FUNGOID DISEASES OF FOTATOKS 



// 



Cause. — Examination of the above felted Lrrowth, 

 microscopically, shows it to be a plantation-like 

 growth of the funous PhytophtJiora infest ans, bear- 

 ing conidia at the tips and sides of the branches 

 (Fig. 2 1, B). These conidia are easily detached 

 and carried away by the wind ; if they fall on a 

 dry surface no further development takes [)lace. 

 Should they fall on the moist surface of the leaf of 



B C D 



Fig. 22. — " Potato Disease" [Pliytophthora infcstans) : (A) A conidium 

 with part of conidiophore (c) attached ; (13) conidium gerniiiiating, 

 having produced a germ tube [t) ; (C) a sporangium showing the 

 division into cells; (D) a sporangium germinating, some of the zoo- 

 spores [z) are already liberated ; notice the cilia or hairs (//) bv means 

 of which they swim about. 



a potato plant, their protoplasm divides, and five to 

 ten zoospores are formed which subsequently be- 

 come liberated. Each zoospore consists ot a 

 minute speck of protoplasm ; bearing two cilia (hairs) 

 by means of which it is able to swim for a time. 

 It soon comes to rest, the cilia are resorbed, and 

 germination commences (see Fig. 22, A, B, C, and 

 D). The germ tube produced either enters a stoma, 

 or digests and penetrates the cell wall ot the leat or 

 stem on which it is found. 



