212 



FUNGOUS DISEASES OF PLANTS 



outset as to cause manifest loss in the final crop. Again, diseased 

 plants may entirely recover. 



The surfaces of diseased roots may be roughened and browned 

 by the presence of the fungus, but the tissues within are usually, 

 in the case of violets, peas, etc., tinted red or pink. Ordinarily 

 the fungus penetrates all parts of the rootlet, but as is common 

 with plants which are not vigorous or obligate parasites, there are 

 no abnormal cell divisions of the host. 



Morphology of the fungus. The mycelium is intercellular, 

 abundantly septate, and at first hyaline. The threads are narrow, 



and the branches are cut off by a 

 septum at a slight distance from 

 the main hypha, somewhat as in 

 Rhizoctonia (Corticium vagum). 

 Three kinds of spores have 

 been commonly found, namely, 

 (i) endosporous conidia; (2) thick- 

 walled conidia, .or chlamydo- 

 spores ; and (3) ascospores. 



i. The endospores are an 

 interesting type of spores formed 

 in chains in terminal branches or 

 clusters of branches (Fig. 83, a). 

 These spores are formed by basi- 

 petal septation as short cylindrical 

 cells within the branch. The tip 

 of the branch is finally broken 

 and they are pushed out by os- 

 motic force, the branch assuming the part of a spore case. The 

 endospores are distinctly hyaline, and as produced in artificial cul- 

 tures, they may remain united in short threads, or cohere laterally 

 as small rafts. Individuals measure about 1020 x 4-5 P- 



2 . The chlamydospores are thick walled, more or less cy- 

 lindrical, brown spores, borne in chains, the early stages of for- 

 mation differing apparently only in size from the endospores. 

 At maturity, however, the short chains, or rather the colored 

 spore cells of these, break up, as shown in Fig. 83, b, measuring 

 about 12 fi in width. 



FIG. 83. CONIDIA AND CHLAMYDO- 

 SPORES OF THIELAVIA 



