THE OCCURRENCE OF PROTOZOA IN PLANTS AFFECTED WITH 

 MOSAIC AND RELATED DISEASES* 



Technical Bulletin No. 58 



RAY NELSON 



GENERAL CONSIDERATIONS 



The mosaic diseases of plants have occupied a prominent place in the field of 

 phytopathological research for many years. Their increasing economic im- 

 portance and wide distribution have attracted the attention of more and more 

 workers in this country and abroad, and today they constitute a major project 

 for investigation in most of the institutions where botanical research is 

 carried on. For several cultivated crops the mosaic diseases of the infectious 

 type are the most serious diseases known. Much progress has been made 

 along the lines of inter-relationships, modes of transmission, overwinter- 

 ing, etc., but, despite the large amount of work done, the etiology of not 

 one of the group has advanced much beyond the realm of speculation. 

 The enzyme theory has its followers, and there are those who lean toward 

 a belief in the bacterial nature of the causal agents, but apparently the 

 concensus of opinion among plant pathologists is, that the mosaic 

 diseases are analogous to certain animal disorders caused by the so-called 

 filterable viruses. Probably no single factor has so retarded research into the 

 nature of the etiological factor of these diseases as the general behef that the 

 organisms are so small that they can pass through filters, the diameter of 

 whose pores is smaller than the visual magnifying powers of the micro- 

 scope. While the trend of evidence has seemed to indicate that the par- 

 ticles of infectious material in the form that they are extracted from the 

 tissues of diseased plants are very small, there is little justification for the 

 conclusion that these filterable forms represent the only ones capable of 

 producing the disease. The extreme polymorphism that is characteristic of 

 the Hfe-cj^cles of many of the protozoa and our lack of knowledge concerning 

 filtration through pressure and suction filters, open for the investigator in 

 mosaic diseases fields for research which have as yet been unexplored. 



Recently interest has been aroused through the work of Matz (l)t, Kunkel 

 (2), and Palm (3), who respectively, have found foreign bodies in the cells 

 of sugar cane, corn, and tobacco plants affected with mosaic. Although no 

 definite organisms have been demonstrated in the tissues of any plant affected 

 with mosaic or the closely related diseases, the work in progress indicates a 

 general belief by many workers in the occurrence of such a factor. 



•The writer is greatly indebted to Dr. G. H. Coons for very valuable suggestions given during the 

 course of this investigation. He also acknowledges the helpful criticism of Dr. E. A. Bessey and his 

 aid in the interpretation of the slides. 



tThe number in parenthesis refers to the number of the article in the bibliography on page — of this 

 bulletin. 



