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Verticillium Wilt

Ravageurs et maladies
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Verticillium wilt.
Photo: IRIIS Phytoprotection (Bernard Drouin - MAPAQ)

Onglets

Description

Summary

The fungi responsible for verticillium wilt live in the soil and may infect a wide variety of ornamental and food species. Leaves wilt suddenly and stems and branches then dry out. Seedlings, herbaceous plants and young trees are more vulnerable and can die off quickly. Verticillium wilt does not necessarily kill mature trees but can weaken them considerably. The infection is transmitted by seeds, cuttings, tubers and contaminated soil and water.

Signs and symptoms

  • The first signs resemble stress caused by drought.
  • Fungi invade plants through wounds and cracks in young, growing roots; once inside, they move through the entire vascular system; as they proliferate, they gradually block the conductive vessels and cause the leaves to dry out.
  • In woody plants (trees, shrubs), the leaves at the tips of some branches (more commonly the lower ones) can be observed to wilt suddenly; the leaves then turn yellow, dry out and drop prematurely.
  • At a later stage, the infection causes the young branches, then larger branches and eventually the whole tree to dry out and die.
  • If you peel away the bark of infected branches, you may see long greenish or brownish streaks in the sapwood; in cross-section, these streaks appear as solid or dotted stains in the growth rings.
  • The entire plant may die in just a few months, or it may take several years; infected trees show delayed growth, with bare-looking branches and smaller leaves.
  • Verticillium wilt does not necessarily kill mature trees, because they are able to isolate the infection. However, young seedlings, herbaceous plants and young trees are more vulnerable and can die off quickly once infected.

Latin name (genus)

Verticillium albo-atrum, Verticillium dahliae

Host plants

Various trees and shrubs, including ash, catalpa, cherry, chestnut, currant, elder, elm, gooseberry, honeysuckle, lilac, magnolia, maple, peach, plum, rose, Russian olive, smoketree, spirea, sumac, tulip tree, viburnum and weigela.

Various herbaceous plants, including aster, castor bean, chrysanthemum, coreopsis, dahlia, foxglove, impatiens, monkshood, peony, phlox, poppy, snapdragon and sunflower.

Certain food plants, including eggplant, melon, mint, potato, raspberry, strawberry, sweet pepper and tomato.

Development cycle

Description and life cycle

Verticillium fungi are microscopic members of the class Deuteromycetes, imperfect fungi, with no known sexual form.

They form thread-like tubular mycelia. Depending on the climate, they also produce conidia, asexual summer spores, and sclerotia, compact masses of mycelia that are capable of surviving in the soil for up to fifteen years.

They overwinter in the soil, in living tissues and infected plant litter, in the form of sclerotia or mycelia.

In spring, the mycelia resume growth and the sclerotia germinate and form other mycelia. The fungi invade plants through their roots and are carried by the sap through the vascular system. This is the primary infection period.

During the growing season, the mature mycelia produce fructifications that release spores inside the plant's vessels and on infected branches. The summer spores spread through the plant's vessels or invade plants through wounds on the trunk and branches. As they germinate, they cause secondary infections that persist as long as weather conditions remain favourable.

Prevention and control

Favourable conditions

Damage usually appears in mid-summer, when temperatures fluctuate between 20 and 28°C. Other factors conducive to these fungi include growing susceptible plants in contaminated soil and growing the same crops in the same spot for several years without rotating them.

Identification

Keep a close eye on plants during the growing season. If in doubt, cut off a wilted branch to see whether there are any dark stains in the vascular system or check under the bark for long brownish or greenish streaks.

Prevention

  • Use healthy material: the infection may be transmitted by seeds, cuttings, tubers and infected plant litter, as well as by contaminated soil and water.
  • Choose resistant species, including apple, beech, birch, crabapple, all conifers, ginkgo, hawthorn, hickory, locust, mountain ash, mulberry, pear, poplar, walnut, willow, etc.
  • Keep plants vigorous by fertilizing them properly and watering them during dry spells.
  • Try not to damage roots with picks, spades, hoes or rototillers and aerial parts with lawnmowers or snowblowers.
  • Rotate crops in the vegetable garden.
  • Keep the site free of weeds, which often shelter the fungi.
  • Plant resistant species in sites where the disease has already appeared, because the fungus is capable of surviving for up to fifteen years in the soil; do not move any contaminated soil or use it for seeding.

Physical control

  • Prune affected trees: remove all dead wood; cut infected young branches back to healthy wood.
  • Sterilize pruning tools regularly with a 70% isopropyl (rubbing) alcohol solution.
  • Remove and destroy all heavily infected trees and plants, including their roots.
  • Never compost infected plant litter.
  • Wait about fifteen years before planting a sensitive species in contaminated soil; plant resistant species instead.

Biological control

None available.

Chemical control

The Montréal Botanical Garden does not recommend the use of pesticides to control this disease.

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