2 3 8 



FUNGOUS DISEASES OF PLANTS 



=**% 



formed within the vessels. These conidia, frequently spoken 

 of as microconidia, are at first cylindrical or elliptical, and with- 

 out septa; but they may become slightly curved and once or 

 twice septate. These are capable of germination and growth 

 within the tissues. On the surface of the host and in culture 

 a type of conidia known as macroconidia may be produced in 

 quantity. These are lunulate or crescent-shaped and from three 

 to five times septate, measuring 30-50 X 4-6 fi (Fig. 99, c). Upon 



the host the conidio- 

 phores arise in loose 

 stromatic tufts known 

 as sporodochia. In cul- 

 ture all gradations be- 

 tween the small and 

 large conidia may be 

 observed. Moreover, an 

 oidium-like stage is 

 Fig. 100. Perithecia, Ascus, and sometimes produced, 

 Paraphysis of Nkocosmospora and ■ the race f this 

 b (After Erw. F. Smith) 



fungus on the melon 

 chlamydospores are not uncommon in old cultures. 



The ascus stage of the fungus has been found both 

 on the host plant and in cultures upon steamed potato 

 cylinders and other solid media in which ascospores were 

 sown. In the case of the cowpea fungus the line of 

 culture work so accurately followed out (Smith) has 

 shown conclusively that the perithecia may be devel- 

 oped from both types of conidia and that the perithe- 

 cium is undoubtedly a stage in the development of the wilt 

 fungus. As the fungus shows considerable differences on the 

 different hosts in regard to the ability to produce perithecia, so 

 it shows also a difference in the ease or difficulty with which the 

 ascus stage may be produced in artificial cultures from the conid- 

 ial stage. The perithecium of the fungus is superficial, more 

 or less scattered, flask-shaped (Fig. 100), and frequently orange 

 vermilion in color, measuring about 250-350 X 200— 300 p. The 

 neck may be straight or slightly curved. The ostiolum is closed 

 until a late period with well-differentiated cells, and within the 



