W. Robinson 133 



disposed granules that are invariably seen in the Phytophthora. Conidia 

 are produced in great abundance, are typically four- or five-celled, curved 

 and slightly pointed at the ends (Fig. 24). Although carefully looked for 

 the mycelium of this fungus has never been observed among the living 

 cells of the host plant. On the other hand it is always possible to find 

 the mycelium of the Phytophthora in the higher parts of the diseased 

 regions, that of the Fusarium always being confined to the lower 

 decaying parts of the stem and to the roots. This Fusarium was 

 isolated and grown in pure culture on Beerwort and Quaker Oat agars. 



Experimental Infections. 



A number of infection experiments was carried out on seedling asters. 

 As a preliminary test, from a number of seedlings growing together 

 in a pot one was selected and a small quantity of mycelium of the 

 Phytophthora from a pure culture of the fungus was placed near the 

 collar. After five days the seedling had completely collapsed showing 

 the ordinary symptoms of "damping off." All the other seedlings in 

 the pot remained unaffected for at least 10 days after the experiment. 

 On cutting off the collapsed seedling and placing it in water, hyphae 

 grew out of the tissues and produced typical sporangia in 24 hours. 



For more critical experiments a number of aster seedlings were 

 carefully uprooted, washed free from soil and laid in Petri dishes with 

 the roots in water. Three of these were then inoculated by placing 

 on each of the roots a piece of mycelium from a pure culture on Quaker 

 Oat agar. Controls were placed in Petri dishes with the roots in water 

 without being inoculated. Zoosporangia were abundantly produced on 

 the mycelium and the seedlings became infected, the hyphae having 

 travelled one inch in the tissues of the hypocotyl at the end of nine days. 

 The tissues were browned and the seedlings were beginning to collapse. 

 The controls appeared quite normal at the end of the same period and 

 no fungus was present in the tissues. 



An exactly parallel series of tests with controls was carried out on 

 seedlings of asters, spores from a pure culture of Fusarium (previously 

 isolated from decaying roots of diseased asters) being used for inoculation. 

 After nine days no change was observable and up to the 19th day, when 

 the tests were stopped owing to the complete collapse of the seedlings 

 in the Phytophthora inoculations, the Fusarium series remained healthy 

 and normal. 



Further series of tests were carried out by inoculating seedlings 



