CH. XV] DISEASES OF PLANTS AND THEIR TREATMENT 137 



from it, with considerable virulence. In the case, which some- 

 times occurs, of a disease absolutely new to the neighbourhood, 

 this may be due simply to the fact that no natural enemies 

 have as yet appeared, but in the case of a disease already 

 known, though rare, in the district, there must presumably be 

 some alteration of the conditions that hinder its growth, or it 

 may be, as in the case of most of the weeds that have at 

 different times spread over Ceylon, that a longer or shorter 

 period of acclimatisation, or of accumulation of energy, is 

 required before a rapid spread can take place. Be this as it 

 may, however, this case of the rapid spread of a "new" disease 

 is the one that most often comes to the notice of the officers 

 concerned with diseases. Here the density is greatest nearer 

 to the edge of the affected area, and is decreasing in the middle. 

 The attack therefore, should be chiefly at the points of maximum 

 density, and outside of these, whereas in the middle, where the 

 density is decreasing in any case, attack will do very little good 

 in comparison. If the disease be left to run its course, it will 

 spread over a greater or less area, reach a maximum density at 

 each point in succession, and then fall off to an average density 

 which may often be so low that most people think that it has 

 entirely disappeared. All these various phenomena, the ap- 

 pearance of a new weed, like the Mexican sunflower in Ceylon, 

 the appearance of a new disease, or the springing up of a new 

 industry, like that of rubber cultivation, behave in the same 

 way, and a recognition of this fact would simplify measures for 

 dealing with them for repression, encouragement, or whatever 

 else was required. 



Just as diffusion may be hindered by the interposition of 

 obstacles, so also may the spread of disease. Shelter belts of 

 forest may be left in the country, or shelter belts may be 

 planted by agriculturists between their fields; trenches may 

 be dug round infected areas, and so on. 



