/web/20061229062031im_/http://www.agf.gov.bc.ca/cropprot/tfipm/images/cherrypm1.jpg) |
Powdery mildew on sweet cherry leaves, causing yellow mottling and
distortion |
![](/web/20061229062031im_/http://www.agf.gov.bc.ca/cropprot/tfipm/images/cherrypm2.jpg) |
Powdery mildew damage on sweet cherry fruit, cultivar 'Sweetheart' |
![](/web/20061229062031im_/http://www.agf.gov.bc.ca/cropprot/tfipm/images/cherrypm3.jpg) |
Powdery mildew growth on the underside of a sweet cherry leaf. Note
black cleistothecia |
Life Cycle
Powdery mildew overwinters as cleistothecia on leaf litter on the orchard
floor, and trapped in tree crotches or bark crevices. Ascospores are released
from the cleistothecia in response to rain or irrigation and provide the
“primary” or first inoculum that infects cherry leaves or shoots in the spring.
In Washington, ascospore release was found to begin one month before bud burst,
and continued until after bloom. There is no evidence that cherry mildew
survives as mycelium in dormant buds, as occurs with apple mildew. Once mildew
colonies have become established, a second type of spore (conidia) is produced.
There are multiple generations of conidia produced all summer, potentially
resulting in a rapid build-up of inoculum and disease levels.
Fruit infection is caused by conidia that are produced on the leaves.
Immature fruit is much more susceptible than mature fruit, and susceptibility
decreases as sugar content increases.
Powdery mildew is favoured by moderate to warm and humid conditions, with
optimal temperatures in the range of 15-25°C. Conidia are not produced below
10°C or above 30°C. Mildew severity is greater in years with frequent showers
in late spring and early summer.
Cultural Control
- Prune for good air circulation. Avoid overly dense plantings.
- Remove
infected water sprouts before full leaf.
- Keep grass mowed short to reduce
humidity in the orchard.
Chemical Control
- Fungicides registered for control of powdery mildew on sweet cherry
include Nova, Cabrio and some formulations of wettable sulphur.
- Critical spray timings for fruit
protection under normal conditions of light to moderate mildew pressure
include fungicide applications at husk fall and about 7-10 days later to
protect the susceptible green fruit.
- Foliage control may not be warranted
in all situations. High density blocks and late maturing or susceptible varieties are more
likely to require fungicide sprays for foliage protection, which in turn will
help to prevent fruit infection.
- Growers who have experienced severe mildew problems in the past should
begin a control program very early in the season in problem blocks. The goal
should be to protect emerging green tissue from airborne ascospores. Begin a
mildew spray program no later than bloom to petal fall, and continue at 7-14
day intervals until the pit hardening stage. Spray intervals can be adjusted
depending on weather conditions and the products selected. More sprays will be
needed in wet years than dry years. Consider protecting highly susceptible
varieties such as Sweetheart and Staccato up to harvest. Caution: sulphur may cause injury during
hot weather.
- Research in Washington has shown that lime sulphur applied to
sweet cherry trees in the fall may be useful to reduce the overwintering
population of the fungus. This will not eliminate the need for fungicides
during the growing season, but reducing initial inoculum will help to delay
the build-up of disease levels and make fungicidal control more effective.
Fall application of lime sulphur was found to be more effective than spring
application in Washington State. In Canada, lime sulphur is registered on
cherry as a “general clean-up” dormant spray, as well as for San Jose scale,
European scale and mites.
Sour Cherry Powdery Mildew
Causal agent: Podosphaera clandestina
Foliage of sour cherries is severely affected by powdery mildew most years.
Visible fruit mildew may be more likely on late-harvest fruit.
Cultural Control
- Avoid dense planting. Prune to increase air movement.
Chemical Control
- Foliage as for sweet cherries
- Spray fruit with wettable sulphur, Cabrio or Nova at husk fall and repeat
in two weeks. Do not apply sulphur later than the end of June.
April 2006