94 TRANSACTIONS OF ROYAL SCOITISH ARBORICULTURAL SOCIETY. 



beech, the fungus has not spread to any great extent on the 

 latter, which is generally free from attack. 



The method of hibernation of the mildew has been the subject 

 of several investigations. It has been shown that the conidia 

 only retain their power of germination for a comparatively short 

 time, and that the mycelium on leaves and shoots does not remain 

 alive throughout the winter. It was finally discovered that 

 mycelium is present between the scales of some of the buds on 

 shoots which have borne infected leaves, and that this remains 

 in a living condition during the winter, growing out with the 

 developing shoot in the following spring, and bringing about a 

 slight infection. Conidia from these shoots then infect the 

 Lammas shoots, which become densely covered by the mildew. 



Perithecia of the oak mildew were first discovered by Arnaud 

 and Foex^ in France, in the late autumn of 1911,- on fading 

 leaves of cut-back shoots of Quercus sessiliflora. These investi- 

 gators have determined that the perithecia belong to Microsphaera 

 alni exiensa, Salm. [M. quercina (Schw.) Burr.), which is found on 

 several species of Quercus in North iVmerica. Griffon and 

 Maublanc, however, consider that the perithecia differ from all 

 known species of Microsphaera, and accordingly assign them to 

 a new species, AT. alphitoides. Perithecia of the mildew were 

 found by the writer in Macedonia in October 19 18, on bushes of 

 Quercus conferta, and these agree closely with those of M. alni 

 extensa.'^ Perithecia were found in Germany in 1920, by 

 Behrens, and by Peyronel, near Rome, in the same year ; they 

 have been assigned to M. alni extensa in both these cases. 



A number of suggestions have been made regarding the 

 conditions under which formation of perithecia takes place. 

 Cold has been looked upon as an important factor as a con- 

 sequence of their discovery in the late autumn. It is also 

 suggested that the chief cause of their formation is the excessive 

 transpiration of the fungus in relation to the supply of food in 

 solution obtained from the host. Evidence in support of the 

 latter suggestion is given by the conditions under which 



^ " Sur la forme de I'Oidium du chene en France," Comptes rendus Ac. 

 Sc. Paris, T. 154, 1912, p. 125. 



■^Mildews on the oak in Europe were occasionally described previously to 

 1907, and in two cases perithecia were found (by Passerini, in Parma in 1875, 

 and by Mayor, in Switzerland in 1S99). It is very questionable, however, if 

 these were identical with the form now discussed. 



^ It is proposed to describe these in detail in a subsequent communication. 



