DISEASES OF CULTIVATED PLANTS 427 



in these lines. Such a statement will not close the march of progress 

 nor make less the need for more knowledge. It is hoped that cultiva- 

 tors of plants, whether farmers, gardeners, horticulturists or florists, 

 will find suggestive statements of information in the bulletin by 

 which they can direct their own efforts to better advantage and cor- 

 rect or broaden their own inferences from observed conditions about 

 them. All such results will not only increase the need for more 

 knowledge, but will furnish impetus to the movements by which we 

 will gain the desired information. 



It is fully apprehended that the host plant is the center of prac- 

 tical as well as economic interest and these statements concerning 

 enzymatic diseases as in the case of peach yellows and mosaic disease 

 of tobacco, diseases transmitted in the seed, soil infesting diseases, 

 and the relation of the spread of certain diseases to leaf biting insects 

 are given as aids in mastering the principles involved. The same 

 aim has governed the discussions upon wounds and wound fungi so 

 especially dangerous with orchard, shade and forest trees. Some- 

 what fuller discussion of atmospheric agencies as affecting the occur- 

 rence and spread of plant diseases, of remedies for diseased condi- 

 tions and of the application of the latter in combatting diseases and 

 a presentation of storage troubles has also seemed desirable. Special 

 attention is called to the host plant in the matter of breeding or se- 

 lection for disease resistance and in the contrasts offered by American 

 and European points of view in plant disease study. 



CONCERNING PLANT DISEASES IN GENERAL. 



As defined in the introduction, a plant is called diseased when it 

 fails to show normal vigor and normal condition of its parts. The 

 manner of disease attack is extremely varied and the conditions set 

 up as a result of disease are accordingly of many different kinds. We 

 learn to recognize disease by the symptoms shown in the plant ; these 

 symptoms will at times be readily interpreted and on other occasions 

 they will prove misleading. Nothing is plainer than the necessity 

 for continuous observation of growing plants if one is to be in a 

 position to interpret the symptoms of disease. 



Diseased conditions may be due to the very obvious attacks of 

 certain parasitic seed plants which lack leaf-green or chlorophyll in 

 their tissues and must subsist on other plants somewhat after the 

 manner of parasitic fungi. The dodders which attack the clovers, 

 alfalfa, onions, etc., belong in the class of parasitic seed plants of the 

 genus Cuscuta. Their seeds are liable to be harvested with the seeds 

 of clover or alfalfa and to be present in the commercial seeds. While 

 these have been treated in the weed manual they require mention 

 here. The seeding plant of dodder first forms a root and sends up- 

 ward a whitish stem which twines about the clover or other stem, 

 and sends sucking branches into the stem interior. These hrmstoria 

 extract food material from the clover stem that is, they rob it of its 

 own substance. Upon the formation of such organs the root of the 

 dodder dies off and the future existence of these twining, ,<trawlike 

 etems is at the expense of the host plant. 



