APPENDIX 455 



OAK CANKER 



{Stereutti quercinum. Potter.) 



Professor Potter has described a disease of the oak 

 caused by a fungus — Stereum quercinum — which proved to 

 be new to science. The destruction arises from the local 

 destruction of the cambium year by year, by which the 

 formation of bast and wood is prevented. The parasite is 

 dormant during the summer and becomes active during 

 the winter season, hence the fungus annually destroys the 

 attempt on the part of the tree to form a callus, and even- 

 tually an open wound is formed which never heals. The 

 Stereum appears to be a wound parasite, an entrance 

 being first effected around the insertion of a dead branch. 

 Its action on the wood is specially noticeable in the brown 

 discolouration of the medullary rays. The appearance of 

 'partridge wood,' so characteristic of the action of Stereum 

 frustulosum, is not produced by S. quercinum. 



Potter, Trans. Engl. Arbor. Soc, p. i, figs. 1-4 (1901- 

 1902). 



CUCUMBER AND MELON LEAF-BLOTCH 



(Cercospora melonis, Cke.) 



This fungus, although first described so recently as 1896, 

 and at the time considered as a rarity, constitutes at the 

 present day the most destructive and widespread of 

 diseases with which the grower of cucumbers, melons, 

 and other economic cucurbitaceous plants has to contend. 

 On the other hand, the disease is purely an artificial 

 creation, being forced into luxuriant growth in the 

 cucumber-house, saturated with moisture, and exhibiting 



