520 DISEASES OF CULTIVATED PLANTS 



should portions of plants that have shown the disease be used 

 for propagation. 



Hall, C. J. J. van, Zeitschr. Pflanzenkr., 13, p. 129 (1903). 



Orchid gummosis. Professor Potter has shown that a leaf 

 spot, occurring more especially on the older leaves of Odonto- 

 glossum uro-skinneri, is caused by a bacterium. The spots 

 are often crowded together, the smallest barely visible to the 

 naked eye, the largest, elliptical in shape, may attain a long 

 diameter of half a centimetre. When large, the spot is raised 

 above the surface of the leaf, and looks like a blister, sur- 

 rounded by a translucent border. A brown mucilaginous 

 substance is present in the tissues under the epidermis in the 

 region of the spots ; deeper down in the substance of the leaf 

 the mucilage is white. It is the accumulation of this substance 

 that raises the epidermis and forms the blister. 



The disease is most prevalent when the plant is grown in a 

 very moist atmosphere, and is very liable to spread. In a dry 

 atmosphere it does not spread, and disease already present is 

 checked. 



Potter, Gard. Chron., March 6, 1909, p. 145. 



Ash canker. The four or five year old stems or branches 

 of young ash-trees are frequently disfigured by the presence of 

 cankered spots, varying in size from small cracks with thickened 

 margins, half an inch long, up to rugged patches forming 

 irregular cavities in the wood, and bounded by irregular out- 

 growths of callus, which may extend for several inches. These 

 diseased spots have been shown by Noack to be of bacterial 

 origin. In some instances, at least, the leaves and leaf-stalks 

 appear to be first infected, the bacteria from thence passing 

 into the wood. On the branches small, discoloured, reddish 

 patches of bark first indicate the presence of the disease. The 

 bark eventually cracks at these points, and the canker gradu- 

 ally increases in size, the disintegration of the tissue being 

 frequently accelerated by fungi that gain an entrance through 

 the wounds in the bark. 



The bacteria are in the form of short cylindrical rods, and 

 are sometimes slightly bent and thickened at the ends. When 

 stained and mounted the rods measure 2-6x0-5 p. 



Noack, F., Zeitschr. Pflanzenkr., 3, p. 191 (1893). 



Ivy canker. Dr. G. Lindau has described a canker on the 

 stems of the ivy caused by a bacterium. At first small dark- 



