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The fire blight is a highly contagious plant disease caused by the bacterium Erwinia amylovora. It first appeared in America about 200 years ago, but has also spread throughout Europe since the 1990s. Since the beginning of the millennium, most of the orchards in Switzerland and the Lake Constance region have been affected.
It mainly attacks pome fruit trees, but also shrubs such as e.g. Hawthorn.
Spread of fire blight bacteria
The infection mainly takes place in spring and summer. The flowers are the main infection sites. The infection is promoted by the normally warm and humid weather at that time. The bacteria can also penetrate through leaves, shoots, branches and wounds in the plant.
The bacteria are transmitted by insects, birds, humans, cutting tools, packaging material or infected plants. Therefore, affected plants should be touched as little as possible and all used tools, including gloves, should be disinfected.
Damage of the fire blightThe leaves and flowers of the affected trees wither and turn brown or black. The shoot tips curve down hook-shaped. At the infection sites bacterial mucus comes out. The bark sinks. The plant looks like burned, hence the disease name "fire blight". Attention! Danger of confusion with Monilia, also called top drought, a disease caused by a fungus.
By cutting the transition between diseased and healthy tissue, a first distinction can be made. In a fungal infection, this transition is sharply demarcated and dry. In an acute bacterial infection, it is out of focus, damp and slightly stained or streaky. If the infection is already long ago, the transition can also be sharply defined. A reliable diagnosis is only possible through a laboratory examination.
Firebrand infected young plants die within two to three weeks. In older plants, the disease spreads over several years, but leads in the end but also here mostly to die off.
Fire blight is notifiable!
Combating fire blight:
Unfortunately, there are not too many options for combating the fire blight.
- The affected branches and twigs must be removed and best burnt on the spot. Because of the danger of spreading, they must never land on the compost. If the infestation is too great, the tree must be felled. If burning is not possible, smaller amounts of cut material can be disposed of in closed plastic bags in the residual waste. Larger quantities are, also well packed, disposed of in the incinerator. Trunks and main branches should be dried before burning. The used cutting tools must be disinfected!
- Chemical pesticides are not permitted to combat fire blight.
- Minerals and plant extracts have proven to be unsuitable.
- In Germany since 2003 the antibiotic streptomycin is used for the prevention. It reduces the infestation by 80%, but is not uncritical, since it has been proven in honey and the development of resistance must be feared.
- As an alternative, yeast preparations, e.g. Candida sake, used. They are applied to the flowers during flowering and prevent proliferation of the fire blight bacteria. They reduce the infestation by 70%.
- It has been shown that older apple trees have the ability to seal off and to heal fire blight infections. They form a bark-like tissue that contains phenolic defense substances. This defense reaction is the stronger, the shorter the retardation is. This means that cutting measures to promote the new shoot, in contrast to the usual practice, should be reduced to the minimum necessary. The removal of unwanted new shoots should also be postponed in the summer. Fertilization with nitrogen should be reduced in order not to further promote shoot growth.
- Pear trees can not develop these defense reactions. For new plantings, varieties that are less susceptible to fire blight should be preferred.
- Affected plants: Woods from the family of rose plants such as apple, pear and quince and their ornamental forms.
- Damage: Flowers and pedicels turn black, but do not fall off. Leaves brown to black. Young shoots curve in a hook shape. Infested areas of the bark reddish to brown discolored. As the disease progresses, the bark ruptures.
- Most dangerous time: spring and flowering
- Defense: Infested trees, especially near fruit-growing areas, must be cleared and destroyed.