86 BOARD OF AGRICULTURE. [Pub. Doc. 



generation, and the fact that theoretically each single spore 

 is able to reproduce the fungus, as a seed does the plant to 

 which it belongs, the wonder is, not that there are so many 

 fungous diseases of plants, but that any plants escape the 

 attacks of these minute but ubiquitous parasites. Nor would 

 they, except for certain features characteristic of the fungi 

 themselves. The first is the comparatively small number of 

 strictly parasitic fungi and the partiality shown by most 

 of these for a certain species or group of species of plants. 

 This in itself is a mighty safeguard. Thus the fungus which 

 causes the "smut" of onions will not attack corn; the 

 " downy mildew" of lima beans is limited absolutely, so far 

 as I know, to that one plant ; the " leaf-spot " of the straw- 

 berry cannot be transferred even to so nearly related a plant 

 as the blackberry. Similar instances might be cited almost 

 indefinitely. So limited are the preferences of parasitic 

 fungi that it is possible to arrange them in a fairly satisfac- 

 tory artificial system, according to the host-plants which they 

 severally afiect. It is evident that in this fact is to be found 

 one cause for the limited spread of fungous diseases. 



Another though far less important factor is the extreme 

 delicacy of that particular form of reproductive bodies which 

 I have spoken of as spores. Since they are borne freely 

 exposed to the air and are produced only during the summer, 

 they might more properly be called summer or aerial spores. 

 Though borne in countless multitudes, characterized by such 

 minuteness that the slightest breath of wind carries them 

 hither and thither often over great distances, requiring but 

 the thinnest film of water for their germination and growth, 

 and therefore being able, under favorable conditions, to cause 

 the rapid spread of disease, sometimes over a large area in 

 the course of a few days, as in the case of the "potato 

 mildew," yet the delicacy of these summer spores is such that 

 they cannot long withstand conditions adverse to their de- 

 velopment. Some degree of moisture, however slight, is 

 essential ; hence we find that the spread of fungous diseases 

 is more rapid in damp, close or foggy weather than when the 

 air is dry. Let the latter condition prevail, and the summer> 

 spores will perish unless protected. This result of course 

 occurs more frequently where sunshine and air have free 



