378 Insects and Potato Foliage 



«> Gross Infestation by Aphis. Chelsea. 



The Chelsea plants were examined by one observer on May 17 

 and their characters noted. They were very different in appearance 

 from those raised at Wisley, being on the whole taller with more slender 

 stems and the leaves much smaller. No less than nine plants had 

 developed two or more slender stems instead of one, thus greatly 

 modifying the general appearance. The characters yellowness, pinkness, 

 and blotch, characteristic of the Wisley series, were entirely absent, but 

 five plants possessed hooded or curled leaves — the margin bent down- 

 wards — a character absent from the Wisley plants. No insects were 

 observed upon the plants on May 17th. 



The plants were examined critically by the other observer for insect 

 characters on June 11. These had been free of all insects except Aphis, 

 which had been allowed to increase on the plants for three weeks 

 unchecked, the plants then being evenly and hghtly infested. The 

 plants were examined individually and the points noted for each (as 

 these proved to be the same throughout it is not necessary to reproduce 

 them in full). The following points should be noted : 



All the plants had colonies of Aphis on the large old leaves. Leaves 

 lightly infested showed no definite symptoms : darkening of the veins 

 occurred in all cases of more than light infestation and the oldest leaves 

 also showed marked yellowness, not necessarily due to Aphis. 



In cases of severe infestation the tip and edges were brown. Old 

 broad leaves were not quite flat but curved: curling was not seen. 

 Small leaves were in many cases sharply curled, but this symptom was 

 not coincident with the presence of Aphis nor with the occurrence of 

 dark veins. Aphis was present in many cases on the small leaves, but 

 on picking out all plants w4th curled leaves these were not markedly 

 infested with Aphis on the small leaves. 



There is no evidence of Aphis selecting plants ; all had Aphis on now 

 or had signs of previous occurrence of Aphis. 



Gross infestation by Aphis. Wisley. 



A series of seedlings raised later than Series I — IV was examined for 

 Aphis on July 24. On this occasion there were three plants with curled 

 leaves and Aphis was absent from two of them. 



A series of 80 plants raised from the original stock of seed, in 

 1914, and exposed to infestation yielded a somewhat different result. 

 Thirteen plants exhibited hooded or curled leaves and all were infested 



