16 DISEASES OF CULTIVATED PLANTS 



aphis ' (Schizoneura lanigera) into this country. The wounds 

 and gouty swellings made on the branches by the ' woolly 

 aphis ' are exactly of the nature required by the * canker ' 

 fungus to enable it to effect an entrance into the living tissues 

 of a branch. When the fungus has once established itself and 

 produced fruit, the 'woolly aphis,' aphides, and other insects, 

 again lend their aid, by carrying the spores from one part of 

 the tree to another. The result is an epidemic of 'canker.' 

 As the result of extended observation on this matter, I think 

 it would be scarcely an exaggeration to say that if we had no 

 'woolly blight' we should have^no ' canker,' that is in the 

 sense of an epidemic. 



A whole volume could be written on the subject of spore 

 dispersion, but perhaps sufficient has been explained to indi- 

 cate the various possibilities by which this object can be 

 attained. The subject cannot, however, be dismissed without 

 some allusion to the part played by man in this connection. 

 When a crop of any kind has been attacked, portions of the 

 diseased plants are constantly left on the ground, even when it 

 is practicable to remove such ; in many instances owing to 

 rapid decay such a course is not possible. In such cases the 

 diseased plants eventually decay and liberate the spores in the 

 soil, where they remain for some time in a living condition, 

 ready to infect any subsequent crop. The matter, however, 

 does not end here ; the infected soil may be, and is, constantly 

 conveyed from one field to another by adhering to the wheels 

 of carts, implements, and in various other ways that will sug- 

 gest themselves. When an epidemic breaks out in plants 

 grown under glass, as in a cucumber house, in some instances 

 many precautions are taken to prevent the disease from 

 spreading to other houses, but it rarely, if ever, occurs to 

 those concerned that spores may be carried on the clothes or 

 hands of workmen, although such is the fact. Of course it 

 may be urged, and with good reason, that it is practically 

 impossible to guard against such cases, nevertheless it is well 

 to be acquainted with the possibilities. 



Too frequently when an endeavour is made to remove and 

 destroy diseased material, the attempt is not a success, owing 

 to lack of knowledge as to the amount of apparently drastic 

 treatment spores can undergo, without material injury. Mil- 

 dewed peas, rusted bean straw, diseased fruit, mangolds, etc., 

 are carefully collected and thrown into the piggery or some 

 similar place, on the supposition that they will either be eaten 



