Ministry of Agriculture

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.

Powdery mildew on apple leaves
powdery mildew russet on apple 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 per 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 mildew 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

  1. Avoid overcrowding of trees and branches.
  2. 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.

  1. Fruit - To prevent fruit infection and subsequent russeting, apply a pink spray of Fontelis (penthiopyrad), Luna Tranquility (fluopyram +pyrimethanil), Nova (myclobutanil), Sovran (kresoxim-methyl), Flint (trifloxystrobin), Pristine (boscalid + pyraclostrobin) or Kumulus (sulphur). Senator (thiophanate-methyl) and lime sulphur are also effective for the control of powdery mildew. Inspire (difenoconazole), Serenade Max (Bacillus subtilis) and PureSpray Green Spray Oil 13E are registered for suppression of powdery mildew.

  2. 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. Fungicides registered for apple powdery mildew must be applied at 10-day intervals or as labels direct, 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 chemical groups 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. Limit sprays of group 3 (Nova, Inspire) and group 11 fungicides (Sovran, Pristine, Flint) to 2 per season for each group.

    WARNING - There is apple scab resistance to Senator in most orchards of the Central and North Okanagan. It is suspected that resistance to Nova is starting to develop in apple scab and powdery mildew. Limit sprays of Nova to once per season and discontinue use if resistance is suspected.
    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.

  3. 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. 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 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
  1. Spray at the pink stage with Kumulus (sulphur), Fontelis (penthiopyrad), Flint (trifloxystrobin), Sovran (kresoxim-methyl), Pristine (boscalid + pyraclostrobin), Inspire (difenoconazole), Nova (myclobutanil), 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.

Fontelis (penthiopyrad), Pristine (pyraclostrobin + boscalid), Quintec (quinoxyfen), Purespray Green Spray Oil 13E (mineral oil) and Milstop (potassium bicarbonate) will provide suppression of powdery mildew. Do not apply sulphur to apricots as it will cause defoliation. If Topas or Indar is 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 Powdery mildew on nectarine fruit

Cultural Control

  1. Provide good air circulation through trees.
  2. Cling peaches, nectarines and seedling peaches are especially susceptible and can serve as a source of infection.

Chemical Control

  1. Apply Kumulus, Quintec, Fontelis, Pristine or Nova at husk fall and repeat after 10-14 days.
  2. Milstop and PureSpray Green Spray Oil 13E will also provide suppression of powdery mildew.

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.

Powdery mildew on sweet cherry leaves, causing yellow mottling and distortion

Powdery mildew damage on sweet cherry fruit, cultivar 'Sweetheart'

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 break, 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

  1. Prune for good air circulation. Avoid overly dense plantings.
  2. Remove infected water sprouts before full leaf.
  3. Keep grass mowed short to reduce humidity in the orchard.

Chemical Control

  1. Fungicides registered for control of powdery mildew on sweet cherry include Quintec, Fontelis, Nova, Pristine, Flint, Cabrio and some formulations of wettable sulphur. Caution: There is evidence that resistance to Nova (group 3 fungicide) is developing in cherry powdery mildew. Limit the use of group 3 fungicides to 2 per season. See resistance management chart.
  2. 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.
  3. 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.
  4. Begin a control program early in blocks with a history of severe mildew problems. 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.
  5. Research in Washington State has shown that lime sulphur applied to sweet cherry trees in the fall may be useful to reduce the overwintering populations 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. 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

  1. Avoid dense planting. Prune to increase air movement.

Chemical Control

  1. Foliage as for sweet cherries
  2. Spray fruit with wettable sulphur, Quintec, Fontelis, Cabrio or Nova at husk fall and repeat in two weeks. Do not apply sulphur later than the end of June.
  3. Pristine and PureSpray Green Spray Oil 13E will also provide suppression of powdery mildew.

Updated January 2013


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