142 DISEASES OF CROP-PLANTS 



scorching of any spore-bearing surfaces. A kerosene blow-lamp 

 is a useful appliance in connection with this scorching, and in 

 starting fires. 



Treatment of the Soil. 



Attempts have naturally been made to arrest the progress of 

 the disease in the soil, or to sterilize situations from which dis- 

 eased trees have been removed, by the application of disin- 

 fectants. 



There is not much scope for the use of disinfectants in the 

 control of this disease. It is safe to say that at the present time 

 there is no substance available which is cheap enough to be used 

 in sufficient quantity to sterilize any considerable area of ground. 

 When dealing with a fungus which penetrates to every part of 

 even the thickest roots, it cannot be expected that chemical 

 agents will kill it out, or, being necessarily of a transient nature, 

 stop its progress, unless all woody material is removed. When 

 this has been done, exposure of the soil to sun and air is probably 

 just as effective as any disinfectant could be. Where it is re- 

 quired to deal with infested soil or accumulations of vegetable 

 matter under shade, as, for example, in the case of the trees with 

 surface infections specified above, it is better to scrape away 

 the surface soil with a hoe than merely to extinguish the fungus 

 with a disinfectant, leaving material liable to reinfection. Such 

 a scraping of the soil, combined with removal of low branches 

 and of any other hindrances to the tree circulation of air, has in 

 the cases under the writer's observation been sufficient. An 

 application of lime or sulphur would give an additional margin 

 of safety. 



The addition of lime to the soil when clearing up a spot from 

 which infested trees have been removed is commonly practised, 

 and is believed to contribute to success in establishing supplies. 

 It seems probable that the results obtained are not so mucn due 

 to the sterilizing effect of the lime as to its action in hastening the 

 disappearance of organic matter. There is a further probability 

 that by neutralizing soil acidity lime produces conditions less 

 suitable to the survival of the fungus. 



If the view here taken of the function of lime is the correct 

 one, the main effect may be obtained by the use of slaked lime, 

 which is the only form that can be conveniently obtained in some 

 islands. Quicklime, because of its additional sterilizing effect, 

 is to be preferred when equally available. 



Where sulphur has to be imported, its price will not enable 

 it to compete with lime, but there would appear to be possibilities 

 in some places of obtaining it cheaply from local sources. Sulphur 

 has a more definite fungicidal action than lime, and one which 

 may be expected to develop slowly and have considerable per- 

 sistence. 



The effect of sulphur on the soil reaction is to increase acidity, 



