54 MASSACHUSETTS HORTICULTURAL SOCIETY. 



the natural groups of the fungi which should be included here may 

 be especially noted the downy mildews (those of grape, potato, 

 cucumber, etc.); the powdery mildews (those of the rose, grape, 

 lilac, etc.); the peach leaf curl, the rusts, smuts, and other well- 

 known groups. In addition to the fact that the above mentioned 

 fungi, together with many others, are able to penetrate vigorously 

 growing plant organs, these especially enumerated have, for the 

 most part, another characteristic which should be noted. They 

 do not kill the tissues which they invade, or at least do not kill for a 

 considerable period of time. All sorts of deformities, or "witches' 

 brooms," may appear as a result of stimulation due to these fungi, 

 but they seem to have a well established relation with the living 

 cells such that when the tissues which they invade are killed the 

 fungus must very promptly cease its growth. 



In contrast to the above, there is that general sub-division of 

 diseases which may be roughly designated as leaf spots, stem rots, 

 and fruit decays. The fungi producing these affections frecjuently, 

 though by no means always, kill the tissues as they penetrate the 

 host. In other cases they only enter and produce disease when the 

 affected parts have suffered some injury, over-stimulation or drying 

 out. In the case of fruits they are proverbially destructive when 

 the fruits approach maturity. In other words, a very large number 

 of the fungi here included are not in very close association with 

 Hving tissues, and are, from several points of view, hemi-parasitic. 

 In nearly all cases these fungi may be grown upon artificial media 

 in the laboratory. It is to be emphasized, however, that many of 

 our disastrous fungous diseases are included in this sub-division, 

 such as the rot of stone fruits, the bitter rot of apple, the various 

 anthracnoses, apple scab, and numerous other well-known horti- 

 cultural maladies. In the case of these fungi in particular, it is 

 not at all surprising that the effects of conditions upon the host is 

 most important in infection. 



Almost as long as fungous diseases have been known, there has 

 existed the behef that such diseases in any given host plant are in 

 some way dependent upon a certain lack of vigor in the plant. With 

 perfect propriety, practical growers and many plant pathologists 

 have held that vigorous, well-cultivated and well-nourished plants 

 mean plants resistant to disease. No one would today question 



