Potato Mosaic. 159 



ing all those in which potatoes are an important crop. During 

 1919 the writer in company with others made a careful estimate 

 of the amount of mosaic in 40 Green Mountain fields in Aroos- 

 took County and the same number of Bliss Triumph fields in- 

 cluding many which were supposed to be above the average in 

 quality. In these fields the percentage of hills that were mosaic 

 varied from one-half to 100 per cent, averaging 28 per cent for 

 the Mountains and 46 per cent for the Bliss. It is apparent that 

 a mosaic-free field of these two susceptible varieties was ex- 

 tremely rare. As will be shown later, there is every reason to 

 expect that the prevalence of the disease will increase unless 

 effective measures are taken to check it. 



Observations upon potato mosaic have been carried on in 

 this country since 1 9 1 2. Active experimental work has been 

 done in Maine and elsewhere since 1916. Tobacco, a species 

 in the same family of plants as the potato, also has a mosaic 

 disease which has been studied for a number of years and is now 

 comparatively well understood. In spite of all this, progress 

 upon potato mosaic has been slow and it has been only recently 

 that the evidence has justified the formation of definite conclu- 

 sions regarding it. This delay has been largely due to certain 

 differences of characteristics between tobacco mosaic and potato 

 mosaic 5 . The mosaic of tobacco is far more easily transmitted 

 from plant to plant than that of potatoes. The natural spread 

 of tobacco mosaic is much more apparent than the natural spread 

 of potato mosaic, since the former occurs early in the season 

 while the latter occurs late, or at least the only known agent of 

 potato mosaic transmission in Maine appears in large numbers 

 too late in the short growing season for the newly infected plants 

 to show the disease. Tobacco mosaic is not transmitted from 

 one generation to another through the seeds, but potato mosaic 

 is transmitted by the tubers, so that the full extent and nature 

 of the spread of the latter in the field during any season can be 



5 A more detailed knowledge of these differences can be secured by 

 comparing the two cited papers upon which most of this bulletin is based, 

 with the following. 



Allard, H. A. Further studies of the mosaic disease of tobacco. 

 Jour. Agr. Research 10:615-632. PI. 63. 1917. 



Chapman, G. H. Mosaic disease of tobacco. Mass. Agri. Exper. 

 Sta. Bui. 175:73-117. 5 pi. 1917. 



