112 



James G. Horsfall 



TABLE 30. Effect of Time on Conidial Germination of Helminthosporium Vagans 



at 25° C. 



Hours elapsed 



Total number 

 of spores 



Per cent of 

 germination 





5. 

 7. 

 8? 

 11! 



21 1 



20.5 

 58.7 

 75.7 

 90.2 

 97.4 



The first visible hydrotic symptom may be observed within four days 

 in the greenhouse. At the end of a week following inoculation, the typical 

 symptoms are well developed. 



Since spores are found in greatest abundance on severely diseased or 

 dead material, the fungus probably pursues a distinct saprogenic existence, 

 although little is known about it. 



Many secondary cycles may occur in one season, because only from one 

 to two weeks need elapse between the generations of spores. 





Control 



All the studies on control have been directed toward testing copper- 

 and sulfur-dust protectants. 



The effect of sulfur upon the germination of the spores of the pathogene 

 was tested in three experiments and the effect of copper-lime dust in four, 

 each of which was made in duplicate. The data from these experiments 

 are summarized in table 31. 



TABLE 31. Summary of the Effect of Copper and Sulpur Dusts on Conidial Ger- 

 mination of Helminthosporium Vagans 



Treatment 



Per cent of 

 germination 



Check 



7 . 2-per-cent copper-lime dust . 

 3.6-per-cent copper-lime dust. 

 Bank's colloidal sulfur dust. . . 



Kolodust 



300-mesh dusting sulfur 



Even the most toxic form of sulfur, Bank's colloidal sulfur undiluted, 

 had little or no effect in inhibiting germination. Copper-lime dust, how- 

 ever, especially when used at full strength, seems to be promising for field 

 experiments. Even the germ tubes that did emerge are abnormal, gnarled, 



