24 ''Reversion" and Resistance to ''Big BucV 



figures but nearly all fruit buds, indicating that the bushes as a whole 

 were not growing as strongly as they should be. Sound Boskoops from 

 Chelmsford which were very strong shoots showed no flower terminals 

 at all while Long Ashton material again gave the higher figure for shoots 

 with wood bud terminals. From the evidence therefore it would appear 

 that shoots making a weak growth from whatever cause usually end in a 

 flower bud and such a condition might be expected to cause reverted 

 wood the following season. 



Weak growth in any twig may be caused by two conditions. The bush 

 as a whole may be making weak growth or the twig itself may be in such 

 a position as to get a poor sap supply. Suckers from a bush usually make 

 a strong growth the first year, a moderate one the second year and com- 

 paratively weak ones in the third and fourth years. In the best pruning 

 method therefore all fourth year wood (if any) and most third year wood 

 is pruned away. Thus none of the weakest wood is allowed to fruit and 

 only part of the rather weak wood, the best and most abundant fruit 

 being obtained from the strong and moderately strong wood. In a 

 healthy bush therefore it is not until the third or fourth year that the 

 terminals are changed into flower buds and so the behaviour of the 

 succeeding year's growth is of no consequence. 



Case 4. Aphis attack. 



The effect of aphis attack in summer is, as stated above, to cause 

 two or three lateral buds immediately below the point of attack to break 

 into growth. Fig. 14, PI. VI, shows a typical example and is of course 

 of comparatively common occurrence. True terminal growth has thus 

 been lost and the terminal buds formed on these lateral outgrowths at 

 the end of the season are practically always flower buds. Consequently 

 next year there is a tendency for reverted wood to be formed. Reverted 

 wood means more lateral wood and consequently less sap is available for 

 the terminals. This results again in a tendency to form flower bud ter- 

 minals and so the process continues. If however the bush as a whole is 

 in very good condition the terminals on the wood formed as a result of 

 the aphis attack may be wood buds in which case there is no immediate 

 tendency to revert. If the growth is not quite so strong but still moder- 

 ately so the bush may recover its condition in the year following aphis 

 attack and form wood buds as terminals and so escape reversion. The 

 immediate effect therefore of aphis attack will depend on the condition 

 of the bush. It is possible that aphis attack may afYect the bush in 

 another way. If as usually happens three shoots appear whore in un- 

 attacked bushes but one is present it is clear that, if every shoot on a 



