428 SCIENCE PROGRESS 



forty-eight hours. In the bean mosaic ^ excised leaves retain 

 their virulence for only about the same time, but the virus (which 

 is transmitted by the seed) will survive any temperature that 

 does not kill the seed. The virus of potato mosaic in the latent 

 condition in the tuber requires apparently a longer time to kill 

 by heat than the tuber itself does.^ There are numerous 

 scattered observations on various properties of the infective 

 agent in several other mosaic diseases, but space does not permit 

 of referring to them. 



Influence of the Environment 



The only other point of general interest in these diseases to 

 which I wish to refer is the influence on them of external condi- 

 tions. This is sometimes so marked as to have led to contro- 

 versy as to whether the whole train of symptoms noticed in 

 certain of these diseases may not be accounted for by unfavour- 

 able environmental conditions without invoking any parasitic 

 agency. Cases in point are the spike disease of sandalwood, 

 which has been proved to be graft-infectious, and the pecan 

 rosette,^ where no transmission from one plant to another has 

 been proved. 



In several cases the air temperature has been found to have 

 a marked influence on the incubation period and symptoms of 

 this class of disease. In the mosaic disease of tobacco the 

 optimum for the activity of the virus, as judged by the length 

 of the incubation period, is 28° to 30° C. and the maximum near 

 36°.* The development of the symptoms is slowed down by 

 lower temperatures proportionately to the growth of the plant, 

 and no symptoms appear at temperatures that quite check 

 growth. Above 36° tobacco continues to grow, but the new 

 growth in infected plants shows no symptoms of disease and 

 even previousl}'' mottled leaves tend to become normal. The 

 virus is not destroyed, however, as the symptoms reappear 

 when lower temperatures are again employed. In cucumber 

 mosaic ^ infection fails when the soil and air temperatures are 

 below 20° C. Above this, the incubation period is shortened 

 as 30° C. is approached. In raspberry mosaic,* leaves put out 



1 Reddick, D., and Stewart, V. B., Transmission of the Virus of Bean 

 Mosaic in Seed, etc., Phytopathology, ix, p. 445, igiQ- 



2 Blodgett, F. M., The Relation of Time and Temperature to the KiUing of 

 Potatoes and Potato Mosaic Virus, Phytopathology, xii, p. 40, 1922. 



3 Rand, F. V.. Pecan Rosette, U.S. Dept. of Agvic. Bull., 1038, 1922. 



* Johnson, J., The Relation of Air Temperature to Certain Plant Diseases, 

 Phytopathology, xi, p. 447, 192 1. 



5 Doolittls, S. P., Influence of Temperature on the Development of Mosaic 

 Diseases, Phytopathology, xi, p. 46, 192 1. 



^ Rankin, W. H., and Hockey, J. F., Mosaic and Leaf Curl of the Cultivated 

 Red Raspberry, Canada Dept, of Agric. Divn. of Bot. Circ, i, N.S., 1922. 



