THE OAK MILDEW. 95 



perithecia had been developed in the Macedonian specimens. 

 The perithecia occurred in considerable numbers on the leaves 

 of bushes fully exposed to the sun, growing on a dry hillside at 

 an altitude of about looo feet. The summer in Macedonia is 

 hot and dry, and the specimens were collected at the end of 

 September after four months' drought, and before the occurrence 

 of any cold weather. 



The determination of the exact systematic position of the 

 mildew is of considerable importance in its bearing on the 

 discussion of the origin of the mildew in Europe. Two sup- 

 positions have been made: (i) that prior to 1907 the fungus 

 existed on some other host in Europe, and that after its transition 

 to the oak as a result of its changed environment it assumed 

 epidemic characters ; (2) that it was introduced from some extra- 

 European country, and that as a result of the change of climate 

 it acquired greatly increased virulence as a parasite. As already 

 stated there is no species definitely known to be infected outside 

 the genus Quercus with the exception of the beech, and it 

 appears that the fungus attacked the latter some time after its 

 appearance on the oak. It has been reported that Pyrus 

 communis and other trees are attacked in Portugal, but there is 

 no confirmation of this statement. The fact that the majority 

 of investigators have assigned the perithecia to Microsphaera 

 alni extensa — a variety widespread on the oaks of North 

 America— lends support to the second hypothesis, which is also 

 strengthened by the occurrence of the European mildew on 

 several American species of Quercus when grown in Europe. 



Before considering the artificial means by which the oak 

 mildew can be combated, two natural enemies of the fungus 

 which may control its distribution must be considered. It is in 

 turn attacked by a parasitic fungus — a species of Cicin'-iobolus 

 — which develops on the mycelium and appears to cause the 

 death of the hyphae — and this, according to Vuillemin, may 

 lead to the limitation of the epidemic. Neger has described the 

 larva of an insect, a species of Mycodiplosis, which feeds upon 

 and destroys very large numbers of conidia. A number of 

 substances have been employed as fungicides which have given 

 beneficial results. Amongst these are hot water, flowers of 

 sulphur, liver of sulphur, various lime-sulphur mixtures, per- 

 manganate of potash, and a solution of common salt in water. 



The careful removal and burning of all the young infected 



