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Anthracnose

Ravageurs et maladies
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Maple leaf affected by anthracnose disease.
Photo: Jardin botanique de Montréal (Lise Servant)

Onglets

Description

Summary

Anthracnose is the name given to a number of diseases caused by microscopic fungi. The infection frequently affects a variety of ornamental and food plants, but the damage usually remains minor. In serious cases, leaves dry out and drop prematurely, fruit rots and cankers form on young branches of trees and shrubs. The incidence of anthracnose fluctuates from year to year, depending on weather conditions.

Signs and symptoms

  • The damage differs slightly depending on the species and the affected parts.
  • The first signs appear on young leaves as discoloured circular spots or irregular brittle lesions.
  • Given favourable weather conditions, the brittle areas spread and may cover entire leaf blades. Leaves appear to have been dried by a hot, dry wind or burned by frost, depending on the season.
  • The fungi then invade the vascular system, causing petioles and leaves to drop prematurely.
  • At an advanced stage, tiny black specks appear on leaves, dark waterlogged lesions on fruit and cankers on young branches. This causes the fruit to rot and the terminal shoots to dry out. The disease sometimes kills annuals.
  • Anthracnose does not usually kill trees, but may weaken them considerably. If many leaves drop, a tree may come into leaf a second time, and this will drain its reserves. Several years of severe defoliation will gradually weaken a tree and make it susceptible to other pests and diseases.

Latin name (genus)

Apiognomonia, Colletotrichum, Discula, Gloeosporium, Glomerella, Gnomonia, Pseudopeziza, etc.

Host plants

Various ornamental plants, including ash, birch, butternut, catalpa, currant, dogwood, elm, hickory, linden, maple and oak.

A few types of vegetables and fruit, including cucumber, grape, melon, pepper, raspberry and tomato.

Development cycle

Description and life cycle

The fungi that cause anthracnose are microscopic members of the class Ascomycetes, spore-sac fungi, and Deuteromycetes, imperfect fungi.

They produce thread-like tubular mycelia that form fructifications producing sexual or asexual spores, depending on weather conditions and the fungus genus.

Ascomycetes 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.

Deuteromycetes have no known sexual form, but they do produce large numbers of asexual spores (conidia).

The fungi overwinter on dead leaves, infected branches, gaps in bud scales and small cankers in the form of mycelia, conidia or ascospores, depending on the species.

In spring, overwintering spores often cause an initial infection when they are dispersed by wind and rain.

During the growing season, cool, rainy weather promotes the development of mycelia and the production of large numbers of asexual spores (conidia). These are dispersed and quickly form fructifications, spreading the infection to new tissues. Spots appear on leaves about ten days later. The fungi continue to multiply as long as weather conditions remain favourable.

In late summer, some genera also produce ascospores, sexual spores that are able to survive drought and winter cold and serve to maintain the genetic diversity of the species.

Prevention and control

Favourable conditions

The incidence of anthracnose fluctuates from year to year, depending on weather conditions. Cool, rainy weather (15 to 27°C) is conducive to the disease, which can clear up quickly once the weather turns warm and drier.

Trees in shady sites are more susceptible than those growing in full sun, because their leaves stay damp longer.

Heavy feeding with high-nitrogen fertilizer promotes the growth of tender shoots that are susceptible to infection.

Spring frost may damage young shoots, making them vulnerable to attacks by the fungi. Trees whose leaves open early, such as ashes, tend to be more susceptible to anthracnose.

Identification

  • Regularly inspect the foliage of susceptible species, particularly young trees, which are less resistant than mature ones.
  • Watch for symptoms to appear mainly on new leaves, young branches and growing fruit, all of which are the most vulnerable tender parts.

Prevention

  • Maintain adequate spacing between plants.
  • Keep trees healthy by fertilizing them properly and watering them during dry spells. Avoid wetting the foliage so as not to promote the dispersal and germination of spores.
  • Prune overly dense trees and shrubs to allow air and light to reach the centre of the plant.
  • Rotate vegetable crops every three years.

Physical control

  • On young trees, pick off the first affected leaves by hand, when the foliage is dry.
  • Rake up and dispose of dead leaves to prevent the disease from spreading.
  • Remove and dispose of any branches with cankers during a dry spell or dormancy. Disinfect your tools between each cut with a 70% isopropyl (rubbing) alcohol solution.
  • Never compost infected plant litter.

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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