Flower, Leaf, Bark, Branch
Damage, symptoms and biology
Fire blight is one of the most destructive diseases of fruit trees in North America. It occurs sporadically and unpredictably and occasionally reaches epidemic levels.
In the spring, infected blossoms suddenly wilt and turn brown. Later, twigs and leaves also turn brown and appear to be scorched by fire, hence the common name. The affected leaves usually remain on the tree well into the winter. Young infected fruits become watery or oily in appearance and exude droplets of clear, milky, or amber colored ooze. They later become leathery and turn brown, dark brown, or black, depending on the species. The shrivelled fruit usually remains attached to the tree.
The bark of branches and stems becomes reddish and water-soaked at the advancing edge of the infection and later cracks and turns black. The shrivelled, leathery infected fruit usually remain attached to the tree.
Fire blight is caused by a bacterium (Erwinia amylovora [Burrill] Windslow et al.) that may enter the tree through the blossoms, leaves, or stem wounds. Usually the disease is spread by bacteria that overwinter in holdover cankers in the main stem and branches or infected twigs. In the spring, just when the blossoms begin to open, the cankers exude drops of bacterial ooze that are disseminated to the blossoms and young leaves principally by rain, heavy dew, or windblown mist. Fire blight may also be spread by pollinating insects such as bees; sucking, chewing, or boring insects; and unsanitary pruning tools. Warm temperatures (24-28 °C) and high humidity are the optimal conditions for infection and disease development.
The bacteria are forced through cracks and bark pores to the bark surface. Here, they form a sweet, gummy exudate called bacterial ooze.
Once in the blossom, the bacteria multiply rapidly in the nectar and eventually enter the flower tissue.
From the flower, the bacteria move into the branch. All flowers, leaves and fruit above that point will die.
Young branch tips can be infected through stomata (breathing holes on the leaves), and lenticels (air openings on branches). More commonly, they are infected through wounds created by pruning, insects or hail damage.
Droplets of ooze can form on these infected twigs within three days. Fruit may be infected through insect wounds. Cankers eventually develop from branch or blossom infections.
In some cases antibiotics can be used to cure fire blight. However, pruning and destroying diseased twigs and branches is the only effective method of control. They should be cut at least 30 cm below the last sign of infected bark.
Canadian Forest Service Publications
Information on host(s)
A severe outbreak of fire blight can seriously damage or kill mature pear, apple or crab apple trees in one season. Mountain ash is equally vulnerable. Other ornamentals such as hawthorn, plum, chokecherry, saskatoon, and spirea may also be affected.
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