16 PLANT DISEASES 



in disseminating spores is most apparent where numerous 

 plants of the same kind grow in close proximity, as in the 

 case of a field of corn, when a gentle breeze is sufficient to 

 bring diseased and healthy plants in contact ; and if the 

 leaves are damp with dew or rain, such conditions are 

 highly favourable to a rapid extension of disease. 



Heavy rain washes spores to the ground, where they 

 germinate and perish in the absence of the proper host; 

 whereas a slight shower or gentle drizzle favours the spread 

 of a disease, by washing spores from one part of a plant to 

 another or on to adjoining plants. 



Insects, birds, snails, and slugs are known to be un- 

 conscious agents in the dispersion of spores, whereas 

 dogs, hares, rabbits, etc., running through a field of corn, 

 potatoes, or turnips, act after the fashion of wind, by 

 bringing into contact adjoining plants. 



Spores or fragments of mycelium capable of growing 

 are too frequently transported from infected to healthy 

 localities by human agency. As illustrations may be 

 mentioned the spores of the ' finger-and-toe ' disease of 

 cabbages and turnips, which are readily transported in soil 

 adhering to cart-wheels, tools, shoes, etc. Spores are also 

 readily transported by clothing ; a gardener moving about 

 among a number of closely packed chrysanthemums can 

 hardly fail to brush off some of the powdery uredospores 

 of the chrysanthemum rust, if such is present, and will 

 as certainly have the spores brushed off his clothing by 

 other plants. 



Parasitic fungi attacking plants grown under glass are to 

 a great extent deprived of those means of spore dissemina- 

 tion so efficient to fungi growing in the open air. This loss 

 is, however, perhaps more than compensated for by those 



