280 THE PRACTICAL APPLICATION OF ECOLOGY 



that resistance is dependent upon mechanical factors, while others 

 believe that physico-chemical influences are the most important. 



It has been recognised as long ago as 1831 that certain sour 

 varieties are much less liable to attack by the aphis in question, 

 and this observation led to utilisation of sour apples as grafting 

 stocks as a preventive measure. In this way the resistant varieties 

 Northern Spy and Winter Majetin came into prominence, but 

 they have not proved an unqualified success in other ways, since 

 dwarf trees result, productivity is not high and they do not react 

 well to adverse local conditions. A number of other resistant 

 varieties have come to be recognised in different countries, but it 

 appears that varieties that prove to be resistant in one country 

 are not invariably so when grown under different conditions in 

 another country. Thiele (1902), for example, states that Northern 

 Spy becomes susceptible in certain regions of Germany, while 

 under English conditions it has been found to be immune to both 

 the root and branch phases of aphis attack. According to Monzen 

 (1926) the variety Jonathan is susceptible in Japan, whereas 

 Theobald showed that it was almost free from aphis attack in 

 England. Similarly, the last-mentioned authority found Cox's 

 Orange Pippin to be subject to attack in England, yet Misra 

 (1920) states that under Indian conditions it is immune. 



It would appear that the resistance in these immune, or slightly 

 susceptible, varieties of trees to attack by the Eriosoma is to a 

 large extent an inherent property, but that this property can 

 either be intensified or reduced by cultural conditions. 



According to Crane (1937) in the results of crossing immune 

 with susceptible root stocks the variety Northern Spy has a higher 

 value in respect to the transmission of immunity than any other 

 of the immune varieties used. The proportion of immune plants 

 obtained is often as large, and frequently much larger, in crosses 

 between susceptibles and immunes than that obtained in families 

 raised from immune crossed with immune. Some susceptibles 

 carry factors or genes concerned with immunity and some do not, 

 and there is a gradation in the inheritance of susceptibility. All 

 the immune stocks used were heterozygous. The results of 

 breeding investigations are not discussed in detail in the present 



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