Erysiphe Polygoni. G. 0. Searle. 291 



Brassicae in spring, though in this case again perithecia are 

 equally rare. 



It would therefore seem more probable from the experi- 

 ments and field observations already described (although they 

 are far from complete) that alternatives No. 2 and No. 4 are the 

 most likely methods by which the mildew exists over the winter. 



One or more of the cultivated Brassicae are always to be 

 found in leaf at all seasons of the year on a farm, and above 

 it has been shown that the conidial stage of the mildew was 

 successfully kept under observation out-of-doors on one or 

 other cultivated Brassica during the autumn, winter and round 

 to the middle of the following May, and that, even after heavy 

 frosts, viable conidia were formed. 



As Swedes and Turnips are not often present on the farm in 

 the spring, it is probable that infection is carried on mainly 

 by " subinfections " on varieties of Brassica oleracea (which are 

 generally to be found throughout the early spring in the form 

 of Cabbage or Kale), aided by persistent mycelium on "volun- 

 teer" plants of Swede, Turnip or Rape. 



It may be argued that it is difficult to understand why, if 

 the mildew is able to attack Swedes and Turnips in the cotyledon 

 stage as has been shown to be the case, it is not usual to find a 

 bad attack before July when the plants are well advanced, but 

 it would seem that this depends largely on climatic conditions, 

 which have not yet been fully investigated. It will be remem- 

 bered that the " subinfections " on Thousand Head Kale did not 

 develop into distinct visible full infections until the middle of 

 May and it is quite possible that infection may take place but 

 the mildew remain in an undeveloped form invisible to the 

 naked eye, until climatic conditions are suitable. Such an 

 explanation would account for the fact, mentioned early in, this 

 paper, that numerous forms of Erysiphe Polygoni appeared 

 spontaneously in the greenhouse some time before they could 

 be found out-of-doors, presumably because the temperature of 

 the greenhouse was more suitable for the full development of 

 the fungus. 



Summary. 



1. In field trials in 1913 no variety, out of seventy-seven 

 varieties of Swedes, Turnips and Rape, was found to be immune 

 to Erysiphe Polygoni DC. Swedes were attacked more severely 

 than Turnips. 



2. In inoculation experiments with cultivated varieties of 

 Brassica campestris and B. oleracea, the form of Erysiphe 

 Polvgoni infecting these varieties was found to be a "biologic 

 form" with this additional distinction that inoculations from 



