S. p. Wiltshire 185 



favourable to the fungus. For instance, the first examples of leaf scar 

 infection of the 1919 growth were not found until January 3, 1920, 

 when three cases only were discovered in a plantation of 230 trees of 

 22 different varieties. In 1920-21 the intensity of infection varied very 

 much, but some of the trees were very severely attacked as early as 

 September 23. It is probable that weather conditions influence the 

 incidence of canker as well as other diseases and one of the conditions 

 most favourable to its spread seems to be rain, which was excessive 

 during the early autumn of 1920. On the other hand, during the 

 previous summer, the trees suffered from a severe attack of aphis 

 and this undoubtedly weakened the trees and possibly rendered them 

 unable to resist infection. 



Whilst, as already pointed out, infection may take place on partially 

 defoUated shoots in the autumn, at the scars of the recently fallen leaves, 

 yet no scar infection has been observed so long as the leaf remained 

 attached to the shoot at that point. The most vigorous trees of each 

 variety retained their leaves longer than the weaker ones and were 

 usually more resistant to canker. Cox's Orange Pippin, Worcester Pear- 

 main and Beauty of Bath each retained their leaves till the end of 

 October. The Medaille d'Or x Kingston Black seedlings referred to above 

 also held their leaves till very late. Some attempt was made to correlate 

 the periods of defoliation with susceptibiUty to canker and although, 

 generally speaking, those varieties which are very susceptible defoliate 

 early and those which are resistant are on the whole late, the differences 

 between the two do not appear to be sufficiently marked to warrant any 

 significant conclusion. 



An interesting point worthy of notice is that the varieties which are 

 susceptible to canker frequently show a similar susceptibiUty to Fusi- 

 cladium dentriticum. This correlation is independent of the infection of 

 scab wounds by Nectria, an account of which will possibly be pubhshed 

 later. One sometimes finds shoots which are scabbed down one side of 

 the stem only and in the shoots of King of the Pippins and Devonshire 

 Quarrenden, the scabbed sides also bear numerous leaf scar infections. 

 The conditions governing scab infection must evidently be similar to 

 those governing leaf scar infection of canker. The side of the stem attacked 

 by the scab and canker fungi is usually that exposed to the prevailing 

 winds, which at Long Asliton come from the south-west. It may be 

 that the lowering of the temperature of the exposed side of the stem 

 retards the formation of the protective phellogen just sufficiently long 

 to allow the fungus to infect. 



13—2 



