THE FUNGI WHICH CAUSE PLANT DISEASE 



309 



when the tissues are soft and actively growing; and that infection 

 is local on the host.^^ 



It is now known that the chlamydospores are capable of^ger- 

 mination without hibernation and that they remain viable one, 

 two, perhaps more 

 years. It was shown 

 by Brefeld in 1895 

 that the chlamydo- 

 spores produce conidia 

 in the air freely. It 

 is these, air-borne, 

 arising from spores 

 on the groimd, ma- 

 nure, etc., which are 

 chiefly responsible for 

 infection. They must 

 reach the plant on a 

 susceptible part and 

 under suitable con- 

 ditions of moisture. 

 The germ tubes from 

 the conidia penetrate 

 the epidermis, grow 

 through or between 

 the cells. Fig. 223, 

 with an irregular my- 

 celium which branches 

 profusely and calls forth great hypertrophy of the surrounding 

 host tissue. In sporing, the mycelium forms a great number of 

 short, slender, irregular branches which make up a close tangled 

 network in the diseased tissue. These slender branches swell, 

 gelatinize, and portions of them round off as spores. Fig. 223. 



U. striaeformis (West.) Niess.^*' ^^^ 



Sori in leaves, sheaths and rarely in the inflorescence, from short 

 to linear, often extending, apparently by terminal fusion, for 

 several centimeters, also occasionally fusing laterally to cover most 

 of the leaf; at first covered by the epidermis but this is soon rup- 

 tured and dusty brown to black, linear masses of spores become 



Fig. 224. — U. zeae. 1, germination after three days in 

 water; 2, similar but in air showing air sporidia. 

 After Clmton. 



