266 FARM FRIENDS AND FARM FOES 



on new ground. In the case of many other diseases, how- 

 ever, the fungus remains in the soil from year to year, and 

 unless crop rotation is practiced, each succeeding season 

 is likely to show greater damage. This is notably true of 

 some of the smut diseases: a garden in which sweet com 

 was grown for many years finally produced few ears that 

 were not injured. This fact makes more plain the lesson 

 of all good agriculture that a rotation of crops is one of 

 the first essentials to success. 



In general, good tillage and fertilization help to prevent 

 injury by fungous diseases. Many plants seem better able 

 to grow without damage by fungi when they are in a 

 vigorous condition. This is not always true, however, for 

 the very rapidity of growth in some plants produces condi- 

 tions favorable to the development of the parasite. Pear 

 trees of quick growth are much more Kable to blight than 

 those of slower growth, and grains growing vigorously seem 

 to be more liable to injury by rusts than those of slower 

 growth. 



Sometimes it is possible to exclude the germs of the para- 

 sites by mechanical means. The various mildews and rots 

 that affect grapes are readily excluded by fastening paper 

 bags over the stems soon after the fruit is set. 



In many instances the easiest way to prevent the spores 

 of fungi from reaching new hosts is through the direct de- 

 struction of the fungus that produces the spores. This is 

 obviously a practical method that may be adopted under 

 greatly varying conditions. In general it involves clean 

 culture and the destruction of refuse materials upon which 

 spores may be developed. The plants of many crops may 

 continue to produce fungus spores after the fruits are 

 gathered, and the burning or deep burying of such plants 

 is very desirable. In the case of species of fungi that 



