92 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



From the consideration of the above results it may be 

 concluded that " blued " is slightly weaker than healthy timber, 

 and it should therefore not be used for structural purposes where 

 failure would result in serious consequences. 



10. The Oak Mildew. 



By Malcolm Wilson, D.Sc, F.R.S.E., 

 Lecturer in Mycology in the University of Edinburgh. 



The mildew of the oaks probably offers one of the best 

 examples within recent years of the sudden appearance and 

 rapid spread of a serious disease of trees, for although first 

 found in Portugal, where it occurred in abundance in 1907, by 

 1909 it had spread to almost all the countries of Europe and 

 had penetrated in-to Asia to an undetermined extent. Its history 

 in this country is somewhat similar ; it appears to have been 

 first observed in 1908, when it was described as widespread in 

 Devon and in the southern counties of England, and within a 

 few years it had spread over the whole of Britain. 



The fungus is one of the true mildews belonging to the family 

 Erysiphaceae, but for some years after its first appearance its 

 systematic position was not exactly determined, owing to the 

 fact that its only form of reproduction was by means of the 

 summer spores (conidia). During this period it was pro- 

 visionally known as Oidium alphitoides. Perithecia were, how- 

 ever, discovered in France in 1911 and proved, on examination, 

 to belong to the genus Microsphaera, bearing a close resemblance 

 to M. alni. 



The mildew^ is found particularly on stool shoots and young 

 plants, but also occurs on older trees. In structure it agrees 

 generally with the other species of the Erysiphaceae. The 

 mycelium is entirely superficial, extending over both leaf 

 surfaces, and forming bladder-like haustoria in the epidermal 

 cells. Experiments have shown that mycelial growth is 

 encouraged by bright light and a fairly high temperature, and 

 that infection takes place most readily when the leaf possesses 

 a high water-content and the cuticle is thin. These conditions 



^ The most complete account of the oak mildew is probably that given by 

 F. W. Neger {Nat. Zeitschrift f. Land-und Forst'virtschaft, xiii. p. i. 1915), 

 whose paper has been largely referred to in writing this article. 



