The Anthracnose Disease of the Raspberry 



173 



The growth of the fungus in culture is very peculiar. Miss Stoneman 

 (1898) states that several isolations were made before she was satisfied 

 that she had obtained the desired organism. The conidia germinate on 

 various kinds of agar within a period of from three to twelve hours (fig. 

 18). From a single conidium one or more germ tubes are developed, 

 which at first grow rapidly. Septa are formed, the cells become swollen, 

 and soon the conidium is indistinguishable from the cells of the mycelium. 

 In tap or rain water, growth ceases at this stage. No appressoria have 

 been noticed. On potato or nutrient agar, the primary germ tubes branch, 

 and their cells, which are globose, form a more or less compact mass. The 

 colony is circular in outline and as the mycelium grows older the color 

 changes to a pale vinaceous pink. Hyphae radiate in all directions from 

 the fungous mass but only for 

 an exceedingly short distance. 

 On media containing a small 

 amount of agar a more nutri- 

 tive growth occurs, the hyphae 

 of which are filamentous, with 

 occasional globose cells. 



The aerial growth of this fun- 

 gus in culture is comparatively 

 slow, and continues as a piling- 

 up of cells forming a wrinkled 

 mass having the appearance of 

 sclerotia (fig. ig). In some cases 

 this mass has a shining appear- 

 ance, while in others fine aerial 

 hyphae are formed over the 

 compact growth. The color of the culture varies from light russet- 

 vinaceous to maroon, while the edges of the colonies are lighter in color due 

 to the younger mycelium. The cells of the hyphae in the young colonies 

 are very minute but become somewhat larger as they grow older. The 

 diameter of the filamentous hyphae is from 1.5 to 4.5 ju, while frequently 

 a globose cell measures as much as 14 /x (fig. 20). 



When the organism is cultured on potato agar containing glucose, 

 the growth is much more rapid and aerial mycelium is formed which 

 gives a pale brown or white downy appearance to the sclerotial growth. 

 With the addition of glucose there is also a reduction, or at least a retarda- 

 tion, in the intensity of color production. On potato agar containing five 

 per cent of glucose, the culture is pale brown with the characteristic red 

 color developing next to the medium. On cornmeal agar the fungus has 

 a brilliant red appearance, while on sterilized bean oods the growth is 



^83 



Fig. 19. CULTURE of plectodiscella ven- 



ETA ON POTATO AGAR CONTAINING I PER 

 CENT OF GLUCOSE 



