Pethybridge — Experiments tuith Phjtophthora. 1^ 



ranged from about 38° to 57° F. ; and it is quite possible tbat the third plant 

 mentioned above, and left in the cool greenhouse, remained healthy, not 

 because the conditions were unfavourable, but merely because " spores " were 

 absent. At any rate there is no evidence against this view. 



Since the evidence in favour of the theory derived from the experiment 

 described was of such an unsatisfactory nature, it seemed to me eminently 

 desirable to carry out another one on similar lines, but with the addition of 

 tlie absolutely necessary control plants. Starting in the early montlis of the 

 year, in order to avoid any possible chance (at least in the early stages) of 

 infection from aerially borne spores, I carried out an experiment of this kind 

 which will now be described. I gladly avail myself of this opportunity of 

 acknowledging with many thanks the assistance of my friend and colleague, 

 Mr. F. W. Moore, M.A., Keeper of the Eoyal Botanic Gardens, Glasnevin, 

 who was good enougli to place at my disposal the necessary room in two of 

 his glasshouses, and who afforded me other facilities for the work. 



The experiment was started on February 11th, and brought to a con- 

 clusion on July 11th, 1910, and thus extended over five calendar months. 

 Six potato-tubers of tlie variety "Champion " attacked by Phytophthora' were 

 halved, thus making twelve sets in all. Six other tubers of the same variety, 

 but perfectly healthy, were similarly cut into twelve sets. Twenty-four pots 

 were filled with virgin loam which had not been previously used for potting 

 purposes. Of these, twelve were sterilized by heating for half an hour each 

 in an autoclave at 120° C, the remaining twelve being untreated. Six 

 diseased and six healthy sets were then planted in the twelve pots of sterilized 

 soil and the same numbers planted in the twelve pots of unsterilized soil. No 

 manure of any kind was used. Of the twenty-four pots (twelve containing 

 diseased sets and twelve containing healthy ones as controls) six were 

 placed at once in a warm greenhouse, the temperature of which ranged from 

 a minimum of 60° F. at niglit with fire-heat, up to 85° F. in the daytime with 

 sun-heat, or at least 65° F. with fire-heat on cold days, and the atmosphere 

 of which was fully charged with moisture, so that the conditions for the 

 development of Pliytophthora were extremely favourable. The soil in all 

 these six cases had been sterilized, and three of the pots contained healthy 

 sets, while the other three contained diseased ones. These pots were not 

 covered at any time with bell- jars. 



The remaining six pots of sterilized soil with three healthy and three 



1 Lest there should be any doubt in the matter as to whether these tubers were actually attacked 

 by Phytophthora or not, it may be stated that I selected them personally from a severely diseased 

 crop, and utilized the remainder of the batch not required for the experiment, for successful demon- 

 stration of the fungus, after suitable incubation, by the members of a large class of agricultural 

 students. 



SCIENT. PHOO. K.B.S , VOL. illl., NO. II. ^ 



