Articles
STEM WITCHES’ BROOM OF FRUITS AND FOREST TREES
Apparently this is caused by some virus or mycoplasma disturbance in the conductive tissues.
Attempts to transmit it by chip budding to annual test plants were unsuccessful.
Different witches’ brooms have been described as occurring on the crowns of various trees.
Bos (1957, 1963) gives some of the literature on this subject.
Here we deal with a type of witches’ broom that occurs only on the stems of fruit and forest trees.
The stems of all trees are usually smooth and cylindrical from the ground up to the twigs.
They are free of branches, tumours and other abnormalities.
During the last five years I have obsereved in the Forest Park of the city of Sofia that the stems of many trees are covered by tufts of thin branches, which die and disappear, but are again formed on the same place the next spring.
Their leaves also die and prematurely drop.
The places on which the tufts of branches are formed annually gradually swell up and form flat or semispherical tumours having a rough surface (Fig.1). Such brooms and tumours are formed only on the stems and are usually never seen on the crowns.
Some trees such as Robinia do not form tufts of branches, but their stems are covered with numerous rough tumours.
The leaves of the broom branches as well as those of the crowns do not show any abnormalities during the whole summer.
The affected trees appear normal but are thinner and some of them begin to die from the top down.
The stems of some of the affected trees are not smooth and cylindrical, but show deep vertical furrows.
Their cross section is not circular but star like.
This disease has not been seen on young trees.
The stem witches’ broom has been seen at various places in Bulgaria, in the parks of the Hague, Holland, and along the roads in England.
In Bulgaria the disease has often been seen in the forests and on trees along streets and highways, but very seldom on the fruit trees under orchard conditions.
However, it is more common on single apple, pear, plum and walnut trees growing among forest trees.
This suggests that mycorrhiza or nametodes may play a role in the spreading of the disease.
Apparently it is not spread by insects.
The most extensive occurrence of stem witches’ broom has been seen and studied in the Forest Park of Sofia.
This park covers an area of over 300 ha.
It was established 70 years ago on the former community pasture land.
It consists of widely different native and imported forest species.