142 



THE FUNGI WHICH CAUSE PLANT DISEASE 



Fig. 97. — Mycelium showing septation and 

 branching. After Stevens and Hall. 



the host's tissue the hyphal threads are thicker, richer in proto- 

 plasm, more septate, and much more branched and crooked 

 than outside of the host. Aerial hyphal filaments when they 

 touch a solid repeatedly branch in close compact fashion form- 

 ing the attachment organs. 



At the exhaustion of the food supply and the consequent term- 

 ination of the vegetative period the mycelium becomes very dense 

 in spots and within these clumps of mycelium the sclerotium 

 forms; at first white, later pink, finally smooth and black (Fig. 95). 

 They are often found in the leaf axils (lettuce), in the pith of 



stems (carrot), etc. Under 

 some conditions, as on un- 

 suitable nutrient media, 

 gonidia are produced. 



The sclerotia can ger- 

 minate at once or remain 

 dormant for one, perhaps 

 several years. On ger- 

 mination they send forth 

 from 1 to 35'* negatively geotropic sprouts which grow to the 

 soil surface unless that be more than about 5 cm. distant. On 

 reaching the light the apex of the sprout begins to thicken and 

 soon develops its apothecium; at first inverted- 

 conidial, soon flat, and finally somewhat revo- 

 lute. Changes in atmospheric humidity cause 

 the discharge of ascospores in white clouds. 



The ascospores germinate readily but the re- 

 sulting mycelium is of such small vigor that it 

 is incapable of parasitism. If the ascospore 

 germinates where it can maintain a saprophytic Fio. 98.— s. Uber- 

 lif e until a vigorous mycelium is developed then piraphys^g."' After 

 the njycelium may become parasitic. Stevens and HaU. 



Both a^cospiores and mycelium are comparatively short-lived. 

 The mycehum can migrate but a short distance over soil. No 

 form of conidia except the apparently functionless gonidia is 

 produced. The fuii^s may be cultivated easily upon ahnost any 

 medium, com-meal-agar is especially suitable. 

 It has been repeatedlyNclaimed that this fungus possesses a 



