Erysiphe Polygoni. G. 0. Searle. 



277 



by a particular abundance of mildew in the Wye district, but 

 little was noticed earlier in the year. A very careful search for 

 the conidial stage of Erysiphe Polygoni on some of its commoner 

 hosts was made during July and August, but it was not found 

 out of doors until August igth on Polygonum aviculare and 

 August 23rd on Swedes. On the other hand it appeared spon- 

 taneously in the greenhouse on Swedes on July 23rd and on 

 Polygonum aviculare on August 2nd. 



This same phenomenon was noted in other cases, the conidial 

 stage on Turnip, Rape, Sonchus arvensis, Linum usitatissimum, 

 Pisum sativum, Trifolium pratense, Onobrychis sativa, Trifolium 

 dubium, T. hybridiim, T. repens, Capsella Bursa-pastoris, and 

 Brassica Sinapis, all appeared in the greenhouse at least a 

 fortnight before they could be found out of doors even though 

 careful daily search was made ; this will be referred to later. 



The first series of inoculations were with the conidia from 

 the Swede, with a view to investigating how far the oidium on 

 Swede was specialised in relation to other hosts of Erysiphe 

 Polygoni, especially other cultivated Brassicae. 



In the following tables the sign + signifies a full infection 

 powdery with conidia and fully visible to the naked eye; the 

 sign o signifies no visible infection; the sign ? signifies that a 

 " subinfection " resulted, consisting of a few hyphae and a very 

 few conidia visible only with a lens. The significance of sub- 

 infections will be discussed later. The sign — signifies that the 

 experiment was discarded owing to infection appearing naturally 

 on other spots than those inoculated. 



Table I. 

 Inoculations with conidia from Swede. 



