THE FUNGI WHICH CAUSE PLANT DISEASE 319 





outgrowths appear from the hyphae at various points. One of 

 these assumes a somewhat spherical form and matures to the fertile 

 spore, while the other branch or branches grow around it, Fig. 235, 

 branching and dividing into joints which eventually round off to 

 form the sterile exterior cells. Spores are 

 known to hve in soil for at least twelve years. ^ 

 A period of rest is necessary before they can 

 germinate. In germination the central spore 

 produces a single short hypha, commonly 

 branched, on which the conidia are borne ter- 

 minally and laterally. Fig. 234. Experiments 

 by Thaxter indicate that infection is subter- 

 ranean. 

 U. occulta (Wal.) Rab.i^*' isi 

 Sori in leaves, especially in the sheaths, Fio. 235.— Successive 

 culms and inflorescence, forming linear stria of gor^baiiT^ A° 

 usually of great length and often merged into ^'^ Thaxter. 

 a continuous stratum of dusty, reddish-black, spore-balls; spore- 

 balls oblong to subspherical, 16-32 /i in length; sterile cells 

 often incompletely covering the spores, hyaline or yellowish, sub- 

 spherical to oblong, usually with distended and uniformly thick- 

 ened walls; fertile cells reddish-brown, oblong to subspherical, 

 often flattened, smooth, 1 to 4 in a ball, 11-18 fi in length. 

 On rye wherever cultivated, though not common in America. 

 The seat of spore formation is most often on the stems or sheaths, 

 though all aerial parts of the plant are susceptible. In the vege- 

 tative parts the fimgus is commonly found in 

 the tissue between the vascular bundles. 

 U. viola (Sow.) F. de W.'^'- i«' i«> 

 Sori on stems, rootstocks, petioles and leaves 

 forming prominent irregular swellings often sev- 

 ^'baiis^^oFu^°oc- eral centimeters in length, rather permanently 

 After covered by the host tissues but upon rupture 

 disclosing black-brown spore-masses; spore-balls 

 reddish-brown, rather irregular, oblong to subspherical, chiefly 

 28-55 /i in length; sterile cells yellowish-tinted with age, 6-10 fi 

 in length; fertile cells light reddish-brown, ovoid to spherical or 

 polyhedral, chiefly 4r-8 in a ball, mostly 11-15 m in length. 



cults 

 Thaxter. 



