14 ^^ Reversion'' and Resistance to '' Birj BuiV 



in summer time. Any cutting back of the terminal in summer time or 

 hard cutting back in winter does not produce the same effect since wood 

 growths then emanate from the buds immediately below the pruned 

 part. 



If the history of a mite attacked bush be followed in detail the 

 following appears to happen in the case of a strong shoot. The terminal 

 bud usually gets infested by migrating mites early in June. Generally 

 only a few are present and the growth from the terminal appears normal. 

 Should many mites succeed in getting in and the bush be in strong 

 growing condition very characteristic leaves are developed. This type is 

 generally known as "oak leaf" by growers. The leaf is thick and dark 

 green and bears no resemblance to the usual shape of the normal leaf. 

 It is a certain sign of a mite infested terminal (Fig. 2; cf. Figs. 3 and 4). 

 In any case the growth from the terminal is checked to some degree. 



This check is sometimes sufficient to force out some of the lateral buds 

 lower down during the current year (PI. IV, Fig. 5). The laterals of course 

 will push according to their strength during the succeeding year. Most 

 of them, where mite is fairly abundant, will also be mite infested, since 

 they are formed at the time when mite migration is most active. They 

 therefore cannot grow out at all. If any escape, and especially if they are 

 not in the process of changing into fruit buds, as they would be normally, 

 they will sometimes make a short outgrowth in the current year and in 

 any case become stronger buds than they would otherwise be (Fig. 5). 

 The lateral buds formed after June, that is those which during the next 

 winter are situated just below the terminal, usually escape mite infection 

 and develop into fruit buds having the structure already described. 



The reason why these often fail to grow out under the stimulus of the 

 check to the terminal bud may be explained thus. At the time of their 

 formation the growth push of the plant is rapidly decreasing (see Tables 

 I and II) and also that the two accessory growing points in the forming 

 fruit bud are very weak. If however the check takes place comparatively 

 early before the lateral buds have changed into flower buds out-growth 

 often takes place (Figs. 5-8). 



The position therefore at the end of the first year is that the terminal 

 bud is "big" as also most of the lower laterals. A few of these have been 

 abnormally strengthened or have made a slight outgrowth while the 

 higher laterals are normal fruit buds. 



The following year when growth begins the terminal, being mited, 

 can make no further growth and there is consequently pressure put upon 

 the buds below. As pointed out, the basal buds have been unduly 



