THE VIRUS DISEASES OF PLANTS 429 



during very hot weather show no symptoms. Total suppres- 

 sion of potato mosaic as a result of cHmate has been found to 

 occur in parts of Canada,^ so that affected plants give a practic- 

 ally normal yield, though they still retain the virus in an infec- 

 tive condition. Thus, under certain conditions the potato may 

 act as a " carrier " of its own mosaic. In curly top of beet the 

 interval after feeding on a diseased plant during which the 

 insect is not infective is shorter at high temperatures than at 

 low. 



The influence of soil and fertilisers is marked chiefl}'' in the 

 direction that in good soils, and especially with heavy manuring, 

 the symptoms may be masked and good yields obtained.* 

 Peach yellows, spike disease of sandalwood, and pecan rosette 

 are amongst the diseases in which the soil factor may be of 

 importance. 



Light may also have an influence on the development of 

 symptoms. Baur showed that the contagium of the infectious 

 chlorosis of Abutilon may be destroyed by growing the plants 

 in darkness, and Chapman that tobacco plants exposed to blue 

 light showed little symptom of mosaic, though the virus was 

 still present. 



General Observations 



The fact that disease can be caused by a filterable virus was 

 first demonstrated in a plant disease — tobacco mosaic. Human 

 pathologists have, however, advanced the study of the possible 

 causative agents beyond the stage reached by plant patholo- 

 gists. Only two observers claim to have detected a possible 

 organism in the cells of infected tissues in plants. In certain 

 cells of necrotic areas in the pith of the maize stem and in the 

 cells of thechlorotic parts of the leaves of maize and Hippeastrum 

 Kunkel ' has described bodies which he compares with the 

 Negri bodies of rabies and with Cytorydes variolce. Some of his 

 figures may perhaps remind one of certain appearances of 

 Rickettsia. Palm * in Java has just reported the presence in 

 the cells of tobacco plants affected with mosaic of bodies agree- 

 ing in every respect with Lipschiitz's Strongyloplasma or the 



1 Murphy, P. A., Investigations of Potato Diseases, Canada Dept. of 

 Agric. Bull., 44, 1921. 



2 Perret, C., Sur les maladies des pommes de terres, Ann. des Epiphyties, 

 vii, p. 304, 1921. 



' Kunicel, L. O. , A Possible Causative Agent for the Mosaic Disease of Com, 

 Bull. Expt. Stat. Hawaiian Sugar Plant. Assoc, Bot. Ser. iii, p. 44, 1921, and 

 Amoeboid Bodies associated with Hippeastrum Mosaic, Science, N.S., Iv, 

 p. 73. 1922. 



* Palm, B. T., De mozaiekziekte van de Tabak een Chlamydozoonose ? 

 Bull.' Deliproef Stat, te Medan-Sumatra, xv, 10 pp., 1922 [English translation 

 attached] . 



