Minnesota Plant Diseases. 207 



proof varieties. It must, of course, be understood that a variety 

 may be immune from one disease and not from another totally 

 different disease. In the selection of varieties this feature must 

 be constantly kept in mind. Moreover, there are usually other 

 features of importance in plant growing such as annual yield, 

 quality, and so on, which are of great importance in the selec- 

 tion of varieties. The best variety is, of course, that which, un- 

 der given conditions, will yield the best sum total results. It 

 is sufficient here, however, to point out the fact that plant dis- 

 eases are very imporant factors in the selection of plant varieties, 

 and that such selection can assist the plant grower in the pre- 

 vention of diseases. 



Prevention of spread. The methods of prevention discussed 

 above all deal with a prevention of infection. They are at- 

 tempts to prohibit the beginnings of a disease. But diseases 

 may sometimes obtain a start and the plant grower may still be 

 able to use preventive methods. The latter now, however, are 

 directed towards a prevention of the spreading of a certain dis- 

 ease — in other words — to prevent epidemics. Such preventive 

 methods usually consist in the burning up of infected plant 

 parts so that the fungus spores or mycelium will be destroyed. 

 The spread of wood rots may be prevented in this way and the 

 infected branches of the black knot of plums and cherries should 

 always be removed and burned. It is well known that a large 

 •number of diseases live over winter in the fallen leaves or dead 

 branches of trees or in the stubble of wheat or straw or refuse 

 piles. It becomes apparent that cleanliness must be an impor- 

 tant weapon in fighting plant diseases. The only successful 

 method of removing such refuse, fallen leaves, etc., is by burn- 

 ing. In the case of field crops fall plowing may also be useful 

 in addition to the burning process. It is not recommended that 

 all straw stacks be immediately burned. It is necessary, how- 

 ever, for the grower of plants to know the disease he is com- 

 bating, and if it is found to winter over on straw or refuse of 

 any kind, measures should be taken to prevent the spread of the 

 disease by destroying its winter abode or by rendering the spread 

 of the fungus from these places impossible. A preventive 

 method already mentioned may be recalled here. It relates to 

 those diseases, as rusts, which live at different times on different 



