16 Fungus Flora of Water Supplies 



taken from luxuriantly growing cultures from three to eight weeks old, 

 on Quaker Oat agar. With Phytophthora parasitica, small pieces of 

 mycelium as before, but bearing numerous ripe sporangia, were placed 

 in sterile water for an hour before the chemical was added, so that nume- 

 rous zoospores were liberated. The strain of Colletoirichum oligochaetum 

 used was isolated from lesions on cucumber leaves from a commercial 

 nursery in 1920. On Quaker Oat agar acervuli are produced, yielding 

 large quantities of spores, and a suspension of these was prepared by 

 pouring 10 c.c. of sterile water into a tube containing a culture, gently 

 shaking and pouring off into a sterile tube. For Cladosporium fulvum 

 suspensions of spores were made in sterile water under conditions as 

 aseptic as possible by scraping the spores from "mildew" spots on 

 freshly gathered tomato leaves. Control plates gave comparatively pure 

 growths of Clados porium fulvu7n. 



In carrying out the tests, the suspension of mycelium or spores was 

 added in 1 c.c. lots to sterile test-tubes. The necessary amounts of sterile 

 water and of solutions of the chemical were added to bring the final 

 volume up to 2 c.c. at the desired strength. After shaking well the sus- 

 pensions of spores and chemicals were allowed to stand for about 20 

 hours. Control tubes were set up in each case and invariably gave rise 

 to good growths when plated. After standing for 20 hours the suspen- 

 sions were transferred to sterile centrifuge tubes, diluted to 10 c.c. with 

 sterile water, centrifuged, and after decanting the top 9 c.c. the remaining 

 1 c.c. containing mycelium or spores was plated — Phytophthora spp. on 

 Quaker Oat agar and Colleiotrichum and Cladosporium on Waksman's 

 modified egg-albumen agar. The plates were incubated at temperatures 

 approximating to the optimum for the particular fungus concerned, 

 viz. Phytophthora parasitica at 29° C, P. cryptogea at 25° C, Collecto- 

 trichum oligochaetum at 25° C, and Cladosporium fulvum at 22° C. 



Sterilisation by heat. 



The suspension of mycelium or spores was placed in a sterile test-tube 

 and heated for one minute in a water bath at different temperatures; the 

 latter being recorded by means of a thermometer in a second test-tube 

 in the water bath. After heating, the suspension was plated out and 

 incubated as in the chemical tests. 



The results in Table II show the limits within which the lethal con- 

 centration lies expressed as parts per 100,000 of chemical in the 

 water suspension. The first number in the columns indicates the con- 

 centration at which the compound begins to have a toxic effect and the 



