Stone Fruit Powdery Mildew

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.

Cherry Powdery Mildew

Symptoms

Sweet Cherry:  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. Chasmothecia (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 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.

Sour Cherry: foliage is severely affected by powdery mildew most years. Visible fruit mildew may be more likely on late-harvest fruit.

Powdery mildew on sweet cherry leaves
Powdery mildew on sweet cherry leaves, causing yellow mottling and distortion
Powdery mildew cherry fruit
Powdery mildew damage to sweet cherry fruit, cultivar 'Sweetheart'
Powdery mildew on the underside of a sweet cherry leaf
Powdery mildew growth on the underside of a sweet cherry leaf.  Note black chasmothecia (overwintering fruiting bodies)

Life Cycle

Powdery mildew overwinters as chasmothecia on leaf litter on the orchard floor, and trapped in tree crotches or bark crevices. Ascospores are released from the chasmothecia 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. 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 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 Vivando (metrafenone), Quintec (quinoxyfen), Fontelis (penthiopyrad), Nova (myclobutanil), Pristine (boscalid + pyraclostrobin), Flint (trifloxystrobin), Cabrio (pyraclostrobin) and some formulations of wettable sulphur. Purespray Green Spray Oil 13E is also registered for suppression of powdery mildew.  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.
  • 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.
  • 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.
  • 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.

Peach and Nectarine Powdery Mildew

Symptoms

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 damage to nectarine
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, Microthiol, Quintec, Fontelis, Pristine, Nova or Vivando at husk fall and repeat after 10-14 days.
  • Milstop and PureSpray Green Spray Oil 13E will also provide suppression of powdery mildew.

Apricot Powdery Mildew

Powdery mildew is not normally a problem on apricots, but may occassionally cause fruit russeting. Primary inoculum is thought to originate mainly from nearby peaches and infected roses.

The fungicides Fontelis, Pristine, Quintec, Purespray Green Spray Oil 13E and Milstop will provide control or suppression of powdery mildew. Do not apply sulphur to apricots as it will cause defoliation.

Updated November, 2016