86 presidential address section c. 



Prophylaxis. 



Very little progress has been made as yet in devising 

 methods of control ; the protective sprays which are successfully 

 applied in combating fungous disease have, with one or two 

 exceptions, proved quite useless in the prevention of bacterial 

 diseases, but to quote from Smith's Conspectus (52) : — " Al- 

 though as regards most of these diseases methods of control 

 must still be worked out, with rapidly increasing knowledge of 

 the biological peculiarities of the parasites causing these 

 diseases, and of the ways in which they are disseminated, light 

 begins to dawn, so that before many years have passed we may 

 confidently expect the more intelligent part of the public to be 

 applying sound rules for the control of these diseases — rules 

 based on the individual peculiarities of the parasites and care- 

 fully worked out experimentally by the plant pathologist." 



It is known that many pathogenic organisms are transmitted 

 on seed, bulbs or tubers ; if it is not possible to ascertain whether 

 these are derived from healthy plants, it is possible in most 

 instances to destroy the parasite by soaking the seed or tubers 

 in a dilute solution of mercuric chloride or of formalin. The 

 source of buds, cuttings and grafts should be carefully examined, 

 and these should not be taken from plants showing symptoms of 

 disease. Diseases such as crown gall and citrus canker are 

 frequently disseminated in this way. Another fruitful source 

 oi infection is the habit of throwing diseased rubbish on the 

 manure heap; fallen fruit and diseased twigs and leaves can only 

 l:e safely disposed of by burning. 



Soil infection can be avoided by a long rotation of crops, and 

 seed l)eds used for rearing seedlings of cabbage, tomato, tobacco 

 and other plants subject to bacterial diseases, need to be made 

 with new earth or should be sterilised before planting. 



In some cases spraying is effective in checking disease, and 

 germicidal sprays have been used successfully in combating shot 

 liole of the peach and walnut blight; in the latter case Pierce 

 reports that the infection was reduced 50 per cent. Spraying, 

 however, it not always successful ; in a number of spraying 

 experiments carried out in connection with the investigation of 

 the mango blight, the fruit on the sprayed trees was as badly 

 diseased as that on the trees kept as controls. 



In diseases affecting trees, surgical methods are often 

 useful ; the amount of spring infection can be materially reduced 

 by cutting out diseased twigs, and cankers and galls on the larger 

 limbs. 



When diseases are known to be transmitted almost entirely 

 by insects, the obvious method of control is to wage a war of 

 extermination against such insects. 



Prevention of disease by selection and hybridisation of resistant 

 varieties has not yet received the attention it deserves. Numerous 

 experiments have been made in the direction of breeding rust- 

 resistant wheats, and plants resistant to other fungus diseases, 



