14 S. E. A. McCallan 



from rotting cherries gave a germination of aboul 94 per cent. As winter 

 approached, the fungus was isolated and grown on potato-dextrose agar. 

 At first the germination was very poor, conidia from the second transfer 

 giving only 10 per cent of germination. Conidia from the third transfer 



germinated better, but it was not until the fourth transfer that the spores 

 again gave their maximum germination, this time about 98 per cent — 

 slightly higher than in nature. It has been found also, with certain fungi, 

 that after many generations on artificial media the spores produced tend 

 to lose their viability. An effort was therefore made to keep the number 

 of generations to a minimum by always transferring from the oldest culture 

 available. The fungus Sclerotinia americana has been grown continuously 

 in culture for thirty months, new plantings being made about every week 

 to obtain spores for germination tests, and yet only the fourteenth genera- 

 tion has been reached and the fungus still maintains its high germination 

 ability of 98 per cent. 



Age of culture 



Although the age of the culture is not a definite indication of the age of 

 the spores produced thereon, there is nevertheless a more or less well- 

 defined correlation. The common practice has been to specify the age 

 of the spores by the age of the culture at the time when the spores are 

 taken for the test. In comparative tests the age of the spores should 

 he known, and preferably they should be of the same age. Doran (1922: 

 316), in an interesting contribution to our knowledge of the factors influenc- 

 ing spore germination, has concisely stated, " Until the spore lias attained 

 a certain age or degree of maturity, it cannot germinate," and also (page 

 318 of same reference), " For all spores there is a maximum time limit, 

 variable with the environmental conditions, beyond which the spore 

 cannot germinate. The spore may be alive and too old to germinate or 

 it may be dead." This definite time limit within which the spore will 

 germinate should be borne in mind. Even within this limit there is a 

 fairly well-defined period of optimum germination; and it is a point of 

 major importance that as the age for optimum germination is passed 

 the spore becomes more susceptible to the toxic effects of fungicides. 



This relationship of spore age to toxic sensitivity in the case of 300- 

 mesh dusting sulfur and Sclerotinia americana is well shown in figure 2, 

 which has been compiled from the results of two experiments. The 

 rapid drop in the sulfur-germination curves points to the extreme impor- 

 tance of the age factor and the necessity of knowing these relations when 

 making fungicidal tests. A similar effect of copper-lime dust on the 

 germination of Macrosporium sarcinaeforme spores is shown in table 1. 

 Doubtless this factor applies also under field conditions, hut this point 

 cannot easily he proved. 



