14 GROWTH OF PLANTS 



mosaic, and rosette mosaic. Some of these diseases were first described 

 before Kunkel discovered M. trimaculata as the vector of peach yellows but 

 more of them have been described since. Much research will be required 

 to determine the natural vectors of these diseases and determine their inter- 

 relations with each other. There is also much to learn about many similar 

 diseases of other stone fruits of the genus Prunus. 



Peach and aster yellows have been described as virus diseases, but are we 

 sure that this is so? Certain it is that we have not proved that they are 

 caused by filterable viruses, as is tobacco mosaic, for we cannot transmit 

 them mechanically by transfer of unfiltered juice from diseased to healthy 

 plants, let alone by the transfer of filtered juice. Such facts as incubation 

 periods in the insect vector, transmission by blending of diseased and 

 healthy tissue (grafting or budding) and not by mere transfer of juice and 

 the extreme heat lability of peach yellows ought to lead to open-mi ndedness 

 on the nature of the inoculum. 



Dr. Kunkel early associated with himself a number of workers of diverse 

 training to work on several of the phases of the yellows and virus disease 

 problems. There were two entomologists. Some of Dr. Hartzell's work on 

 yellows diseases is described above. He has also shown that peach yellows 

 is not transmitted by pollen as was already known for seeds. Dr. Irene D. 

 Dobroscky made a cytological study of the salivary glands and alimentary 

 tract of aster yellows-infected Cicadula sexnotata and found no evidence of 

 the presence of organisms or other visible changes. She also showed that 

 cranberry false blossom was transmitted by Euscelis striatulus Fallen but 

 not by some other insects feeding on cranberries or by mechanical means. 

 She found no cytological changes in the glands and alimentary tract of 

 viruliferous insects and suggested control of the disease by spraying to con- 

 trol the vector. Dr. F. 0. Holmes, a protozoologist, studied the cytology of 

 intracellular bodies of Hippeastrum mosaic, the Herpetomonas bancrofti in 

 latex of Ficus, movement of tobacco mosaic ^^dthin the plant, and several 

 other phases of tobacco mosaic. He also attempted to photograph virus by 

 use of ultraviolet light. Dr. Helen Purdy Beale, a plant pathologist, at- 

 tempted the culture of organisms from tomato mosaic, found that slugs 

 do not transmit tobacco mosaic, and studied the multiplication of tobacco 

 mosaic in isolated leaves. She discovered a crystalline form of virus in the 

 tobacco leaf. She also made an extensive study of serum reactions of viruses 

 which developed a new technique for plant virus research. Dr. W. C. Price 

 showed that tobacco plants acquired immunity to the ring spot disease 

 and made a study of local lesions in the bean leaf inoculated with tobacco 

 mosaic. Dr. C. G. Vinson and Mr. A. W. Petre, biochemists, isolated, 

 purified, and crystallized tobacco mosaic virus. They concluded that it 

 acted like a chemical rather than an organism. When heated, the juice con- 

 taining the virus gives a precipitate at 85° C (185° F), also another precipi- 

 tate at 90° C (194° F). Only the first contains an appreciable amount of 

 nitrogen, and the juice shows little virus after removal of the first precipitate. 



