CHAPTER 1 



Early Problems 



Yellows and Virus Diseases of Plants 



At the time Dr. L. O. Kiinkel started his researches at the Institute, 

 "yellows" were very obscure plant diseases. Peach yellows was described in 

 1791 by Judge Richard Peters of Philadelphia. There were violent outbreaks 

 of it; some of. them destroying whole orchards, in 1791, 1806-07, 1817-21, 

 1845-58, 1874-78, 1886-88, and 1920. The symptoms of the disease are as 

 follows: the fruits ripen early mth deep red color of both skin and flesh, 

 and the fiesh is bitter; the leaves are yellowed, rolled, and drooping; the 

 new shoots are thin and wiry, growing upright and bearing narrow yellow 

 leaves; the buds that remain dormant on healthy trees grow prematurely 

 on diseased trees, producing witches' broom effect; the diseased tree be- 

 comes worthless and dies in two to six years. The disease is limited mainly 

 to southeastern Canada and northeastern United States. Some outbreaks 

 have occurred as far south as Texas and as far west as Arkansas and Ne- 

 braska. The disease had been fairly extensively studied by able pathologists, 

 including Penhallow (1882-83) and Erwin F. Smith (1888-94), but neither 

 the causative agent nor the method of transmission had been learned. 



Dr. Ermn F. Smith described aster yellows in 1902 and suggested that it 

 might go to other composites closely related to the China aster. The causa- 

 tive agent and method of transmission of this disease was likewise unkno\vn 

 when Dr. Kunkel started his work on yellows diseases. He had recently re- 

 turned from his research on sugar mosaic, the insect vector for which he had 

 discovered. Dr. Kunkel did not seem enthusiastic about the problem, al- 

 though his later discoveries in the field did much to enhance his already high 

 standing as a plant pathologist. Recently the author once accused him of 

 showing little en husiasm for this new undertaking. His answer was that he 

 never liked to undertake a new problem. Anyone who knows Dr. Kunkel, 

 able and a bit phlegmatic as he is, would know that his lack of enthusiasm 

 did not augur lack of future accomplishment in the problem. Prof. L. R. 

 Jones, a member of the Board of Directors of the Institute, had some 

 doubts of the wisdom of having a scientist undertake such an obscure prob- 

 lem. He might spend many years on it without success. 



Dr. Kunkel started on aster yellows searching for possible insect vectors 

 by the methods he used in discovering the insect vector of sugar cane 

 mosaic. About six weeks after he got the insect isolation cages, he announced 

 that of the several insects feeding commonly on asters, one and only one 



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