8 9 



irrow any plant tliat we wish in any particular spot, and of course 

 at the >ame time the reason why some plants, and even some 

 varieties of plants, thrive more luxuriantly in some particular 

 locality than others. I 'nless we know definitely what these con- 

 ditions are, and can alter them to suit the requirements of plants, 

 it is mere waste of time and energy to endeavour to grow plants 

 where they will not readily thrive. 



Thus. Mime plants which we wish particularly to grow in 

 our orchard may be found to be highly susceptible to certain 

 fungus pests whereas others of a different variety are practically 

 immune in the same locality. The latter, however, when grown* 

 elsewhere may or may not exhibit such immunity. This is well 

 illustrated in the case of rust in wheat, for it is found that wheats 

 which are highly immune to. rust in Europe, when grown in 

 South Africa are highly susceptible to the same rust. The same 

 holds good in the case of the susceptibility of certain varieties of 

 apples to the " scab " fungus Fusicladium. In one locality some 

 particular variety is regarded as immune to the fungus, whereas 

 in another locality it may prove highly susceptible to the same 

 fungus. These facts suffice to show that no disease can be. 

 caused by a parasite alone, since its power for injury or other- 

 wise is affected by local conditions as well as by the host plant. 



In the same way it is fairly safe to assume that no single 

 factor in the local conditions of a plant can be regarded as respon- 

 sible for disease, for it is impossible for one change to take place 

 without involving other concomitant disturbances. These facts 

 bring us to a consideration of the treatment of fungus diseases in 

 plants. 



At the present time three distinct methods of coping with 

 fungus diseases are in vogue, viz. : Plant-breeding, plant-feeding, 

 and the application of fungicides. 



In recent years the tendency has been to devote more atten- 

 tion to the plant itself, and much has been accomplished by the 

 breeding and selection of immune varieties. In South Africa as 

 yet little has been done in this direction, but as soon as farming 

 and fruit-growing become more firmly established, progress will 

 be made as a natural consequence. 



Farmers and fruit growers will from experience learn what 

 varieties of plants are best suited to their own particular locality 

 and requirements, and it is from this material that they should, 

 with the aid of the plant breeder, endeavour to raise supplies for 

 future years, always with an eye to improvement. As a general 

 rule, plants which exhibit immunity to fungus pests are lacking 

 in most of the other desirable qualities, and consequently the task 

 of the plant breeder is to combine immunity with such other 

 characters as arc required in the crop under cultivation. In 

 matters of this sort it is largely a question which the farmer, and 



