WINTER PRUNING EXPERIMENTS WITH APPLE TREES. 153 
varieties whose wood becomes badly infected by the fungus. ' Allington, ' 
' Bismarck,' and ' Newton Wonder ' appear so far to be extremely 
resistant to this form of infection ; it has been difficult to find a single 
scab pustule even on the wood of the unpruned trees. So far as one 
can judge, the unpruned and long-spur pruned trees of these varieties 
have an equal chance of infection. It is worth noting, also, that even 
with ' Cox's Orange ' the scab pustules are usually much less numerous 
on the wood of the long-spur pruned trees than on the unpruned trees, 
even before the pruning is done. 
In the case of mildew, of every variety examined the unpruned 
trees had at least three times as many wood infections as the long-spur 
pruned trees, and in some cases more than ten times as many. Here 
again it might be thought that the difference is mainly due to the 
cutting out of centres of infection in the pruning ; but all wood 
infections have been rut away from all the trees, pruned and unpruned, 
each winter, and the new mildewed shoots similarly removed each 
May ; this would at least tend to equalize chances of infection. 
The case of canker again shows that the difference in susceptibility 
is not solely due to the cutting out of sources of infection in the pruning. 
Many of the unpruned ' Worcesters ' had, in the winter of 1920-21, a 
very large number of bud infections of canker, in some cases running 
into the hundreds. Generally there were older, spore-producing 
infections, which had obviously acted as centres of the new bud 
infections ; but in a few cases, especially of ' Grieve,' there were numerous 
new bud infections even where no old spore-producing cankers could 
be found. One or two of the long-spur pruned trees had old spore- 
producing cankers, yet on none of them could more than a single bud 
infection be found, and most had none at all. This difference in the 
case of canker may be partly due, judging from the observations 
recorded by Wiltshire,* to the markedly later leaf-fall of the long-spur 
pruned trees. But if this were the chief cause one would expect to 
find numerous bud infections on the long-spur pruned trees in the 
spring ; actually they seem to be no more numerous than in the 
autumn. 
One can only conclude, I think, that the greater freedom of the 
long-spur pruned trees from these diseases is largely due to their 
greater power of resistance to infection. It remains to be seen whether, 
and to what extent, this greater power of resistance is brought about 
by a thicker cuticle or differences in the structure of the tissues. 
Effect of " Open Centre " Pruning. 
The differences between the " unpruned " and " open centre " 
trees are as yet small, but some of them are significant, and most of 
them may be expected to increase as the trees get older. This is the 
more likely, since the method of pruning the open centre trees was 
considerably modified in the winter of 1920-21. Previous to that time 
* Annals of Applied Biology, viii. 3 and 4, November 1921 
VOL. XLVII. 
M 
