76 PRESIDENTIAL ADDRESS— SECTION C. 



and useless. Should infection occur later, when the outer tissue 

 is beginnin^^ to harden and active growth has ceased, the orga- 

 nism does not spread and the points of infection appear as very 

 minute, superficial, dark-coloured areas scattered over the sur- 

 face of the nut ; these late infections, therefore, do very little 

 damage. 



Even in the presence of young tissues, however, stomatal 

 infection does not occur unless the atmospheric conditions are 

 suitable; in many cases infection only takes place when there is 

 rain or heavy dew, so that there is a film of water on the leaf. 

 This has an important bearing on epiphytotics of diseases caused 

 by bacteria in South Africa. To refer once more to the walnut 

 blight : in a certain orchard in the Eastern Province the disease 

 is very severe, in seasons when there are abundant misty rains 

 in the spring and early summer, when the nuts are growing 

 rapidly ; but when the rains are delayed the damage caused by 

 Wight is almost negligible. This probably accounts for the fact 

 that, in the Oudtshoorn district, where the rainfall is slight and 

 takes place in winter, the walnut blight is unknown. 



In studying the mango blight ( 14 ) it was noticed that the 

 first fruit infections appeared about three weeks after the first 

 steady rain of the season, and citrus canker is almost latent 

 during the early spring, but becomes extremely active after 

 rainy weather in the summer. 



Recent studies (45) on the blackleg in the potato show that 

 a high temperature and low precipitation tend to diminish the 

 disease, while a low temperature and high precipitation produce 

 conditions favourable for it. The same may be said for the 

 bacterial spot of citrus occurring in the Western Province. (19) 

 This disease is only known to occur where there is abundant 

 rainfall during the winter, when the fruit is ripening. 



Some parasites, in contrast to those previously mentioned, 

 whose activities are limited to young tissues, are able to attack 

 and destroy mature organs. To this class belong the cabbage rot 

 organism, which destroys the pith of cabbage stems and turnip 

 roots, and numerous soft rot organisms such as Bacillus atrosep- 

 ticus, which attacks potato tubers and reduces them to a slimy, 

 evil-smelling mass in the course of a few days._- Organisms 

 causing typical vascular diseases also attack mature tissues, 

 occupying the vessels for long distances and not infrequently 

 causing the wilting and death of the whole plant. 



How Infection Occurs. 



Infection may occur through wounds; in this country serious 

 consequences often follow hailstorms, which bruise the tissues 

 and open the way for the entrance of parasites. The first 

 serious outbreak of the mango blight was after the great hail- 

 storm of 1906, which seriously damaged the trees ; and in more 

 than one instance there is evidence that the crown gall organism 

 has entered the ]:)lant through wounds caused by hail. 



