Abstracts. 223 



TRANSMISSION OF THE MOSAIC DISEASE OF 

 IRISH POTATOES.* 



This paper summarizes results of the continuation of ex- 

 periments which were concerned with certain phases of potato 

 mosaic, in particular transmission, and which were conducted 

 mostly in northeastern Maine. 



Transmission from season to season occurs in tubers pro- 

 duced by diseased plants. The parent plants may appear healthy 

 throughout the season if infected late. Their diseased progeny 

 show symptoms varying greatly in severity but averaging about 

 the same for different lots and varieties. The severity of symp- 

 toms shown by obviously diseased parent plants is not changed 

 much in their progeny. The percentage of disease in the progeny 

 of apparently healthy parents can be reduced by selection of 

 hills, tubers, or seed-pieces, but the reduction is too slight and 

 uncertain to be of value. 



The infectiousness of the disease was demonstrated by ef- 

 fecting transmission by means of tuber grafts, stem grafts, and 

 inoculation with juice. This was done in part in the field with 

 plant-lice eliminated, because previously reported experiments 

 upon infectiousness had not been performed under such condi- 

 tions and because plant-lice had been shown to be carriers of 

 the disease. Juice inoculation was attempted from one variety 

 to another and was successful. 



Plant-lice, as had been found and reported previously, were 

 a reliable means for transferring the disease. As more condi- 

 tions favored the dispersal of these insects from diseased plots 

 to healthy plants, there followed greater spread of the disease. 

 One such favorable condition was growth in the open field in 

 contrast to growth under insect cages in the field and in contrast 

 to growth in a greenhouse with plant-lice controlled. Other 

 favorable conditions were greater proximity to diseased plots, 

 greater interseasonal abundance of plant-lice, and later dates of 

 harvesting together with seasonal increase of plant-lice. 



*This is an abstract of a paper by Donald Folsom and a cooperating 

 member of the Bureau of Plant Industry, having the same title and 

 published in Journal of Agricultural Research, Vol. XIX, No. 7, p. 315- 

 337. 



