9 o 



especially the fruitgrower, has to decide for himself as to whe- 

 ther it pays him best to grow such varieties as he particularly 

 wishes, but which may be liable to certain fungus diseases, and 

 which it may be necessary to spray repeatedly if he is to reap 

 any crop at all, or whether it would not, in the long run, be more 

 profitable to grow those varieties which are not susceptible to 

 disease, but perhaps command a lower market price on account 

 of less desirable qualities. 



In other parts of the world much has already been done by 

 breeding crops immune to certain fungus pests, and there is no 

 reason why similar results should not be obtained in South 

 Africa. 



The second method of controlling plant diseases, although of 

 very recent origin, opens up a field of enquiry which seems to be 

 full of promise. The problem is that of attacking the fungus 

 through the food supply of the host. . This phase of the subject 

 has been brought about mainly through the now common use of 

 artificial manures. 



It has been a matter of general observation that crops which 

 had received dressings of manures in which there was a large 

 supply of available nitrogen invariably suffered more from cer- 

 tain fungus pests than those which did not receive such treat- 

 ment. 



Definite experiments carried out both in the field and the 

 laboratory confirmed these observations and established the fact 

 that such manures as nitrate of soda and sulphate of ammonia 

 considerably increased the susceptibility of plants to attacks of 

 such fungi as rusts and mildews, whereas salts of potassium, 

 especially potassium phosphate have a marked tendency to de- 

 crease the amount of fungus attack. Following up this line of 

 enquiry it has also been found that the addition of even traces 

 of certain mineral salts such as those of lithium and magnesium 

 to the soil tend to decrease and increase respectively the virulence 

 of certain fungus attacks. If, as would seem at present, only 

 the merest traces of such elements are required to produce such 

 marked effects in the virulency or otherwise of the fungus at- 

 tack, it is possible that further research in this direction may 

 lead to results of considerable practical importance. 



The third method of treatment entails the application to the 

 plant of a wash or powder commonly known as a fungicide. 



This mode of controlling plant diseases has been in use for 

 some considerable time, and although it may often serve the end 

 in view, it has many disadvantages and leaves much to be desired. 



The object of the fungicide is merely to prevent the germina- 

 tion of such spores as may be present on the plant, or may fall 

 on it later. It is the greatest mistake in the world to imagine 



