46r PROTECTION OF PLANTS, 19)6-17 



ones of the same size. The resulting plants certainly are different for they 

 show in a more advanced stage the disease which affected the parent plant. 

 But it is not always possible to differentiate the one from the other. Mosaic 

 in advanced stages may have the appearance of curly dwarf. Leaf-roll 

 however, usually has rather distinct symptoms. The reduction in yield 

 in such advanced stages is very marked. The few tubers usually produced 

 are often no larger than hickory nuts. It is not definitely known whether 

 these diseases are contagious, and, if they were not there would be no further 

 danger from such badly affected plants as the tubers are too small to be 

 used for seed purposes. Quanjer^ believes that leaf roll is contagious to a 

 certain extent and that the soil in which affected plants are grown can 

 become contaminated by the virus so that healthy tubers planted there may 

 produce diseased plants as a result of this soil contamination. 



Evidently elimination is the only method of control. Even that can- 

 not be entirely depended upon, for apparently health}', high -yielding 

 strains have somietimes produced affected individuals in later plantings. 

 Professor F. C. Stewart^ has given a number of instances of this kind and 

 the writer observed a similar case in northern Vermont in 1916. An intelli- 

 gent grower, having secured after several years selection from high-yielding 

 strains a quantity of potatoes which he intended to place on the seed market, 

 found to his disappointment that a liarge proportion of them (over 30%) 

 showed curly dwarf. He said that he had found a small percentage of such 

 plants the preceding year but had rogued them out. It is probable that 

 the plants originally selected contained the elements of this disease or were 

 latently affected and, although the progeny produced well for several years, 

 the break was certain to come. Such an experience, however, tends to 

 discourage further efforts toward seed improvement. Quanjer^ also states 

 that selection cannot be depended upon to reduce the extent of leaf roll. 



In New York State, leaf roll and curly dwarf seem not to be very pre- 

 valent. Mosaic, on the other hand, is very commonly met with on white 

 sprout potatoes, but the writer has never observed it on the blue sprout 

 varieties. Plants may become very much dwarfed by it and not uncom- 

 monly the vines of rather large plants become more or less prostrate. 

 Other plants may show the mottling plainly but otherwise are as large and 

 produce as well as neighboring healthy vines. Then there are plants so 



^ Quanjer, Dr. H. M., Lek, H.A.A. van der, and Botjes, J. Oortwijn, Nature, mode of 

 dissemination and control of phloem-necrosis (leaf-roll) and related diseases i.a. Sereh. 

 Reprint from "Mededeelingen von de Rijks Hoogere Land-Tuinan Boschboun school" 

 10:84-138, English translation, pis. 1-12, 1916. 



^ Stewart, F. C. Observations on some degenerate strains of potatoes. New York 

 (Geneva) Agr. Exp. Sta. Bui. 422:319-357, pis. 1-12, 1-916. 

 »L.c. p. 109-111. 



