VEGETABLE PARASITES AND WEEDS 175 



grow upon the surface of their host, drawing their nourish- 

 ment through the epidermis; others, Hke the oat smut 

 and the bacterial bhghts, grow within the tissues of the 

 plant upon which they feed. Most of the fungi send 

 their fruiting parts to the surface of the host plants, where 

 they produce spores (52) in immense numbers. The 

 bacterial developments more often take the form of slimy 

 exudates which may ooze from the surface as with fire 

 blight of pear. Such fungous spores are most frequently 

 disseminated by the wind, and the bacteria by insects, 

 although water and other agencies, including man, may 

 also play important parts in their distribution. 



322. Methods of controlling plant diseases are numer- 

 ous and varied, but those most effective may be grouped 

 into three classes : 



(a) Removing and destroying the affected parts. 

 (6) Preventing infection, chiefly by the use of a fungi- 

 cide. 



(c) Increasing the resistance of the host to the parasite. 



323. Destruction of the affected parts is especially im- 

 portant with the bacterial diseases, such as fire blight of 

 pear and apple {Bacillus aviylovorus) , where spraying is in 

 general useless. This method is applicable with certain 

 conspicuous fungi which spread rapidly from part to part, 

 as brown-rot of stone fruits (Sclerotinia fructigena) and 

 corn smut ( Ustilago zeae) and also with certain fungi 

 which tend to spread through the plant tissues, like the 

 red rust (Gymnoconia interstitialis) of raspberry and black- 

 berry and the black knot (Plowrightia morhosa) of plum 

 and cherry. 



The affected part should be removed as soon as dis- 

 covered and burned at once, to destroy any spores or 

 germs of the parasite it may contain or which might 



