THE OAK MILDEW. 93 



are especially found in shoots which develop about midsummer, 

 (Lammas shoots), and it is these which suffer most severely from 

 the disease. In severely infected shoots the mycelium is not 

 confined to the leaves, but also spreads to the stem. Chains of 

 conidia of the usual form are produced on the mycelium, most 

 abundantly on the upper leaf surfaces, more sparingly on the 

 under surfaces, and in small numbers on the stems. These are 

 easily detached and are carried by the wind, spreading the 

 infection to neighbouring trees. 



As a result of a severe attack the leaves become discoloured 

 and fall prematurely, and brown patches of dead epidermis 

 appear on the infected stems. Diseased shoots fail to ripen 

 thoroughly, and in consequence are usually killed by the frosts 

 of the following winter. As would be expected photosynthesis 

 is seriously interfered with by the attack, and as a result both 

 height and girth increase are diminished in infected trees. The 

 effects also of insect attack are intensified by the mildew, for in 

 cases of defoliation the shoots subsequently produced from 

 resting buds are particularly liable to infection by the fungus. 



The mildew attacks a considerable number of species of 

 Quercus, including Q. sessifiora and Q. pedunculata ; a number 

 are also described as immune. The susceptibility of a species 

 may, however, vary in different regions, e.g. Q. Cerris is 

 virulently attacked where native in southern Austria, but is 

 almost immune in Central Europe. In some species the whole 

 of the leaves are liable to attack, while in others only young 

 foliage is infected ; in others again, only the leaves of Lammas 

 shoots become diseased. In southern Macedonia the writer 

 observed that Q. conferta was strongly infected both on the old 

 and young leaves, while Q. coccifera, growing in close proximity, 

 was immune. The chestnut {Castanea vesca) has been stated by 

 some to be attacked by the oak mildew, but Neger has shown 

 by experiment that it is immune. The beech is the only species 

 outside the genus Quercus which is infected by the mildew, and 

 in this the attack appears to be confined to stool shoots. Its 

 occurrence on this host was recorded in France in 1908, and it 

 has since been found in a number of countries, including the 

 South of England.^ Although the experiments carried out by 

 Neger have proved the identity of the mildew on the oak and 



1 Cotton. Transactions Brit. Mycol. Soc, vol. vi. p. 198, 1919. 



