CHAPTER XXXIX 



ORNAMENTAL PLANTS SUBJECT TO DISEASE 

 AND INSECT PESTS 



The purpose of the list of plants given below is not to discourage any 

 prospective planter, even though the list is a formidable one, nor to 

 catalogue all the ornamental plants which may be affected by insects 

 or disease. It should be remembered that ornamental plants, like 

 animals, are much less subject to disease, and less likely to be harmed 

 also when in good condition. Thus the average person who grows 

 ornamental plants will, if he keeps them from being damaged by 

 mechanical factors such as wounds, lack of food and water, poor soil 

 aeration, and so on, not need to worry very much as to whether they will 

 succumb easily to disease or insect attacks. 



The true diseases of plants are either physiological, such as tip burn, 

 due to over-transpiration of water during a hot wind, or parasitic. 

 The parasitic diseases either kill by the secretion of toxins and enzymes 

 which destroy plant tissues, or these toxins and enzymes cause ex- 

 cessive tissue growth or diversion of food substances of the plant to the 

 use of the parasite. When the plant tissue is killed a rot, blight, or leaf 

 spot appears and when the second effect takes place the result is a gall, 

 leaf curl, rust, or smut. The first effect, which results in the immediate 

 death of the plant tissue, is of course the most harmful. Anthracnose 

 is a disease caused by one definite sort of fungus or parasite. 



While the diseases of plants are not something new, since the his- 

 torical writings of the ancients mentioned rusts, cankers, and smuts, 

 the study of how to combat them is not only a new but an everchanging 

 subject. Accordingly, the reader is urged to consult the latest bul- 

 letins of his State Experiment Station or of the U. S. Department of 

 Agriculture before embarking very far upon a programme of spraying or 

 of otherwise combating these diseases. 



Not all insects which live on or about plants are harmful. Thus 

 the honey bees, the "lady-bugs," and many others are beneficial since 

 they either pollenate the flowers or eat other harmful insects. The 



382 



