108 James G. Horsfall 



four physiologic strains postulated from the cultural studies, three were 

 produced by mutation corresponding to cultures 128 and 244, and the 

 typical condition, as represented by culture 231, was secured as a reversion 

 from a mutation. A strain corresponding with culture 242 was not ob- 

 tained by mutation, but a new one, 271 aa, appeared. 



Effect of temperature on cultural characters. As one of the corallaries 

 of the studies of the effect of media, culture 127, representing the typical 

 fungus, was grown in triplicate plates containing 20 cubic centimeters 

 each of Coons' synthetic medium at various temperatures. The diameters 

 of the thalli were measured daily to the nearest millimeter. A duplicate 

 experiment of shorter duration revealed the same results. 



Both experiments demonstrate that the best growth occurred at 25° C, 

 with a minimum somewhere below 3° and a maximum between 30° and 

 35° C. 



The data from the first experiment are presented in the graphs shown 

 as figures 25 and 26. In figure 25 is shown the growth curve based on the 

 final diameter of the thalli. The peak is very sharp. The daily growth 

 rate at the various temperatures is plotted in figure 26. 



Life history. Although the complete life history of Helminthosporium 

 vagans is not known exactly, yet enough data have been accumulated to 

 warrant their inclusion here. 



In the spring two primary sources of inoculum exist. To judge from a 

 single year's observations, the fungus lives over winter in lesions on living 

 leaves. Also, mycelium and spores were overwintered in test tubes of 

 oat agar on a north window ledge where the temperature reached as low 

 as 5° to 10° F. on several occasions. In the spring the agar had liquified 

 from freezing, but transfers gave typical cultures of the fungus. The 

 vegetative mycelium is capable therefore of living all winter, at least 

 under the conditions of this experiment, and of growing again in the spring. 

 This indicates the possibility of its living in refuse or in living leaves of 

 blue-grass. No perfect stage has yet been found, although immature 

 perithecia which may belong to this fungus have been observed on over- 

 wintered infected leaves. Conidia from dead leaves probably serve as 

 the chief inoculum. 



An experiment was made in duplicate to test the effect of temperature 

 on spore germination. Counts after twenty-four hours revealed that the 

 spores had germinated 98 per cent or better in all temperature chambers 

 from 3° to 35° C. Obviously the fungus will germinate over a very wide 

 range of temperature if sufficient time be allowed. It is interesting that 

 germination occurs well at 35° C, whereas no appreciable growth occurs 

 when plate cultures are incubated at that temperature. At 9° and at 

 3° C. there is a hint of shortening of the tubes, but that is a retardation 

 of growth rather than a reduction of germination. 



