336 ^PP^^' ^'^*^^ Pear Scab Ftmrji 



Fischer^ states that Venfuria cannot attack a completely uninjured 

 fruit without previous injury to the epidermis. The attack is dependent 

 on the weather, cold nights followed by warm days cause injury to 

 the epidermis. Small cracks arise which can be seen with the aid of 

 a lens, and he explains the conditions favouring infection, as due to 

 the injuries of the cuticle brought about by them. 



Voges^ notes the presence of a "gelatinous envelope" round the 

 appressoria which he suggests are functionary in attachment of the 

 fungus. 



Wallace (loc. cit.), by boihng up the leaf with caustic potash and 

 thus releasing the cuticle, gathered that the germ tube of an ascospore 

 bores its way directly through the cuticle and continues to grow 

 between it and the epidermis. His figure, which, as far as the writer 

 is aware, is the only one published of the entrance of the germ tube 

 into the host, shows the hypha growing through a crack without the 

 formation of an appressorium and without showing any difference 

 between superficial and sub-cuticular mycelium. 



For investigation of the problems of disease resistance connected 

 with any pathological fungus, it is important first of all to determine 

 the actual details of infection by the fungus. Having ascertained the 

 facts of infection, knowledge as to the fate of the fungus after penetra- 

 tion into the tissues of the host is required. The work described in 

 this paper deals primarily with those two subjects. In the case of the 

 disease under investigation, a detailed study of the germination of the 

 spores of V. inaequalis and V. pirina, the method of penetration into 

 the hosts and the fate of the infecting hyphae was made. The methods 

 of investigation adopted, the results and their bearing upon the above 

 questions are described in turn below. 



Methods. The germination of the conidia was examined by the 

 method of drop cultures, conidia being obtained both from pure 

 cultures of the fungi on artificial media and from affected leaves and 

 fruits of apple and pear. The same sources served for a supply of 

 conidia for the inoculation experiments. The inoculations were carried 

 out on cut shoots and fruits under bell jars or on shoots which were 

 enclosed in lamp chimneys held in position by clamps on the trees. 

 The atmosphere was kept saturated with water vapour, and the inocula- 

 tions were kept moist by means of addition of sterilised water from time 

 to time. 



1 Fisolier, Pjlanzmkraukheilen, xix, 432-434. 



* Vogcf<, Zcilschr. Pflanzenkrctulchdten xx, 385-393. 



