448 Scientific Proceedings, Royal Dublin Society. 



The longest period during which the fungus was definitely proved to have 

 retained its vitality in soil was forty-four days. It is worthy of note that 

 with two of the tubes (Nos. 2 and 7) infection was just as easy and certain 

 after the lapse of this period as at the beginning. From the same two tubes 

 infection of potato slices was again secured sparingly after sixty-six days, and 

 a still further trace of infection was obtained from No. 2 after seventy-four 

 days, but in none of these cases was P. infestans recovered. Pieces of tubers 

 inoculated after the last two intervals from other tubes (sixteen in number) 

 and with similar non-contaminated soil (eight in number) remained sound. 



The conditions under which the fungus survives longest are evidently 

 determined more by moisture than by temperature. The longest periods of 

 activity were found in tubes which were either moist to begin with or were 

 kept in a saturated atmosphere, irrespective of the temperature up to and 

 including 20° C. At 30° C. the temperature becomes important in spite of 

 the abundance of moisture. The most fatal conditions are those which com- 

 bine high temperature and dry soil. 



Attempts at direct examination of contaminated soil. — Direct examination 

 of Ihe soil failed to give an explanation of these findings. It was concluded, 

 after careful search, that the fungus did not survive in the form of mycelium. 

 On the other hand, apparently perfect conidia, among others, were still seen 

 in soil to which they had been added more than three week^previously. 

 The majority of the conidia observed under such conditions appeared, how- 

 ever, to be shrunken but ungerminated. Others were empty, and from the 

 presence of apical openings were concluded to have produced zoospores. No 

 germ tubes were seen. Soil samples such as these, when placed on potato 

 slices, infected them freely, while similar slices not inoculated remained sound, 

 so that in the apparent absence of mycelium it was concluded that the 

 conidia themselves had the power of retaining their vitality for comparatively 

 long periods. 



An attempt to prove this by placing fresh conidia on twelve dry cover 

 glasses in a small chamber, the atmosphere of which was saturated with 

 water-vapour, proved only partially successful. Twelve other cover glasses 

 similai'ly provided witli conidia and prepared at the same time were placed in 

 a similar chamber which was kept dry. The period of life was tested by 

 infecting potato slices (with controls) from the fifth day onwards. The 

 conidia from the moist chamber produced disease regularly until the ninth 

 day, but not afterwards. Neither the conidia from the dry chamber nor the 

 controls produced disease on any day. A later experiment in which the 

 moist chamber was closed with a water-seal led to no better result. 



