loo Minnesota Plant Diseases. 



Epidemics. When a fungus disease becomes particularly- 

 abundant and devastates great fields or forests of certain host 

 plants there arises an epidemic. There have been notable epi- 

 demics of continental extent in historical times just as there 

 have been famous plagues attacking man. Potatoes have many 

 times been decimated by the blight, forests have been threat- 

 ened by the honey mushroom ; the mallow rust has swept over 

 Europe and America damaging many kinds of mallow; while 

 year after year one may read of epidemics of grain rusts and of 

 smuts. These epidemics are more widespread in some years 

 than in others. This last season (1904) has seen wheat-rust 

 epidemic almost throughout the northwestern United States 

 and Canada. In Ceylon the coflfee disease has ruined hundreds 

 of cofifee plantations. A remarkable fact in these epidemics is 

 that the fungi which produce them may have been present a 

 long time previous to the epidemic without exciting any great 

 amount of damage. It is well known, for instance, that potato 

 blight, wheat rusts, mildews and smuts are always with us, but 

 that not every year furnishes epidemics. It is therefore evident 

 that other factors besides the immediate cause or fungus fac- 

 tors must be present. Of these, weather conditions are usually 

 the most important factor. Potato blight never thrives in dry 

 weather or on plants in sandy soil, but is at its best when the 

 weather for days is misty and moist so that the fungus can form 

 its swimming spores and distribute them rapidly. It is just in 

 such weather as this, and particularly after a warm growing sea- 

 son, when the leaves are swollen with moisture and rich in food 

 material, that blight strikes. Several of such epidemics of enor- 

 mous extent have been known. It is also a well-known fact 

 that wheat rust often follows upon very moist springs and early 

 summers. In fact many people still think that the wet weather 

 causes rust. And they are not altogether wrong, but the effect 

 of the weather is not exactly as such people imagine. Warm, 

 moist weather is just the kind of weather which is favorable to 

 the development of the summer spores of the rust fungus and 

 the fungus grows luxuriantly, producing in two weeks or less 

 another crop of summer spores, thus multiplying an hundred or 

 a thousand-fold in this short time. 



