Shoot symptoms are similar to those in blossoms but develop faster. Ooze droplets are initially creamy white, becoming amber-tinted as they age. During periods of high humidity, small droplets of bacterial ooze form on water-soaked and discolored tissues ( see example on fruit, Figure 7). Similar symptoms often develop in the base of the blossom cluster and young fruitlets as the infection spreads internally ( Figure 2). Later these tissues shrivel and turn brown to black. The floral receptacle, ovary, and peduncles become water soaked and dull, grayish green in appearance.
Blossom clusters and young shootsīlossom symptoms are first observed 1-2 weeks after petal fall. Generally, symptoms of fire blight are easy to recognize and distinguishable from other diseases. Symptoms of fire blight can be observed on all above ground tissues including blossoms, fruits, shoots, branches and limbs, and in the rootstock near the graft union on the lower trunk. Today, fire blight is an important disease of apples and pears in many parts of the world. It was the first bacterium proven to be a pathogen of plants. HOSTS: Apple, pear, several rosaceous ornamentalsĮrwinia amylovora is a native pathogen of wild, rosaceous hosts in eastern North America. Other ornamentals such as hawthorn, plum, chokecherry, saskatoon, and spirea may also be affected.Johnson, K.B. Canadian Forest Service PublicationsĪ severe outbreak of fire blight can seriously damage or kill mature pear, apple or crab apple trees in one season. They should be cut at least 30 cm below the last sign of infected bark. However, pruning and destroying diseased twigs and branches is the only effective method of control. In some cases antibiotics can be used to cure fire blight. Cankers eventually develop from branch or blossom infections. Fruit may be infected through insect wounds. More commonly, they are infected through wounds created by pruning, insects or hail damage.ĭroplets of ooze can form on these infected twigs within three days. Young branch tips can be infected through stomata (breathing holes on the leaves), and lenticels (air openings on branches). All flowers, leaves and fruit above that point will die. Once in the blossom, the bacteria multiply rapidly in the nectar and eventually enter the flower tissue.įrom the flower, the bacteria move into the branch. Here, they form a sweet, gummy exudate called bacterial ooze. The bacteria are forced through cracks and bark pores to the bark surface. Warm temperatures (24-28 ☌) and high humidity are the optimal conditions for infection and disease development. Fire blight may also be spread by pollinating insects such as bees sucking, chewing, or boring insects and unsanitary pruning tools. 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. Usually the disease is spread by bacteria that overwinter in holdover cankers in the main stem and branches or infected twigs. The shrivelled, leathery infected fruit usually remain attached to the tree.įire blight is caused by a bacterium ( Erwinia amylovora Windslow et al.) that may enter the tree through the blossoms, leaves, or stem wounds. 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 fruit usually remains attached to the tree. They later become leathery and turn brown, dark brown, or black, depending on the species. Young infected fruits become watery or oily in appearance and exude droplets of clear, milky, or amber colored ooze. The affected leaves usually remain on the tree well into the winter. Later, twigs and leaves also turn brown and appear to be scorched by fire, hence the common name. In the spring, infected blossoms suddenly wilt and turn brown. It occurs sporadically and unpredictably and occasionally reaches epidemic levels. North America Damage, symptoms and biologyįire blight is one of the most destructive diseases of fruit trees in North America.
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