450 



FUNGOUS DISEASES OF PLANTS 



Characters of the fungus. The mycelium varies considerably 

 in form, depending upon age, or the conditions under which grown. 

 In diseased tissues where there is abundance of water, or in 

 pure culture, the young hyphae develop branches, which are usually 

 inclined at an acute angle to the direction of growth of the 



parent branch, although subse- 

 quently the two may grow par- 

 allel. The branch is usually 

 somewhat narrowed or con- 

 stricted where united with the 

 main hypha, and a septum is 

 formed at a distance of several 

 micromillimeters from the 

 point of origin of the branch. 

 The hyphae may be almost 

 hyaline when young, but very 

 generally become yellowish 

 brown with age. Furthermore, 

 in age the branches appear to 

 be more at right angles, at 

 least, so far as the origin is 

 concerned. Upon many host 

 plants, and especially when the 

 fungus is grown in pure cul- 

 tures, a short tufted growth of 

 the mycelium may occur. The 

 hyphae of these tufts are brown, 

 closely septate, constricted at 

 the septa, and often branched 

 in an irregular dichotomous 

 fashion (Fig. 222, b). In the 

 latter case the hyphae readily 



break up into short hyphal lengths, consisting of a single cell or 

 more, and these cells are able to germinate within a few hours 

 when placed in fresh nutrient media. Germination is commonly 

 by means of a germ tube protruded from a septum. A germ tube 

 may even, in some cases, pass through a neighboring cell. It would 

 appear that the fruiting stage usually develops upon living plants. 



FIG. 221. RHIZOCTONIA ON BEAN 



STEMS AND PODS. (Photograph by 



M. J. Barrus) 



