24 BULLETIN 64, U. S. DEPARTMENT OF AGRICULTURE. 



Tliis author finds that the phloem strands of leaf-roll plants are 

 shrimken and the walls thickened and Hgnified, resultmg in such a 

 disorganized condition that the translocation of elaborated food mate- 

 rials from the leaves to the tubers for storage is prevented or inter- 

 fered with. 



This shrinkage of the phloem strands can be detected before exter- 

 nal signs of leaf-roll appear, but it is not present in false leaf-roll due 

 to mechanical injury, wet soil, bacteria, overfertilizing with kainit, 

 etc., nor in the curly-dwarf disease. It is discoverable first after the 

 young shoot from a diseased tuber has broken through the ground 

 and formed several leaves. Each new branch is in the beginning 

 healthy, but the diseased condition soon manifests itself. It can be 

 traced upward, as the plant grows, to the tips of mature diseased 

 shoots, and even to the petioles and midribs of the leaves and to the 

 flower stems, but not on lateral leaf veins. The same pathological 

 condition can be traced downward in the underground portion of the 

 stem to the mother tuber, but it rarely appears in the stolons and 

 never in the young tubers. 



This shrinkage of the phloem affords an explanation of many of 

 the results of leaf-roU, including the thickened stems and formation 

 of aerial tubers, which takes place when the products of photosyn- 

 thesis can not be translocated. It may be connected with the 

 "endurance of the mother tuber'' and with the higher percentage 

 of nitrogen in the latter, since these compounds can not move so 

 freely in the shrunken phloem. The reduction of growth and the 

 lessened yield are attributable to the same cause. The roUing of the 

 leaves is a natural reaction of the plant to a stem injury or stoppage. 



The observations of Quanjer led him to the conclusion that leaf- 

 roll is hereditary and not parasitic and that the presence of fungous 

 mycelium, bacteria, tyloses, and vascular discoloration are not char- 

 acteristic symptoms of the disease. 



It is left undetermined how this phloem shrinkage is brought about. 

 It has not yet been produced experimentally, nor have remedial 

 measures been foimd, but the need is emphasized, as pointed out by 

 Sorauer (1913), for more experimental work, under controlled condi- 

 tions, on the influence of the several natural environmental factors 

 on the potato plant. 



NONCOMMUNICABILITY OF LEAF-ROLL. 



That leaf-roll is not communicable from diseased to healthy plants 

 is the conclusion to be drawn from all available evidence. Appel, 

 Werth, and Schlumbergcr (1910) report grafting a great number of 

 diseased sprouts on healthy ones and vice versa. These were put in 

 the greenhouse and union took place. The scions gradually died, 

 however, after the plants were brought into the open air. During 



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