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Powdery Mildews

Ravageurs et maladies
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Powdery Mildews.
Photo: Ministère des forêts du Québec (Lina Breton)

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Description

Summary

The fungi responsible for powdery mildews are microscopic in size, with bright white mycelia. Infected plants are covered with powdery or downy spots, sometimes with tiny black specks. Powdery mildews rarely kill mature trees, but may severely damage certain ornamental and food plants. Damage usually appears in mid-summer, spreading quickly when days are warm and dry and nights are cool, as well as when the air is very humid.

Signs and symptoms

  • The first sign is a thin film or whitish spots, usually on the upper surface of leaves.
  • In the primary stage, or in a mild infection, the damage remains superficial, and it is easy to rub off the spots to reveal the still-healthy leaf surface.
  • At a later stage, the white spots become thicker and look downy or powdery (summer spores).
  • In serious or recurrent infections, the down may invade all the tender parts of the plant and cause them to dry out.
  • Buds that were infected the previous year fail to open or produce already infected organs: young leaves and flower buds dry out and drop or produce stunted, deformed flowers and fruit.
  • Late in the season, the spots become covered with small fruiting bodies that turn black as they mature (winter spores).
  • Powdery mildews rarely kill mature trees, but may severely damage certain ornamental and food plants. The infection interferes with photosynthesis and flower and fruit production.

Latin name (genus)

Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca, Uncinula, etc.

Host plants

Various trees and shrubs, including azalea, catalpa, chestnut, cinquefoil, crabapple, currant, hawthorn, honeysuckle, hydrangea, lilac, linden, maple, oak, peony, poplar, rose, viburnum, Virginia creeper and willow.

Various herbaceous plants, including aster, begonia, chrysanthemum, coreopsis, cornflower, larkspur, monarda and rudbeckia.

Certain food plants, including cucumber, grape, strawberry, sugar beet and tomato.

Development cycle

Description and life cycle

Powdery mildew fungi are microscopic members of the class Ascomycetes, spore-sac fungi.

They produce white, thread-like chains of mycelia, which grow mainly on the upper surface of leaves. These obligate parasites feed on the cells by means of tiny suckers. They produce two types of spores: conidia (asexual) and ascospores (sexual). The latter are usually arranged in groups of eight in an ascus, a small sac inside which meiosis occurs.

They overwinter on buds, branches and fallen leaves in the form of mycelia, conidia or ascospores, depending on the species and climate.

In spring, the overwintering mycelia and spores cause an initial infection and are dispersed by wind and rain.

During the growing season, the mycelia proliferate and produce large quantities of asexual spores (conidia). These spores are dispersed and may germinate quickly, even on dry plant tissues. Once anchored, the mycelia are able to grow under a wide range of climatic conditions.

In late summer, the fungi form small black fruiting bodies containing sexual spores (ascospores) that are able to survive drought and winter cold and serve to maintain the genetic diversity of the species.

Prevention and control

Favourable conditions

Damage appears from mid-summer (July) to late fall. An infection will spread more quickly when days are hot and dry and nights are cool, as well as when the air is very humid. These pathogens thrive in damp, poorly aerated and shady sites.

Drastic pruning and heavy feeding with high-nitrogen fertilizers promote the growth of vulnerable, tender tissues.

Identification

Regularly inspect the foliage of susceptible plants and those growing in less than ideal conditions. The disease is often recurrent: a plant that has already been infected will remain more susceptible.

Prevention

  • Choose resistant species and cultivars, including:

    Lilac: Dwarf Korean lilac 'Palibin', Korean lilac 'Miss Kim', Japanese lilac, Preston lilac 'James McFarlane'.

    Monarda: 'Gardenview Scarlet', 'Marshall's Delight', 'Petite Wonder' and 'Violet Queen'

    Phlox: 'Bright Eyes', 'David', 'Orange Perfection', 'Prime Minister' and 'Starfire'

    Rose: 'Alexander Mackenzie', 'Henry Hudson', 'Jean-Pierre Ferland' and 'Thérèse Bugnet'

    Zinnia: 'Old Mexico', 'Orange Star' and 'Star White'

  • Plant susceptible species in sunny, well-aerated spots; follow spacing recommendations.
  • Keep plants vigorous by fertilizing them properly and watering them during dry spells.
  • Prune overly dense plants to allow air to circulate and sunlight to penetrate to the centre of the plant.
  • Divide crowded perennials.
  • Avoid drastic pruning and heavy feeding with high-nitrogen fertilizers, both of which promote the growth of tender tissues that are more susceptible to infection.

Physical control

  • Remove and dispose of infected parts of annuals and perennials; disinfect pruning tools regularly with a 70% isopropyl (rubbing) alcohol solution.
  • In hot, dry weather: spray the foliage with a hose to slow the growth of the fungus; avoid wetting the foliage late in the day, so as not to encourage other fungal diseases.
  • Never compost infected plant material.

Biological control

None available.

Chemical control

In case of a serious infection in the previous year or years, as a preventive measure, apply a low-impact pesticide with sulphur, calcium sulphide or calcium polysulphide (lime sulphur), citric acid and lactic acid or QST 713 strain of Bacillus subtilis as the active ingredient. Read the product label carefully and follow the manufacturer's recommendations.

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