Potato Mosaic. 183 



get all the diseased plants may leave them in the field long enough 

 to serve as centers of infection if aphids appear early and in- 

 crease rapidly. Therefore two or more inspections are prefer- 

 able, so that diseased plants may be removed or "rogued" soon 

 after they show that they are diseased. 



In 191 7 three one-fifth-acre plots, each consisting of 6 rows 

 next to an all-mosaic plot, had from 32 to 49 per cent of the 

 hills mosaic. Two — a Green Mountain and a Bliss Triumph — 

 were rogued three times and in 191 8 the stocks were mosaic in 

 11 and 16 per cent of the hills respectively. The third lot, Green 

 Mountain, was rogued once and in 1918 was mosaic in 13 per 

 cent of the hills. Thus roguing in 1917 made a considerable 

 reduction in the mosaic percentage in spite of the proximity to 

 all-diseased plots — no plant being more than a few rows away — 

 and of the abundance of mosaic hills in the lots. In 1918, the 

 year of excessive abundance of aphids, these three lots of stock 

 were planted together, one lot only being next + o a half-mosaic 

 plot, and of course they contained fewer mosaic hills. Roguing 

 in that year was not so effective, since the mosaic percentage 

 increased to from 20 to 30 per cent shown in 1919. This is 

 undoubtedly due to aphid dispersal and indicates the need of 

 isolating the seed plot. 



ISOLATION OF SEED STOCK. 



If healthy stock of a susceptible variety is grown near to 

 mosaic stock it will acquire more or less of the disease. On the 

 other hand, if it is grown so far from mosaic stock that no in- 

 sects, especially aphids, will come to it except from weeds or 

 healthy potatoes, it may be expected to remain healthy, judging 

 from the evidence at hand. Tests of this method are under way. 

 As far as is known, such an isolated seed plot, even if partly 

 mosaic, may produce healthy potatoes if it is rogued completely 

 of mosaic hills early enough. 



Recommendations for the Control of Potato Mosaic. 



Dp not expect to control or reduce potato mosaic by means 

 of sprays, seed treatment, or soil treatment, except by spraying 

 methods which control plant lice. Use as healthy stock as can 



