304 



Journal of A gricultural Research voi. xiii. no. s 



LIFE-HISTORY STUDIES 



The inoculation period for this fungus is usually from three to five 

 days. Infection may occur either on the upper or lower surface of the 

 leaves. In a few cases the writer has observed germ tubes entering 

 through the opening of the stomata. Whether this is the only method 

 of infection was not determined. For observing this phenomenon 

 pieces of inoculated tissue were fixed and stained with eosin or sectioned 

 and stained with Fleming's triple stain. The mycelium penetrates the 

 tissues of the leaf, causing them to collapse somewhat. In from 8 to 12 

 days spore formation is found to be taking place on the lower surface 



of the leaf. The conidiophores 

 arise from mycelium growing 

 on the surface. 



The fungus appears to over- 

 winter as mycelium in the 

 tissues of the host. Whether 

 the spores may also live over 

 a winter has not been defi- 

 nitely determined. Viable 

 spores were obtained from 

 both cultures and leaves that 

 were exposed outside over a 

 winter, but it was impossible 

 to determine whether these 

 spores had overwintered or 

 had been produced in the 

 spring. Spores taken from 

 culture media and placed out- 

 side in paper packets failed to 

 germinate in the spring. 

 Fig. 3.-Myceiiuni of siemphyiium cucurbitacearum: A. The sporcs may be Scattered 



Mycelium from a string-bean-agar culture la days old. ^ wind, rain, inSCCtS, auimals, 

 X 1,050. B-C, Mycelium from spores germinated in -' i- U4-I 



nutrient bouillon, 72 hours old, showing fusions of the or garden tOOlS. When Slightly 



hyphae. Xi,i9o. wcttcd, the spores have a 



tendency to stick together, owing apparently to surface tension phe- 

 nomena. This would allow them to be easily carried by insects or 

 animals. The spores may be easily spattered about by raindrops. To 

 test this point, the writer inoculated the upper leaves of three plants; 

 and as soon as spore formation occurred, the plants were sprayed 

 with water. In a few days infection was general on the lower leaves, 

 while healthy plants sprayed as controls remained free from disease. 

 The spores may also be carried short distances at least by strong winds. 

 The writer has caught spores on the moistened slides several inches from 

 leaves held in the current of an electric fan. 



