POTATO WILT, LEAF-EOLL, AND RELATED DISEASES. 23 



showed this appearance, the question was again investigated with an abundance of 

 material, and it turned out that the discoloration had no connection with the disease, 

 but that it might be produced by the weather conditions during the vegetation period. 

 It was further proved that the discoloration is not present in the most pronounced final 

 stages of the disease. Since the vascular discoloration is frequently associated with 

 the appearance of mycelium, it will be taken up again in the chapter on the causes of 

 leaf-roll. When Spieckermann states, as a characteristic symptom of leaf-roll diseased 

 plants, that the vascular bundles are not discolored, and in particular that there is no 

 yellow color of the vascular ring, he only means that this api^earance can not be uti- 

 lized as a character of the disease, for the discoloration may naturally be present in 

 diseased plants just as in healthy ones. 



HEREDITARY NATURE OF LEAF-ROLL. 



The true leaf-roll is inheritable. The tubers from diseased plants 

 produce diseased progeny as a general rule. This affords a means 

 of distinguishing from genuine leaf-roll those temporary conditions 

 which give rise to a similar appearance of the plants. All those 

 who are best acquainted with the trouble agree as to the results of 

 planting diseased seed stock, though there are different explanations 

 therefor. 



This point is one of capital importance in the control of the disease 

 and of great interest in its bearing on the nature of the disease. It 

 will be further discussed on another page. 



CHEMICAL COMPOSITION OF LEAF-ROLL POTATOES. 



It was early suggested by Sorauer that the leaf-roll potatoes ex- 

 hibited a more active oxidase reaction than the healthy ones. This 

 was determined by Griiss and later more thoroughly by Doby (1911, 

 1912), who proved that leaf -roll potato tubers gave a higher reaction 

 with respect to oxidase, peroxidase, and tyrosinase; also that they 

 had a slightly higher ash content and less starch and protein. 



The full significance of these results is not yet understood. It 

 would seem that katabolism is more rapid in the diseased plants, yet 

 the biochemist could hardly determine by analysis which of the 

 samples given him were healthy and which diseased. 



We hope that more light will be shed on this subject through the 

 early publication of the work of Dr. H. H. Bunzel, of the Bureau of 

 Plant Industry, who m 1912 and again in 1913 has made a study of 

 the leaf-roll material at Houlton, Me., using a method and apparatus 

 designed to give more accurate results than any previously available. 

 (Bunzel, 1912.) 



NECROSIS OF PHLOEM STRANDS. 



It has recently been pointed out by Quanjer (1913) that the physi- 

 ological and structural viewpoint has been neglected by investigators 

 of leaf-roll and that the small attention given has been principally 

 devoted to the xylem, in the search for fungi, rather than to the 

 phloem. 



