Potato Mosaic. 165 



controls, each consisting of healthy scion and stock, remained 

 healthy, showing that the operation of grafting was not the 

 cause of mottling. 



Since aphids, or plant lice, were very abundant in 1918 and 

 had not been controlled in previous grafting experiments, final 

 tests were made both in the field and in the greenhouse with 

 these insects eliminated 10 . In the greenhouse any vigorous new 

 growth that was made by a healthy part of the graft became 

 mosaic unless both stock and scion were healthy. In the field 

 under cloth insect cages, healthy stocks produced mosaic shoots 

 in spite of the fact that the mosaic scions soon died because of 

 shading. That this was due to the grafting is indicated by the 

 absence of mosaic in the stock at the time of grafting or at any 

 time in ungrafted check stalks in the same hills. 



In the preceding experiments either the Bliss Triumph or 

 the Green Mountain variety was used. One test with Irish Cob- 

 blers — very resistant to mosaic — was made in 1919 with mosaic 

 scions upon uncaged stocks. The result was that mosaic ap- 

 peared in the new branches from the grafted stalks while un- 

 grafted stalks remained healthy. 



Tuber grafting was attempted in the following manner. A 

 supposedly mosaic tuber and a likewise supposedly healthy tuber 

 were split lengthwise into halves. The freshly cut surfaces of 

 one-half of each were immediately brought into contact and the 

 two halves securely bound together and planted in this condi- 

 tion. The remaining halves of the healthy and diseased tubers 

 were planted separately as checks. In 14 such cases the sep- 

 arately planted or check halves produced diseased and healthy 

 plants in accordance with the supposed condition of the original 

 tubers. Likewise all of the diseased tuber halves in the at- 

 tempted grafts produced mosaic plants. On the other hand only 

 11 of the healthy tuber halves in the attempted grafts produced 

 healthy plants while 3 produced mosaic plants. Examination 

 showed that in the case of the 3 last mentioned actual union had 

 been established between the cut surfaces of the healthy and 

 diseased tuber halves. No such union had occurred in the 11 

 cases where no transmission of the disease occurred. 



10 This precaution had been made necessary by the results of certain 

 insect experiments which will be described later. 



