206 Minnesota Plant Diseases. 



out soil by the action of the clover root tubercles. In other in- 

 stances the rotation is effective in preventing disease ; the latter 

 can usually not infect the alternate crop and thus may be eradi- 

 cated before a new crop is planted. This is often a very effect- 

 ual method of preventing disease. By "pure cultures" are meant 

 the great fields of one kind of plant, such as the common wheat 

 fields of our own four neighboring states. The plants in such 

 fields are all at about the same stage of development. When 

 such a disease as a rust obtains a start in these fields the winds 

 rapidly spread the spores and no obstructions are raised to the 

 wholesale scattering. The result is a veritable epidemic. As 

 long as large, unprotected fields of this sort are planted just so 

 long will there be a tendency toward epidemics of rusts. The 

 planting of such pure cultures therefore carries with it undoubt- 

 ed risks. This is also true in forest culture where mixed for- 

 ests have in this respect advantages over pure unmixed ones. 



Fertilisers. The manuring of soils may under some condi- 

 tions bring dangerous diseases. In some smuts the spores may 

 continue to live and grow for a long time in the nutrition fur- 

 nished by the manure and may be introduced into the field by 

 this means. Old manure is preferable to fresh manure, since in 

 the former fungi may have died out or become enfeebled. The 

 fresh manure may contain the more vigorous fungi. Of course 

 it must not be assumed that manuring of fields is therefore al- 

 ways injurious, but two points must be noted : first, the kind of 

 manure used and its source; and second, the prevalence of such 

 diseases as thrive in manure, e. g., certain smuts of grains. In 

 other words, manuring may furnish favorable disease condi- 

 tions. That it does so always or even commonly is not implied. 



Selection of varieties. The selection of varieties is becoming 

 more and more important for success in plant growing. The 

 immediate objects of such selection may be various, e. g., in- 

 crease in yield, quality of yield, hardiness, etc. It has already 

 been pointed out that plants of a given species may vary in their 

 susceptibility toward certain diseases. The cause of such im- 

 munity or predispositoin is not understood in many cases, but 

 certain facts of immunity are undoubted. It is possible, there- 

 fore, to select varieties of plants which may show successful re- 

 sistance toward prevalent diseases in other words, disease- 



