CHAPTER XXIX. 



FUNGUS AND OTHER DISEASES OF PLANTS 



Fungi differ from plants possessing green leaves in not being 

 able to obtain their food from the soil; they can only derive 

 nourishment from the bodies of plants or animals, either living or 

 dead. Those fungi that live on dead vegetable matter, as humus, 

 decaying wood, roots, etc., are called saprophytes; these do not 

 injure living plants, unless they alter their mode of obtaining nutri- 

 tion, as they sometimes dQ, and become parasitic. Fungi which 

 grow on living plants are known as parasites; the greater number 

 of these are very minute, and are only visible to the naked eye 

 when present in great numbers. All parasites cause disease, 

 important or not, according to the extent to which they occur and 

 the harm they do to plants or crops. 



Preventing the spread of diseases. The methods available 

 for combating diseases are more or less regulated by the life 

 history of the fungi concerned, and may be grouped under the 

 following heads: (1) by destroying the plant tissues that contain 

 the vegetative forms or the reproductive spores of the fungus; (2) by 

 spraying with a fungicide to prevent the germination of spores 

 deposited on leaves, stems, or fruits; (3) by avoiding conditions 

 that are known to be favourable to the spread of the disease; (4) by 

 raising disease-resistant varieties; (5) by prohibiting the impor- 

 tation or transport of plants or seeds from diseased -infected coun- 

 tries or localities. 



Destroying plant tissues. The most certain means of eradi- 

 cating a disease is to burn the affected plants, or cut and burn the 

 diseased portions of these. When the latter course is adopted, 

 the cut surfaces of the living plant should always be painted with 

 tar or other substance that will prevent the germination of fresh 

 spores which may fall on them. When burning is not practicable, 

 .as through excessive moisture, the diseased parts should be buried 

 -with lime. Lime not only hastens decay, but also prevents local 

 souring of the soil by the addition of large quantities of decaying 



