1 66 DISEASE IN PLANTS. 



favour the spread of any disease is provided by 

 growing any crop continuously in " pure culture " 

 over large areas. This is sufficiently exemplified 

 by the disastrous spread of such diseases as Wheat- 

 rust, Larch-disease, Potato-disease, Phylloxera, 

 Hop-disease, Sugar-cane disease. Coffee-leaf disease, 

 and numerous other maladies which have nov/ 

 become historic in agricultural, planting, and forest 

 annals. Providing the favourite food-supply in 

 large quantities is not the only factor of an 

 epidemic, but it is a most important one in that 

 it not only facilitates the growth and reproduction 

 of a pest, but affords it every opportunity of 

 spreading rapidly and widely. 



Moreover, Nature herself shows us that such 

 pests are kept in check in her domain by the 

 struggle for existence entailed by innumerable 

 barriers and competitors. As matter of experi- 

 ence also it is found that rotation of crops, planting 

 forests of mixed species, and breaking up large 

 areas of cultivation into plantations, fields, etc., of 

 different species afford natural and often efficient 

 checks to the ravages of fungus and insect pests. 

 Over and over again it has been found that a 

 fungus or an insect which is merely endemic so 

 long as it is isolated in the forest, where its host 

 is separated from other plants of the same species 

 by other plants which it cannot attack, becomes 

 epidemic when let loose on the continuous acres so 

 beloved of the planter. And the same reasoning 

 applies to the success of such pests on open areas 

 from which the birds or other enemies of the pest 



