1921.] Protection Against Fungi from Abroad. 659 



of which there are two or three wild kinds, are also often dis- 

 torted and injured by the black galls. 



The knots occur sometimes at the end of twigs but more 

 often on the smaller branches, where they grow most frequently 

 along the branch in a slow spiral. Occasionally they entirely 

 encircle the bough, forming a complete girdle, and the bough 

 soon dies. 



These black galls so check the nourishment of the branch on 

 which they occur that the branch usually dies within a couple of 

 years. The lumps or excrescences are light yellow-brown in the 

 spring and early summer, turning black and charcoal-like in 

 the autumn. The fungus produces spores of two kinds, a summer 

 (conidial) spore from the light brown mass and a different winter 

 (perithecial) spore from the knotty, black autumn stage. 



These four diseases are of outstanding danger, but there are 

 several other well-known diseases on fruit-trees, cereals and 

 ornamental plants in various parts of the world which may, if 

 introduced in the British Isles, become extremely troublesome, 

 but which at present do not appear to warrant legislative action. 

 They will, however, not be forgotten. 



Another question is ever present which requires further 

 research, namely, that of various strains in the same species 

 of fungus. It is possible that different strains are concerned in 

 the Nectria Canker and Monilia Brown Rot of x^pples in America 

 and in Europe and hence different results show in the two con- 

 tinents. It is possible also that the notorious Siereum pur- 

 jiureuTYij the cause of Silver Leaf in Europe, is not absolutely 

 identical on both sides of the Atlantic and hence does not atta-ck 

 Plums in the United States and Canada as it does in Britain. 



The Ministry is indebted to Professors H. H. Whetzel and 

 M. F. Barrus of Cornell University for the photographs of Fire 

 Blight and Black Knot of Plum and to Professor C. B. Orton 

 for obtaining sanction from the Pennsylvania Chestnut Tree 

 Blight Commission to use the photographs of Chestnut Blight 

 accompanying this article. 



F 2 



