12 DISEASES OF CROP-PLANTS 



less intensive cultivation resulting from the increase in size of 

 estates under the factory system, the reduction of humidity 

 by clearing away trees, the diminished supply and increasing 

 cost of the labour required for hand cultivation, and perhaps 

 above all to the treacherous facility with which the benefits of 

 organic manures can apparently be obtained by the use of 

 chemical substitutes. 



The passing of estates into the hands of mercantile rather 

 than agricultural proprietors (a frequent consequence of agricul- 

 tural depression) with the break in traditions involved, has often 

 led to a visible decline in agricultural conditions and thus to 

 a corresponding increase in disease. 



There is a further cause of debility disease very frequently 

 encountered : the planting of a crop in a situation which from 

 soil or climate is somewhat unsuited to it. If a district or an 

 island is successful with a given crop, attempts are always 

 made to extend it to marginal or supposedly similar locations. 

 Under the less suitable conditions production is smaller, and it 

 too frequently happens that the need for greater attention to 

 agricultural practice in order to offset the defective conditions 

 cannot or will not be understood or the expense cannot be 

 afforded. In such a case the soundest advice that can be given 

 with reference to the diseases which invariably occur is to plant 

 a crop more suited to the situation. It is advice which is fre- 

 quently required, seldom given, and never heeded until necessity 

 dictates it. 



Returning to more definitely parasitic diseases, there are 

 various natural causes producing broad effects in the prevalence 

 of disease. The disastrous potato blight epidemic in Ireland 

 in 1845, the social consequences of which have had a marked 

 effect on British and American history, was brought on by a 

 period of dull and humid weather. Nearer at hand a week or 

 two of heavy rain in St. Vincent has more than once brought 

 about the destruction of the visible cotton crop by boll-rotting 

 fungi. An instance of the complexity of the factors sometimes 

 involved is afforded by the history of internal boll disease in the 

 same island, where the eradication of casual trees of two species, 

 by interfering with the breeding of an insect, has saved the later 

 pickings of cotton from a fungus disease which regularly destroyed 

 them. 



The Principal Types of Fungus Diseases. 



Every part of a plant is liable to attack by fungi, but each 

 individual parasite affects only certain parts or stages in a 

 certain way, and usually produces a disease of a constant and 

 recognisable type. 



Leaf Diseases. 



Fungus infections on leaves commonly give rise to localised 



