20 



James G. Horsfall 



plants. There seems to be little doubt of the ability of the fungus to 

 produce the disease. 



Life history. The complete life history of the fungus has not been 

 worked out. Evidently the sexual stage is unknown as yet. 



Gentner (1918), Krakover (1917:315), and Milburn (1915:72) report 

 the fungus in the seed. It would appear that the occurrence of the fungus 

 within the seed argues for the possibility that primary cycles sometimes 

 are initiated by rain splashing spores from the seed up on young foliage. 

 Krakover (1917) placed soil from an infected field in pots in the green- 

 house. Sterile seed when planted failed to produce infected plants as 

 long as he watered the plants from below; but when he watered them 

 from above with the hose, they immediately became infected, indicating 

 that splashing rain is an important factor. Probably this is the chief 

 inoculating agent, the spores coming either from the diseased seed or the 

 infected soil that constitutes the inoculum. 



The effect of some of the factors governing spore germination has been 

 tested by the writer. 



In studying the effect of temperature on germination, the spores almost 

 invariably have been allowed to remain in each temperature chamber 

 until all the spores germinable at that temperature grow. This gives a 

 flat curve when the results are expressed graphically, but it indicates to 

 what extent spores may be expected to germinate under given temperature 

 conditions. Although it is desirable to learn the minimum time for maxi- 

 mum germination at each temperature, this has not always been done. 



In one experiment conidia were germinated in temperature chambers 

 ranging all the way from 3° to 35° C., with intervals of 6 degrees. Ger- 

 mination after forty-two hours was essentially equal in all the incubators, 

 being from 96 to 99 per cent. The germination at 3° C. was 97.6 per 

 cent and that at 35° C. was 99.2 per cent. Obviously the spores are 

 capable of germinating over a very wide range of temperature. 



TABLE 5. Effect of Time upon Conidial. Germination of Macrosporium Sarcinae- 



FORME AT 3° AND 25° CENTIGRADE 



