I 



POTATO WILT, LEAF-KOLL, AND RELATED DISEASES. 33 



scribed, and the effect on the plant is the same, though possibly in 

 the western cases there have been more pronounced abnormalities 

 in stolon and tuber formation than are described in the German 

 literature. These effects are illustrated in Plate VIII, figure 2, and 

 Plate IX, figure 2, which show the numerous stolons, often thick 

 and white, bearing many small tubers, frequently strung along like 

 beads. The few tubers which attain any size are generally clustered 

 around, the base of the stem, as in Plate IX, figure 2, This clustering 

 is characteristic of leaf -roll. Kornauth and Reitmair (1909) say: 

 ''The stolons are greatly shortened. Many times the tubers are 

 attached directly to the stem." 



AERIAL TUBERS. 



Aerial tubers are very frequent, and there is often a thickening of 

 the upper stem and leaf petioles which seems to be another result 

 of the plant's efforts to store starch above ground. (PI. IX, fig. 1.) 

 This is a distinct phenomenon from the formation of aerial tubers 

 due to lesions on the stem caused by Rhizoctonia, for the leaf-roll 

 cases show no trace of fungous injury. Neither of these characters 

 is constant, however. Mr. Fritz Knorr informs us that ''in 1911 the 

 greater percentage of the plants took on this stoloniferous character 

 and a smaller portion developed the aerial tubers; this year (1912) the 

 reverse was the case. We had but few of the stoloniferous plants 

 and very many of the aerial tubers." 



These are reactions of the plant to the abnormal physiological 

 conditions accompanying the leaf -roll, which are in turn influenced 

 more or less by moisture and food supply and by weather factors. It 

 is easy to understand how aerial tubers are produced by the fungus 

 Rhizoctonia, which causes lesions on the stem near the soU line and 

 thus prevents the translocation of starch from leaves to tubers, for 

 we can produce the same result by a mechanical injury, i. e., "gird- 

 ling" the stem or by rooting a cutting from a potato shoot in such a 

 manner that no node is covered by soil and stolons can not, in conse- 

 quence, be formed. 



In those leaf-roll diseased plants which form aerial tubers there 

 are no below-ground fungus lesions, and some other force, such as the 

 phloem shrinkage described by Quanjer, must be acting to hinder the 

 storing of starch in the tubers. 



There is evidence, as mentioned in the paragraph on the relation 

 of enzyms to leaf-roll, which suggests that there may be unusual 

 katabolic activities going on in the diseased plants, which would 

 consume the carbohydrates formed in photosynthesis, leaving little 

 or none to be laid by in the tubers during the period of leaf-roll 

 prevalence. If, at a later date, under the influence of favorable 

 ,• weather, for example, an excess of starch was again formed in the 



