382 Insects and Potato Foliage 



Field observations on Aphides. 



Aphides were very closely under observation at Wisley in 1912. 

 On July 5 it was found that some varieties were badly infested and 

 others not at all. These Aphides were not observed on the Duke of 

 York ; a few were found on Arduthie Early and Ringleader, associated 

 with discoloured veins, whilst there occurred among a single row of 

 Sharpe's Express, twelve plants with dead foliage, eight with foliage 

 undoubtedly killed by Aphides and six with foliage still green. In 

 several cases the original injuries caused by Aphides had received 

 secondary extension owing to a period of wet weather — the portion of 

 the leaf-blade next the discoloured mid-rib first decayed, the decay 

 spread and ultimately involved the whole leaflet. In spite of the 

 presence of numerous Coccinellids, the Aphides appear to have got the 

 upper hand. 



Certain varieties grown in another part of the Wisley Garden and 

 including the Duke of York, the Sutton Flourball, Up-to-Date, Lang- 

 worthy, Northern Star and President were scarcely affected by Aphides, 

 although these were present throughout the season. Coccinella was 

 abundant here also but appears to have held the Green fly in check. 



Diseased plants received at the Wisley Gardens from Woking on 

 July 2, 1912, were stated by the sender to have stopped growing at 

 quite an early stage, and bore only six tubers of very small size. The 

 plants showed similar symptoms to those which appeared on the Sharpe's 

 Express variety at Wisley — discoloured veins and a secondary extension 

 owing to bad weather. Living Aphides were present on the specimens 

 and the disease was certainly caused by them. 



Similar specimens with remains of Aphides upon them were received 

 from Midhurst on July 6. The sender had been advised that owing to 

 disease in the haulms, potatoes should not be grown on the same ground 

 for two years. The haulms were not diseased, the condition of the 

 plants being again due to Aphides. 



Owing to the kindness of Dr RedclifEe N. Salaman we were enabled 

 to study the experimental races of potatoes raised by him at Barley in 

 1911 and 1912. It was observed in 1911 that the foliage of certain 

 races was badly diseased, whilst the foliage of other races remained 

 almost entirely healthy. The chief symptoms of disease took the form 

 of brown blotches, dead leaflet-ends, and perforations, but no attempt 

 vv^as made to ascertain the cause of these symptoms then. 



