SUBSEQUENT CARE OF THE PLANT 167 



squash-bug or stink-bug, and the leaf -hoppers ). 

 We may classify injurious insects again, without 

 reference to their mode of taking food, into 

 those which live and feed on the outside of the 

 plant, and those which, as borers and apple- 

 worms, burrow and feed inside the tissue. 



292. Of fungous pests, the farmer may recog- 

 nize two groups, those which live wholly on the 

 outside of the host (as the powdery mildew of 

 the grape, pea mildew), and those which live 

 wholly or in part inside the tissues (as apple- 

 scab, black-knot, potato mildew). Most inju- 

 rious fungi are of the latter kind. Fungous 

 troubles are nearly always marked by definitely 

 diseased spots on the leaves or twigs. 



293. Physiological or constitutional troubles 

 are those which affect the whole plant or an 

 entire leaf or branch, and the cause of which is 

 not apparent on the exterior. These troubles 

 may be due to germs or bacteria working within 

 the tissues (as pear-blight), or to some difficulty 

 in the nutrition of the plant. These troubles 

 are generally not marked by definitely diseased 

 spots or blemishes, but by the gradual dying of 

 an entire leaf, branch or plant. 



36. The preventives and remedies 



294. Keeping the plants vigorous and healthy 

 is the first step towards the control of pests and 



