July 1, 1920 Transmission of Mosaic Disease of Irish Potatoes 331 



INTERSEASONAL INCREASE 



It was very apparent that aphids, which seemed as abundant as the 

 flakes in an ordinary snowstorm when they were migrating in the late 

 summer, were unusually numerous in 191 8. Consequently it is of interest 

 to compare the relative interseasonal increase in mosaic in the same 

 stocks from 1917 to 1918 with that from 191 8 to 191 9. It has been 

 demonstrated in several greenhouse experiments already discussed that 

 aphids may transmit mosaic without the symptoms being shown until 

 the progeny of the inoculated plants is grown the following season. 

 It is considered that they may do likewise in the field during the latter 

 half of the summer, which is usually the only time when they are abundant 

 on potatoes in northern Maine, although there are more species in the field 

 than were used in the various experiments. 



Previous experiments seemed to indicate that the percentage of mosaic 

 in susceptible varieties could be materially reduced by roguing the 

 diseased plants from the plots as soon as the mottling appeared upon 

 the vines. However, before it was fully demonstrated that insects were 

 capable of transmitting mosaic the plots usually were arranged in 

 such a manner that insect transfer could take place very readily. In view 

 of this situation, it is possible to note the effects of those agents of 

 transmission upon the performance of a few of the plots, each including 

 X acre, which were rogued during the last three seasons. Table VII 

 records the observations on these plots. 



Table VII. — Relation of aphids to increase of mosaic from season to season 



Variety. 



Green Mountain . . . 



Bliss Triumph 



Green Motmtain . . . 



Stock. 



Location. 



Next to 100 per cent mosaic stock. 



do 



do 



Per- 

 cent- 

 age of 

 mo- 

 saic. 



Treatmpnt. 



Rogued three times, 



do 



Rogued once 



Niunber 



of 

 aphids. 



Few. 

 Do. 

 Do. 



Variety. 



Stock. 



1918. 



Location. 



Per- 

 cent- 

 age of 

 mo- 

 saic. 



Treatment. 



Number of 

 aphids. 



Per- 

 cent- 

 age of 

 mo- 

 saic. 



Green Mountain . . I A 



Bliss Triumph I B 



Green Mountain . . ' C 



Six rows from 45 



per cent mosaic 



stock. 

 Nine rows from 45 



per cent mosaic 



stock. 

 Next to 4S per cent 



mosaic stock. 



Rogued three times. 



....do 



do 



Very abundant 



do 



do 



