TORREYA 



Vol. 43 December 1943 No. 2 



Viruses in Relation to the Growth of Plants* 



L. O. KUNKEL 



About twenty-four years ago, Nishimura (10) reported that Physalls 

 alkekengi allowed the tobacco mosaic virus to multiply within its tissues but 

 showed no symptoms of disease. Since that time other masked carriers of 

 plant viruses have been studied (2, 3, 9). We now know that practically all 

 potatoes produced in this country carry the X virus but that, unless it occurs 

 in combination with some other potato virus, no well defined symptoms are 

 produced (3). We also know that some of the mutants of ordinary tobacco 

 mosaic virus cause no obvious symptoms in tobacco (2). But, while it is true 

 that some viruses multiply in some plants without causing symptoms by which 

 a disease can be readily recognized, it is doubtful whether there are any really 

 symptomless carriers. All viruses that become systemic and multiply within 

 a plant probably cause some injury. However, the injury may be slight and 

 easily overlooked unless control plants are available for comparison. Some 

 virologists have gone so far as to suggest that there may be viruses capable 

 of stimulating rate of growth in plants, but if such viruses exist they have not 

 been discovered. 



From viruses that cause exceedingly mild diseases, it is possible to pass 

 by gradual steps to viruses that are lethal. We may, in fact, do this without 

 going outside of the tobacco mosaic virus group. When masked strains of 

 tobacco mosaic virus are propagated in tobacco, they are sooner or later re- 

 placed by mild mottling strains some of which approach ordinary tobacco 

 mosaic virus in severity. Similarly, when severe strains are propagated in 

 tobacco, they are replaced by milder strains some of which approach tobacco 

 mosaic virus in mildness. All except the so-called masked virus strains cause 

 marked stunting and other symptoms of disease. The masked virus strains 

 cause stunting but no other well marked symptoms. Thus, the tobacco mosaic 

 viruses and all other plant viruses may be classified as growth-depressing en- 

 tities. This, however, does not mean that they depress rate of growth in all 

 tissues. 



The Fiji disease virus of sugarcane causes well marked galls in phloem 

 tissues (4). The cranberry false blossom virus, with which the writer has 



* Read at the 75th Anniversary Celebration of the Torrey Botanical Club at the Boyce 

 Thompson Institute for Plant Research, Inc., Wednesday, June 24, 1942. 



ToRREYA for December (Vol. 43, 87-183) was issued February 10, 1944. 



87 



