14 



' Unfavou/rable Soil and Atmospheric Conditions. 



Soil and atmospheric conditions may be conveniently considered 

 together, and may be regarded as those causes which are due to the action 

 of the non-living environment. The spotting is frequently attributed to 

 unsuitable soils, especially those of a light nature, although httle ex- 

 planation is given as to why such soils should bring about this peculiar 

 condition. In Europe it is thought that it is the sudden drying-up of 

 these soils in times of drought that is responsible for the trouble, but in 

 the hght of my own observations it seems only reasonable to suppose that 

 this is due to the fact that in such a climate the spots are then more 

 noticeable, owing to the drying-out of the groups of cells which have been 

 cut ofE from their water supply through the bursting of the cells bordering 

 on the vascular bundles at a much earUer period. There is little doubt 

 but that those chmatic conditions which bring about excessive trans- 

 piration are largely conducive to the production of the spotting. 



Unfavourable Grafting. 



Before we can clearly understand any disease, we must be thoroughly 

 acquainted with the processes by which it is brought about, and the un- 

 ravelling of this particular disease is complicated by the fact that httle 

 reliable information can be obtained regarding the history of the plants 

 that bear the diseased fruit. Especially for the elucidation of a trouble 

 of this kind is a thorough knowledge of the whole plant's working system 

 almost essential, and when it is remembered that the stock has probably 

 been grafted over two or three times or even more, then will it be reahsed 

 that the problem is more intricate than would appear at first sight. For 

 instance, if the stock contains a greater number of vessels than its scion, 

 then it is only natural to expect that the normal physiology of the latter 

 may be deranged through the anatomical pecuharities of the former, and 

 vice versa. Now, most of our apple trees in South Africa are grafted on 

 Northern Spy stocks, a tree which is notoriously susceptible to bitter pit, 

 and we are here again face to face with the question as to how far the stock 

 is able to influence its scion ; whereas we find that Colonial varieties 

 propagated by rimners are free from the trouble. In a climate with such 

 varying and rapid extremes of temperature as is experienced in South 

 Africa, there must be perfect harmony between root and shoot system, 

 so that the one may regulate to a nicety any abnormal state of affairs set 

 up within the tissues of the other. For instance, if a scion with a very 

 high rate of transpiration be grafted on a stock of relatively slow root- 

 absorption power, or if a scion with a low transpiratory action be grafted 

 on a stock of great absorption power, it stands to reason that the working 

 relations between the two will become somewhat distorted. 



Alechanical Injury. 



The chief theory under this heading that has been advanced to 

 account for the trouble, is that the injury was brought about by blown 

 sand. It requires very httle, however, to disprove any such proposi ion, 

 when it is remembered that a great number of the spote do not take their 

 origin from the exterior, but are found deeply seated in the internal flesh 

 of the fruit. 



