94 



SCIENTIFIC AGRICULTURE 



November, 1921 



With regard to locality it is interesting 

 to note that with the same variety the 

 symptoms differ according to whether the 

 variety is planted in a northerly or cooler 

 district, or in a more southerly, warmer 

 place. In the cooler locality mottling is 

 pronounced whereas in tjie southerly, 

 warmer district mottling is reduced or al- 

 together obscured. That the plants are 

 still diseased, however, is proved by the 

 fact that if tubers from a plant grown un- 

 der the latter conrlitions be planted furthei- 

 north mottling is again pronounced. 



In addition to the above symptoms it is 

 to be noticed that plants severely affected 

 with mosaic will show this in weaker stem 

 development and in a generally unthrifty 

 . eondition. 



Effect on the Plant. 



One effect of the disease ha.s already 

 been mentioned, namely the modified 

 growth of the palisade cells in light green 

 areas. Other cells are affected to a less 

 noticeable extent, but the general effect 

 is that the tissues whioh should be manu- 

 faetnring a maximum amount of starch 

 are not able to do so and further a pro- 

 jiortion of that manufactured is undoubt- 

 edly used by the causal principle. T^liis is 

 postulating that the causal principle is a 

 parasite. Hence the amount of food avail- 

 able for tuber formation is reduced ac- 

 cording to the severity of the disease and 

 the duration of it in the particular plants. 

 The tubers are therefore reduced in size 

 or in number, or both. . In the case of late 

 infection there is not likely to be any 

 noticeable difference in the tubers. Mur- 

 phy has estimated t^at severely diseased 

 plants yield only 57.8 per cent of a crop 

 as compared with healthy plants of the 

 same varietj^ under the same cultural con- 

 ditions. 



Infection. 

 Gussow has shown that by grafting the 

 disease may be readily transmitted and 

 Folsom et al. have obtained similar re- 

 sults in Maine. The writer in unpublisihed 

 work has found aphids to be fertile sources 

 of infection, and Folsom et al. have carried 

 on extensive experiments in Maine proving 

 clearly tjiat plant lice (Myzus pfraicae 

 Sulz. and Macrosijihum aolanifolli Ash- 

 mead) are carriers of the disease from 

 plant to plant. The writer has found that 



aphids are also most important in the trans- 

 mission of clover mosaic and it is probable 

 that they play a part in most mosaic dis- 

 eases. 



Other potato pests, such as the Colorado 

 potato beetle and the flea beetle, are not 

 found to be carriers of the disease. 



Experiments also indicate that the seed- 

 cutting knife is not important in trans- 

 mitting the disease. The same applies to 

 contact in the field between healthy and 

 diseased plants. Soil has not been found 

 to transmit the disease from one season 

 to the next, but it may be possible that 



Plate 4.— A leaf of potato showing mosaic 

 mottling. Note at 1 a flea-beetle hole and at 

 2 a typical light green area. 



volunteer plants arising from diseased tu- 

 bers left in the soil will give rise to centres 

 of infection the following season. 

 Varietal Susceptibility. 



Little is yet known of this p^ase of the 

 question and work on it is difficult be- 

 caiLse of the carrying over of diseased tu- 

 bers which came from apparently healthy 

 plants. It is known, however, that on the 

 w^hole the Irish Cobbler variety is less 

 susceptible to the disease while Grreen 

 Mountain is particularly susceptible. 

 Control. 



Three points in conti'ol are empliasized. 

 If the grower prefers to use jhis own .seed 

 the seed plot should be isolated from other 



