462 Scientific Proceedings^ Royal Dublin Society. 



Summary. 



The conidia of Phiitophtliora infestans when mingled with soil and kept 

 out of doors may remain viable and capable of infecting potato tubers for a 

 period of between three and four weeks. 



The vitality of the couidia is retained longer in loam than in a quick- 

 drying medium like silver sand. 



In experiments indoors conidia survi\'e longer in comparatively dry to 

 somewhat moist soil than in very wet soil, the respective periods being forty 

 and twenty-six days. 



Temperatures up to and including 20" C. have no ill effect on the conidia 

 as long as suificieiit moisture is present. At 30° C. the temperature is not 

 too high for the conidia to survive for twenty-six days if the atmosphere is 

 saturated. The most fatal conditions are those which combine lack of 

 moisture and high temperature. 



Under the most favourable conditions indoors, conidia in soil infected 

 tubers just as freely after forty-four days as when fresh. 



Conidia when placed in soil survive without the production of obvious 

 mycelium. The attempt to preserve ungerminated conidia in air saturated 

 with water- vapour met with comparative failure, nine days being the apparent 

 limit of existence. 



Initially contaminated surface soil, after the continuous passage through 

 it of large quantities of water, may remain pathogenic to potatoes for a period 

 exceeding four and a half days, and generally infects potatoes more freely 

 than the water after passing through 10 em. of soil does at any time during 

 the period. 



As having a possible bearing on the resistance of conidia in soil, their 

 method of germination was re-examined in the light of previous work, which 

 is summarized. 



At a temperature of 10°-15° C. the conidia germinate freely, and 

 generally produce zoospores. A small number of germ tubes may, and often 

 do, result ; while exceptionally they may be in the majority. Nutrient solu- 

 tions, such as soil filtrate and 1"5 per cent, glucose in water, are without 

 material influence in favouring tube production ; but such solutions hinder 

 somewhat the perfect formation of zoospores. Abundant oxygen and the 

 absence of competing organisms favour zoospore production. 



The tendency to produce germ tubes at low temperatures seems to be 

 innate in the conidia themselves. Conidia which have remained for some 

 time ungerminated in air and those which have had their germination in 

 v^ater delayed for any reason seem capable only of giving rise to germ tubes. 



