386 Soil Factors on Disease Resistance 



was given — on Sept. 30th — and duriDg the winter and spring no irriga- 

 tion water at all was applied. As a result these plots were practically free 

 from green-fly, the trees grew vigorously and the foliage showed all the 

 characteristic appearances of healthy peach trees. These plots stood out 

 in marked contrast to many of the trees to be seen in the Civil Station 

 when in 1917 the ravages of green-fly were far above the average. 



3. Three lines of almonds (a deep-rooted tree), and a plot composed 

 largely of various stocks including plums, almonds and peaches, which 

 were clean cultivated in 1916 and which were remarkably healthy and 

 quite free from green-fly, were sown with shaftal (Trifolium reswpitiatum) 

 right up to the stems in August and September, 1916. Several waterings 

 were applied to these trees during the winter and spring. All the almonds, 

 the seedling peaches, and some of the plum stocks became badly affected 

 by green-fly soon after the leaves appeared at the end of March, 1917. 

 By May, the attack was severe and practically all the young growth was 

 affected. In this case, trees free from green-fly in 1916 lost in a single 

 season all their immunity as a result of winter watering. 



4. A number of almond and peach trees — grown under a system of 

 furrow irrigation by which over-watering is almost impossible — were 

 planted in the autumn of 1916 close to one of the lines of almonds which 

 was over- watered during the winter. The object of this was to obtain 

 another demonstration of the fact that insects like Aphides are unable 

 to attack healthy plants. The over-watered trees were all affected by 

 green-fly which in no case spread to the plants which had been watered 

 by furrows and which had obtained an abundant supply of air for their 

 roots. 



These results, which have been repeated on several occasions at 

 Quetta, suggest that the control of green-fly must be sought elsewhere 

 than in the destruction of the insect. Soil-texture and soil-aeration in 

 Baluchistan are markedly improved by a winter fallow and are known 

 to suffer from over-watering during the cold season. This appears to 

 affect the development of the roots of fruit trees in the spring by inter- 

 fering with soil-aeration. A change in the sap seems to result after 

 which the trees become attacked by green-fly. The connection between 

 winter irrigation and green-fly has been obtained so frequently and is 

 so definite that more detailed investigations of the soil, of the root- 

 system and of the sap of the trees affected by green-fly is urgently called 

 for. If, as appears possible, it is found that the insect can only attack 

 trees in an abnormal condition, the prospects of the efficient control of 

 this pest becomes much more hopeful. 



