274 ANNUAL, REPORTS OF DEPARTMENT OF AGRICULTURE. 



carried on principally at Aroostook Farm, Presque Isle, Me. ; Holm- 

 del, N. J.; Virginia Truck Experiment Station, Norfolk, Va. ; 

 Spooner Branch Station, Spooner, Wis. ; Louisiana Agricultural Ex- 

 periment Station, Baton Kouge, La. ; the substation at Troup, Tex. : 

 Oklahoma Agricultural Experiment Station, Stillwater, Okla. ; Ar- 

 kansas Agricultural Experiment Station, Fayetteville, Ark.; and 

 at Eagle, Colo. Very gratifying results have been obtained in the 

 maintenance of high vegetative vigor and yield in members of the 

 Rural group in the long-time growing tests under irrigation at the 

 Greeley station; in fact, the potatoes which are being produced as a 

 result of these tests are eagerly sought by growers for seed purposes. 

 In addition to the segregation of the healthy high-producing strains 

 of seed in several of the important potato-producing regions stock 

 of valuable strains has been secured from high-yielding disease- 

 free seed gi'own in Canada. 



Potato diseases. — An important development in the investigations 

 of the past year has been the differentiation of another distinct type 

 of the so-called degeneration disease of potatoes, namely, spindling- 

 tuber, which, like mosaic and leaf-roll, is transmitted from season 

 to season by seed tubers and from plant to plant by aphids. Here- 

 tofore this type of trouble has been designated as " running out " or 

 " running long " and ascribed to senescence due to prolonged asexual 

 propagation or to unfavorable soil, climatic, or cultural conditions. 

 It occurs in the leading commercial varieties and spreads about as 

 readily as mosaic. Besides reducing the yield about 50 per cent, the 

 disease affects very materially the marketability and apparently the 

 culinary quality of the tubers and is therefore a worse trouble than 

 mosaic or leaf-roll. Control measures apparently will be the same as 

 for mosaic and leaf-roll. 



Further studies of mosaic have shown that there are three distinct 

 types (mild mosaic, rugose mosaic, and leaf -rolling mosaic) and 

 that fertilizers high in nitrogen and potash mask the mottling symp- 

 tom somewhat but do not reduce the amount of the disease. Con- 

 trol experiments have shown that the percentage of healthy plants 

 increases as the distance from diseased plants increases and that the 

 percentage of infection fluctuates seasonally and regionally with 

 aphid infestation. 



Further observations on the relation of leaf-roll and net necrosis 

 have confirmed previous conclusions that net-necrosis tubers pro- 

 duce leaf-roll plants but that not all leaf-roll plants develop from 

 tubers with net necrosis. This disease is apparently much less gen- 

 eral in seed-growing sections in the East than mosaic or spindling- 

 tuber. Besides these three distinct types of virus diseases there are 

 combinations of two or more of these types which are also trans- 

 mitted by aphids. 



SWEET POTATOES. 



The variety collection of sweet potatoes which has been main- 

 tained by the department for many years is being used as the basis 

 of careful selection and development work, with the idea of produc- 

 ing high-yielding strains of all of the important commercial varieties 

 cultivated both in the northern and southern portions of the sweet- 

 potato producing area. The selection work which has been carried 

 on in connection with the variety collection has resulted in the devel- 



