30:? NINETEENTH REPORT. 



The lowest temperature at which observations were made was 6°C^. 

 A light aerial mycelium appeared within four days. Some spores though 

 comparatively few in number were produced in 7-9 days. At the same 

 time a healthy plant inoculated with the fungus was placed in the incuba- 

 tor beside the culture tubes. The first signs of the disease appeared on 

 the fourth day, and the typical spots bearing a few spores were observed 

 on the eighth day. The humidity in this incubator varied from 8-5 to 98% 

 of saturation. The light was very diffused, entering the incubator through 

 a glass door facing a north window. Although the temperature was low, 

 conditions favored the fungus more than the plant, which was practically 

 dormant. The liigh humidity favored the germination of the spores, 

 hence, infection was to be expected. 



At 9-12 C. (ice box), the fungus grew slowly, making about the same 

 amount of growth as at 6°C. 



At room temperature, (20-24°C.), the fungus made its optimum growth. 

 At this temperature all culture studies were made. 



At 30 C, growth was a little retarded. The spore development was 

 fair. 



At 33-34°C., growtli appeared like that at 30 C, but microscopic 

 examination sliowed that no spores were formed. As occasional young 

 conidum appeared at this temperature, but on the whole spore foi'mation 

 was retarded. If cultures kept at this temperature were removed to 

 room temperature spores developed within one to three days even after 

 they had been kept at the higher temperature for 19 days. 



At 37°C., growtli proceeded no farther than the germination of the 

 spores. Removal to room temperature brought about normal growth 

 even if tlie cultures were kept in tlie incubator for eleven days. This 

 temperature may be considered as the inhibiting one. 



Thermal Death-point: 



Duplicate tubes of liquid and agar media were inoculated with one- 

 tenth cc. of a water suspension of spores and heated at a given tempera- 

 ture carefully controlled in a water bath, during a period of ten minutes. 



No growth developed in any of the cultures heated at 60° C. for 10 

 minutes. After 10 minutes at 56°C., growth in all media was normal. 

 With exposure for 10 minutes at 57 and 59°C., growth was present but 

 much retarded. Microscopic examination showed many ungerminated 

 spores. The thermal death-point, therefore, must lie between .'59 and 

 60° C. for lO minutes, but many spores are killed at that time limit 

 between 57 and 59^ C, though some always seem to survive. This experi- 

 ment was repeated twice witli the same results. 



'This temperature was secured in tlie constant temperature ice box described by Coons 

 (1916). p. 727. 



