342 AlWUAL REPORT SMITHSONIAN INSTITUTION, 1940 



has been made with classification, new cases that arise can often be 

 placed at once in their proper categories, while the inevitable debate 

 that centers around the doubtful cases often leads to the suggestion 

 of new experimental approaches. 



In the case of the fungus and bacterial diseases the problem is 

 primarily one of determining under what circumstances the insect 

 becomes a carrier. The insects whose feeding secretions are re- 

 sponsible for plant diseases present much more difficult problems, but 

 these fall into perhaps two general categories: What is the nature 

 of the secretion and how does it affect the plant's physiology, and, 

 What factors of insect nutrition and plant susceptibility are involved ? 



The trend in virus research is unmistakable. The nature of the 

 virus, whether living or nonliving, will continue to occupy the at- 

 tention of workers for a long time. All viruses may ultimately be 

 classified as nonliving proteins with some of the characteristics of 

 crystallinity, but there is a possibility that the viruses as we now define 

 them may include both living and nonliving things as well as those on 

 the borderline between highly complex chemical molecules and living 

 matter. This is an exciting prospect for those whose privilege it is 

 to pursue knowledge for its own sake and important for those whose 

 services are devoted to the control of virus diseases. The general 

 problem is so complex, however, that workers in very widely separated 

 fields of study are involved. It is a far cry from a farmer's field to 

 the microscope specially designed to reveal the structure of crystals, 

 but workers in these two fields must somewhere get together if the 

 problem is to retain its fundamental unity. The entomologist is in 

 a position to make material contributions to the problem by adding 

 to our knowledge of the relationship between insects and viruses. 



Investigations on control methods will continue to follow as diverse 

 lines as heretofore. Fungus and bacterial diseases will probably be 

 attacked by reducing the numbers of the insect carriers and eliminat- 

 ing sources of infection. The diseases caused by the toxic secretions 

 of insects will continue to be controlled by reducing the numbers of 

 insects, but there is a promising line of attack in learning what factors 

 make a plant susceptible to this sort of injury. It might then be 

 possible to reduce this susceptibility by cultural or other methods. 



Some progress has been made in breeding plants, not only for 

 resistance to organisms and viruses but also to insect attack. This 

 offers promise as a part solution of the problem, but resistance to the 

 virus seems to be the most fruitful line of attack on the immediate 

 problem of how to reduce losses by virus diseases. Protecting plants 

 against viruses by inoculation with other viruses is a fascinating pros- 

 pect even though fraught with many complications. Finally there is 

 the possibility that chemical treatment of plants to protect or cure 

 them of infection may become a reality. 



