Witches Broom is a peculiar type of gall in which there is an over-production of whole organs, resulting in a broom, or nest-like habit of growth. This name is in general use in North America. In England, the disease is called thunder-bushes, bull-boughs, bull-wood and bull - branch. The Germans refer to it under the name Hexenbesen, while the French call it Balai du Sorciere.

The disease is very common and destructive in Europe, where both sweet and sour cherries are affected. Although the English sweet cherry {Prunus avium) is commonly cultivated in eastern United States, the disease was not recorded until 1886, when it was reported from Germantown, Pennsylvania. About ten years later it was found in Long Island at scattering points. The Witches Broom disease of cherry is not a disease well-known to American cherry-growers. It occurs sparingly in New York, but is said to be common in Oregon. It has also been observed in a few other states, including Washington, Minnesota, Pennsylvania and New Jersey. The disease is of most interest perhaps because of the fact that the pathogene spreads so slowly, and on account of the peculiar effect on the cherry. Its history thus far in this country does not indicate that the disease should ever be feared by cherry-growers. Records show that the disease may affect in this country the following cherries: English sweet cherry {Prunus avium), sour cherry (Prunus Cerasus), wild black cherry {Prunus serotina), wild red cherry {Prunus pennsylvanica), choke - cherry {Prunus virginiana). Several varieties and species of plums are also affected.

Symptoms

The twigs and leaves are susceptible to this disease. On the former it produces a very striking deformity, although not a killing, of the affected parts (Fig. 51). Infected branches become more numerous and are more or less elongated (Fig. 51). Authorities disagree on the matter of whether affected twigs are thickened. In some cases the twigs become so numerous that they are not able to bear their own weight, and as a result the abnormal portion of the tree droops. In such cases the tips of the branches usually turn upward (Fig. 51). The over-production of branches at a local region results in a broom-like growth. Some of the brooms are large enough and so conspicuous that they are very noticeable from a distance. This is especially true when the tree is bare of leaves. These broom-like growths are also conspicuous at blossoming-time; for they bear few or no flowers, hence the affected portion stands out in bold contrast to the remaining blossoming - branches. Leaves also come out on brooms before they do on healthy branches.

Affected leaves take on a crinkled shape and a reddish discoloration. The disease on leaves is usually referred to as Leaf Curl, and the abnormalities exhibited are very much like those of the peach Leaf Curl (see page 277). Affected leaves fall prematurely and later new foliage appears. On their lower surfaces a whitish coat may be found prior to defoliation.

Fig. 51.   Witches Broom on cherry.

Fig. 51. - Witches Broom on cherry.

Cause

The causal fungus, Exoascus Cerasi, is very closely related to the fungus causing peach Leaf Curl and to that of plum-pockets (see pages 277 and 373, respectively). Its full life-history is not known. The mycelium invades the twigs, living in them from year to year. Both the bark and wood are attacked. The pith-cells, medullary-ray cells and outer cells of the bark (hypodermis) are greatly increased in number as a result of stimulation set up by the parasite. The sclerenchyma fibers, on the other hand, are fewer or lacking. The stimulating effect of the fungus brings about a condition whereby, instead of flower - buds being produced as in ordinary cases, abnormal twigs are developed, giving the broomy aspect already described. The mycelium invades the leaves, causing them to curl and fall. Finally, before defoliation, the fungus forms a fruiting layer of . asci with ascospores on the lower surface of the leaves. These bodies give to this surface the whitish appearance already mentioned.

Control

Fortunately the disease is not usually of sufficient importance to require urgent attention. Little experimental work has been done on which to base recommendations for American conditions. Since the brooms bear neither flowers nor fruit and are a source of trouble, they should therefore be removed. The cut should be made several inches below the lowest point in the diseased portion. It is said that this method readily controls the disease.

References

Stewart, F. C. Witches' brooms on cherry trees. New York (Geneva)

Agr. Exp. Sta. Rept. 14: 532-533. 1896. (See also Garden and Forest 8: 269. 1895.) Jackson, H. S. Diseases of drupaceous fruits. Leaf curl or witches' broom. Oregon Crop Pest and Hort. Bienn. Rept. 1911 - 1912:

250. 1913. Atkinson, G. F. Leaf curl and plum pockets. Exoascus cerasi (Fuckel) Sadeb. Cornell Univ. Agr. Exp. Sta. Bul. 73: 326 - 327.

1894. Stewart, F. C. Notes on New York plant diseases, I. New York (Geneva) Agr. Exp. Sta. Bul. 328: 340. 1910. Lawrence, W. H. Some important plant diseases of Washington.

Witches broom of cherry. Washington Agr. Exp. Sta. Bul. 83:25. 1907