290 Transactions British Mycological Society. 



These results show that, under cultural conditions, 



1. Conidia from " subinfections " on varieties of Brassica 

 oleracea are quite capable of giving full infections on varieties 

 of B. campestris. 



2. Conidia from "subinfections" on varieties of Brassica 

 oleracea when sown on B. oleracea sometimes give full 

 infections, though more usually they give "subinfections" or 

 medium infections which cannot quite be classed as full in- 

 fections. 



It is hoped to be able to carry out soon a much longer series 

 of similar inoculations, including also hybrids between Brassica 

 campestris and B. oleracea, so that definite conclusions may be 

 drawn as to the relative infection powers of conidia taken from 

 "subinfections" on B. oleracea and full infections on i^.c«m/)es/m. 



(C) In the case of Erysiphe Polygoui on cultivated Brassicae 

 the method of over-wintering must be restricted to one or more 

 of the following: 



I. Perithecia. 



^ Persistent mycelium. 



3. Re-infection in the spring direct from other hosts of 

 Erysiphe Polygoni, or by spores from " bridging species." 



4. "Subinfections" within the genus Brassica. 



The perithecial stage of the fungus is very seldom found on 

 the cultivated Brassicae. In fact the writer, though making 

 constant search, failed to discover perithecia on these hosts 

 during the whole course of the investigation. Of course this 

 does not imply that they never occur, but they are of such 

 rare occurrence that alternative No. i above can be ruled out 

 as a negligible method for the fungus of over-wintering. 



In face of the experiments described in this paper and the 

 large number of other experiments which have been carried out 

 with other species of Erysiphe, all of which go to prove the 

 extreme specialisation of parasitism of this genus of fungi, 

 it seems permissible to lay down definitely that alternative 

 No. 3 is also highly improbable; even though one takes into 

 consideration the possibility of "biologic forms" breaking down, 

 either under certain climatic conditions which, as. far as the 

 writer knows, has not been shown to occur by any investigator, 

 or of their breaking down by reason of various injuries to the 

 host, as has been demonstrated by Salmon (8, 9). 



Even in the case of such a very closely related species as 

 Brassica Sinapis there is a certain amount of doubt as to the 

 free transference of the mildew to the species B. campestris 

 and B. oleracea though it is possible that Charlock does help in 

 a small degree in the reinfection of varieties of the cultivated 



