104 James G. Horsfall 



After extensive investigations a method of obtaining conidial production 

 in culture was perfected. 7 The spores thus produced were used for inocu- 

 lation trials in which the typical leaf symptoms were produced within nine 

 days on blue-grass. The pathogene was reisolated from these lesions. 



Physiologic specialized on. Christensen (1922) and Stevens (1922) showed 

 that Helminthosporium sativum P. B. K. causing the Helminthosporium 

 disease of cereals was a composite species of several physiologic strains. 

 Ch istensen in a later paper (1926:12) separated several forms by their 

 reactions on culture media, rate of growth, thermal relations, amount of 

 sporulation, zonation, the readiness with which they mutate, and lastly 

 by their pathogenicity. 



A test for the presence of such strains in the species Helminthosporium 

 vagans Drechs. was undertaken in a small way by the writer. During the 

 summer of 1927 several isolations of the fungus were made from blue-grass 

 in as many localities in New York State as possible. The culture number, 

 the place of collection, and the date of these isolations, are shown in 

 table 28. 



TABLE 28. Isolations cf Helminthosporium Vagans 



Single spore cultures of all the strains were made except for number 271 

 which was made as a tissue planting and never wa converted to a single 

 spore culture. 



Tests of the reaction of the various strains to four culture media were 

 made. Coons' (1916) synthetic medium with 2-per-cent agar, oatmeal 



' The fact that the fungus was found to sporulate by placing transfers on oat or potato agar on a north 

 window ledge according to the method of Jones (1916), encouraged the writer to investigate the stimulus 

 to spore production, the results of which are described briefly. It appears that light is the factor usually 

 limiting spore production in the laboratory where the cultures were usually kept in darkness or semi- 

 darkness. Transfers in quadruplicate to oat agar were made in March, 1928, for all the possible combi- 

 nations of the following conditions: sealed, unsealed, covered with black paper, uncovered, on north win- 

 dow ledge, and in the greenhouse. Spores were produced in abundance within two weeks only in those 

 tubes that were uncovered whether sealed or not. No spores could be found in those from which light 

 had been excluded. The sealed tubes remained moist, while the unsealed ones dried somewhat, indi- 

 cating that moisture is unimportant. Ultra-violet light was not a factor, inasmuch as it will not pass 

 through the glass of the test tubes. Direct sunlight is not important, because the tubes on the north win- 

 dow ledge were in diffuse light only. Alternating temperature does not appear to exert much effect, 

 since spores were produced in a subsequent experiment when the transfers were placed in constant 

 temperature incubators heated by incandescent bulbs. 



