698 



Journal of Agricultural Research 



Vol. XX, No. 9 



The striking outcome of this comparison is the marked retardation in 

 unsterilized soil extract and the complete inhibition in. onion leaf and 

 onion scale extract. Even when the last two were diluted with 10 parts 

 of water no germination occurred. As pointed out in a previous note 

 by the writer (38), further experiments have shown the presence of at 

 least two distinct substances in onion tissue which are probably respon- 

 sible for inhibition of spore germination. A more detailed study of this 

 phase and its relation to the parasitism of the fungus will be included in 

 another paper. Cooked soil extract, soil decoction, and onion decoction 

 stimulate germination and promote rapid growth of the germ tubes. 

 It is evident that the cooking of the onion scale removes or destroys the 

 substances which are unfavorable for spore germination. 



Relation of temperature. — Since conidia were found to germinate 

 well in distilled water, this medium was used for studies of the effect of 

 temperature on spore germination. A large number of tests were run at 

 a gradation of temperatures ranging from i° to 35 C. Spores were 



found to germinate between the 

 limits of 4 and 32 . Appressoria 

 developed in germination drops 

 throughout the same range of tem- 

 perature. At 35 to 37 slight swell- 

 ing of the spores took place, giving 

 them the appearance of ' ' involution 

 forms," but normal germination 

 did not occur. Figure 5 is a 

 graphic representation of the effect 

 of temperature as indicated by per- 

 centage of conidia germinating in 

 distilled water at 12 hours. Best germination occurred at about 20 , but 

 good germination occurred between 13 and 25 . 



The temperature range for spore germination thus coincides closely 

 with that of fungous growth. The point of optimum development is 

 comparatively high, and this fact is significant in exolaining the occur- 

 rence of the disease in the field. 



Fig. 



so ss 30 



— Relation of temperature to spore germina- 

 tion of Colletotrichum circinans. 



EFFECT OF DESICCATION 



In order to interpret more fully the development of the disease in the 

 field and the overwintering of the causal organism, the effect of desicca- 

 tion on conidia and stromata was studied in the laboratory. 



On conidia. — Studies were made on conidia as they occur (1) in 

 masses on the fruiting body on the host, where they are embedded in 

 the mucilaginous material which surrounds them, (2) in similar masses 

 on potato agar, and (3) in water suspension, where the spores are sepa- 

 rated from one another, approximating to some extent conditions as 



