184 Apple Canker Fungus — Leaf Scar Infection 



cider varieties such as Kingston Black, Cap of Liberty and Strawberry 

 Norman. There is, however, marked difference in the susceptibility of 

 varieties. A striking instance of this was afforded in one of the planta- 

 tions at Long Ashton where varieties growing adjacent to each other 

 have behaved in characteristic manner during the last two winters. A 

 record of the number of new infections occurring on the different varieties 

 in the winter 1919-20 was as follows: Cox's Orange Pippin, no bud infec- 

 tion; King of the Pippins, 49; Worcester Pearmain, 1 ; Devonshire Quar- 

 renden, 149; Beauty of Bath, nil. A particularly interesting point about 

 this result is that Cox's Orange Pippin is usually regarded as extremely 

 susceptible to canker, whilst Worcester Pearmain is by no means a re- 

 sistant type. In view of the normal susceptibihty of the King of the 

 Pippins it is interesting to note that in another plantation a httle dis- 

 tance away a row of this variety showed no canker at all. Anyone 

 drawing conclusions from the behaviour of the trees on the above- 

 mentioned plot would be completely misled as to the susceptibility of 

 the varieties concerned. The explanation of this behaviour is at present 

 obscure, but there are obviously many factors, such as, for instance, the 

 vigour of the tree, the influence of the rootstock, the condition of the 

 soil, or some inherent quality of the sap, about which we know very 

 little at present and which may supply the key to the mystery. 



The period of infection in the case of the Kingston Black x Medaille 

 d'Or seedhngs is limited almost completely to the spring. The first in- 

 fection of the 1919 wood of these seedlings was found on March 31, 1920, 

 but after this date infection proceeded rapidly. During the winter 

 1920-21 three infections of the 1920 wood were found as early as Febru- 

 ary 1921. On most varieties, however, a large percentage of the infection 

 takes place in the early autumn immediately after defoliation. During 

 the last autumn (1920) infection of the lowest leaf scars of that year's 

 growth occurred in many cases before the upper leaves had fallen. The 

 rapidity with which infection took place was very striking, in some cases 

 practically every shoot being attacked before defoliation was half com- 

 pleted. Generally speaking, therefore, there are two periods when leaf 

 scar infection is especially active — (a) in the early autumn, and (6) in 

 the spring. It is not suggested, however, that infection entirely ceases 

 at other times of the year, in fact it appears to continue throughout the 

 summer and with certain varieties during the winter, but the spring and 

 autumn are periods when infection is markedly prevalent. 



Comparing the severity of the attack of the winter 1919-20 with 

 that of 1920-21, it would appear that the latter season was specially 



