METHODS OF PREVENTION" 13 



Selection of Resistant Varieties. — It has long- 

 been observed that with many crops certain varieties are 

 more liable to fungous diseases than others. Tlie reason 

 of this, in many cases, is not well understood, but the 

 fact will lead the careful cultivator to select such varie- 

 ties as prove most resistant, other things being equal, in 

 his locality. Dr. B. D. Halstead has recently shown 

 that plants with variegated foliage are much more sub- 

 ject to fungus attack than those with green leaves. 



Rotation of Crops. — The spores of the great ma- 

 jority of parasitic fungi pass the winter on or in the soil 

 where the crop attacked was grown, and if a crop of the 

 same kind is planted there the next season it is almost 

 certain to be infested. The onion smut is a striking- 

 example of this. In consequence, the proper rotation 

 of crops is not only desirable, but often imperatively 

 necessary. The rotation should be so arranged that 

 each crop will not be liable to attack from the diseases 

 of the preceding crop. 



Clean Culture. — Injurious fungi may be largely 

 destroyed by keeping the farm premises clean and free 

 from weeds and rubbish. Burning potato tops, old 

 tomato vines, and similar refuse, will destroy millions of 

 spores. Some fungi which infest cultivated crops also 

 grow upon weeds, and, hence, keeping the ground free 

 from the latter may prove of great benefit to the former. 

 Care should also be taken not to distribute the spores by 

 means of stable manure. 



Mechanical Exclusion. — It is often practicable 

 to prevent the spores from reaching the crop by methods 

 of mechanical exclusion. The most familiar example of 

 this is to be found in the practice of bagging grapes to 

 prevent their rotting. Small paper sacks are tied over 

 the bunches as soon as the fruit is well formed, and 

 allowed to remain until it rii^ens. The sacks exclude 

 the rot spores, and also prevent insect attack. 



