316 



BACTERIAL DISEASES OF PLANTS 



also the one who proved the stem-blight known as black-arm 

 and the angular leaf-spot to be due to the same organism (Wash- 

 ington hothouse inoculations of 1905). The inclusion of black- 

 arm in Mr. Orton's account of the bacterial cotton-blight 

 ("Sea Island Cotton: Its Culture, Improvement and Diseases," 

 U. S. Dept. Agric, Farmers' Bulletin 302, 1907, pp. 41-42) 

 was due to these experiments, some of which he had seen. 



The first indications of the disease are minute water-soaked 

 spots. On the half -grown leaves these spots are chiefly between 

 the veins and more or less hmited by the latter, the result as 



Fig. 240. — Cotton leaves inoculated in spring of 1915, using a pure culture 

 plated from the lower leaf of Fig. 239. Time, 32 days, cool house. Stomatal 

 infection; veins also invaded. 



they enlarge being square or variously angled small spots (Fig. 

 238) which are translucent at first, then brown. These spots 

 by their number and their one-sided enlargement and coa- 

 lescence may seriously injure the attacked leaves, greatly reduc- 

 ing the amount of assimilating surface and causing them to 

 become yellow and to fall early. The bracts and the veins or ribs 

 of the leaf are also subject (Fig. 239). Contrary to Atkinson's 

 supposition, infection often takes place very early, i.e., when the 



