DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 



559 



black or very dark brown at first, but changing to a lighter shade, as the 

 spots grow older. The first diseased part of the leaf looks as if water- 

 logged, and in a few days, the diseased part of the leaf peripheral to the 

 central spot fades, or bleaches, to a yellow, or grayish-white. Here 

 the disease may stop and the plants recover, the diseased areas separate 

 from the healthy tissue and fall out leaving holes in the leaves. The 

 disease may spread, however, until the whole leaf is destroyed. 



Fig. 203. — -Violet leaves affected with leaf-spot {Alternaria viola). {Photo, by 

 Heald, F. D. and Wolf, F. A., Bull. 135 (,Sci. Ser. 14), Univ. of Tex., Nov. 15, 

 1909.) 



The majority of the spots are free from fungous spores except under 

 conditions favorable to their development. Some spots produce spores 

 in abundance, especially upon the central, or older portions of the spots. 

 The spores are borne in chains on dark brownish hyphae that arise 

 from the diseased surface. The conidiospores are clavately flask- 

 shaped, muriform, strongly constricted at the septa, which are variable 



