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Black Knot

Ravageurs et maladies
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Black Knot.
Photo: Jardin botanique de Montréal (Pascale Maynard)

Onglets

Description

Summary

This fungal disease, which is very common in humid regions, attacks mainly cultivated and wild plum and cherry trees. It is caused by a microscopic fungus belonging to the genus Dibotryon. Infected twigs and branches are marked with dark black, rough, cylindrical galls (knots). Once a twig or branch has been completely encircled by a gall, the parts above the knot die back. Planting less susceptible cultivars and regularly pruning infected branches are ways of controlling this parasitic fungus.

Signs and symptoms

  • The initial infection starts in early spring, but the first sign of any symptoms does not appear until several months later: in late summer on susceptible cultivars or the following spring on more resistant cultivars.
  • The first thing you will see is a slight swelling at the base of young shoots or on branches.
  • During the growing season, the fungus develops and expands within the vascular system. The swollen branches crack and downy greenish growths fill the cavities. This velvety appearance indicates that the fungus is producing and releasing spores.
  • In damp weather, the knots may exude large quantities of gum.
  • In late summer, the knots turn brown and corky looking before turning hard and coal black. They may grow to 20 cm in length and 4 cm in diameter. They gradually encircle branches and twigs of all sizes. Once they have been completely girdled, the parts above the knots die back.
  • A serious infection can kill a large number of branches and twigs; a small branch may be killed in the first year, while larger branches will die back over several years. Knots may also form on the trunk, considerably decreasing the value of the wood.
  • The disease usually moves inward toward the trunk, rather than toward the tips of the branches. Severely infected trees stop growing and producing fruit and may die. Old knots, often invaded by boring insects, provide entry points for other fungal diseases.

Latin name (genus)

Dibotryon morbosum (syn. Apiosporina morbosa)

Host plants

A few fruit and ornamental trees, including plum and cherry, and more rarely apricot and peach.

Some plum cultivars are particularly susceptible, including 'Bluebell', 'Bluefre', 'Bradshaw', 'California blue', 'Damson', 'Earliblue', 'Early Italian', 'Fellenburg', 'Methley', 'Myrobalan B', 'Ozark Premier', 'Stanley', 'Valor', 'Vanier', 'Veeblue', 'Vision' and 'Voyageur'.

Development cycle

Description and life cycle

Fungi in the genus Dibotryon are microscopic members of the class Ascomycetes, spore-sac fungi.

They produce thread-like mycelia and two types of spores: conidia (asexual) and ascospores (sexual), grouped together in asci, small sacs inside which meiosis occurs.

The life cycle may be completed in one year or extend over two years.

The fungi overwinter in infected branches and old knots, in the form of mycelia or immature ascospores.

In early spring (March to June), the mature spores are released into the air and dispersed by wind and rain. Given favourable weather conditions, they germinate quickly and the mycelia proliferate in the tissues. This primary infection period lasts about two months and causes new infections on other branches or other trees. The first symptoms appear in late summer or the following spring, depending on the cultivar's resistance.

Over the summer, the knots grow, becoming olive-green and downy looking, indicating that the fungus is producing summer spores (conidia). This relatively brief stage causes other, more limited infections.

In fall and winter, the knots turn hard and coal black. They shelter the winter spores (ascospores), which will be released the following spring, to contaminate new branches or new trees.

Prevention and control

Favourable conditions

Rainy springs tend to encourage the development of the mycelia and the production and release of spores, the primary inoculum source. Infected wild plum and cherry trees nearby are likely sources of contamination.

Identification

Inspect branches in spring and regularly during the growing season.

Prevention

  • Choose resistant cultivars, including:
    • Plums: 'Burbank', 'Early Golden', 'Formosa,' 'German Prune', 'President', 'Santa Rosa' and 'Shiro'.
  • Keep plants healthy by fertilizing them properly and watering them during dry spells.

Physical control

  • When plants are dormant (late fall and winter), remove any infected branches before they begin growing in spring. By pruning off old knots you will prevent overwintering spores from contaminating the new shoots.
  • During the growing season, remove knots as they form.
  • Cut branches back about 10 cm below any knots.
  • Carefully clean your tools between each cut with a 70% isopropyl (rubbing) alcohol solution.
  • Avoid touching healthy parts after touching infected tree parts.
  • Never compost infected plant litter; immediately dispose of all trimmings, because the fungus continues to produce spores.
  • Cut down severely infected trees.

Biological control

Various insects and fungi are parasites on the knots, but they are not sufficient to stop the growth of this fungus.

Chemical control

As a last resort, use a low-impact pesticide with calcium sulphide or calcium polysulphide (lime sulphur) or sulphur as the active ingredient. Read the product label carefully and follow the manufacturer's recommendations.

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