POTATO WILT, LEAF-EOLL, AND RELATED DISEASES. 21 



planted were still firm and sound at harvest time and that they were 

 even larger in size. This observation has been verified by others, 

 and the endurance of the seed tuber is considered by Appel and 

 Schlumberger to be one of the symptoms of leaf-roll, though they 

 point out that this varies greatly on different soils. They further 

 show that the enlargement of the seed piece is not a symptom of 

 disease, but that the same thing occurs with healthy plants. The 

 significance of these observations relative to the endurance of the 

 seed piece has not been made fully clear, though the discovery of 

 Quanjer that the phloem strands in the stems of leaf-roll plants are 

 shrunken and lignified suggests that the seed piece remains because it 

 can not be used up by the plant. Its endurance argues against the 

 relation of parasites to leaf-roll, since if the disease were caused 

 by fungi or bacteria at the root, one would reasonably expect the seed 

 piece to be decayed. The writer is unable to state whether it is the 

 case also in the United States that the seed piece endures longer 

 with leaf -rolled than with healthy plants. Certainly sound hills 

 are to be found with sound seed, and many leaf -rolled plants have been 

 dug whose seed piece had decayed. According to the writer's 

 observation, sound mother tubers are to be found as often in the 

 curly-dwarf disease as with leaf-roll. 



Tlie effect of leaf -roll on the tubers is strongly marked. In general,, 

 the yield is very much reduced (PL XIII, fig. 1) Appel (1907) states: 



The growing apprehensions find their confirmations at the harvest. The diseased 

 hills have numerous tubers very much smaller than normal, so that the yield is only 

 about half that of a healthy field. If one uses these potatoes again for seed, the greater 

 part fail to develop, and an uneven stand is the result. Others only germinate without 

 sending their shoots through the earth, but branch below ground and form a consider- 

 able number of roots, so that frequently the seed tuber lies in a more or less thick tangle 

 of roots and thin shoots. Stronger tubers succeed in growing, but the stem remains 

 weak, the leaves are from the beginning considerably rolled, and, according to the 

 variety, more or less colored. These colors are in this stage of all gradations from dark 

 red to blue-red. Few or no tubers are found in such hills, so that a complete crop 

 failure results. 



Appel and Schlumberger (1911) say: 



Badly diseased plants have often no tubers or only a few. When there is a setting of 

 tubers, these are almost always very small. They are borne frequently on shortened 

 stolons, or clustered on the underground part of the stem. This shortening of the 

 stolons is occasionally very commonly observed, as for example, in 1908, in Eisgrub 

 with the varieties Eduard Lefort and Long Six Weeks. This character is, however, - 

 not constant. More often hills occur where the stolons are normally developed but 

 bear a great number of small tubers the size of hazelnuts. 



A striking circumstance which must here be given special attention is that slightly 

 diseased hills ujider certain conditions give an exceptionally high yield, which, how- 

 ever, falls rapidly in succeeding years. Such an example was afforded by the variety 

 Modell dimng its cultivation in Grobzig. This sort had just come in 1907 from the 

 breeder, and had been distributed by the German Potato-Culture Station to its experi- 

 mental fields. It showed itself to be diseased on all fields, but, notwithstanding this. 



