Plant Diseases 



429 



Ue of Forest Pathology, U.S.D.A. 



Fig. 267. Effects of chestnut bark disease. A familiar sight everywhere within 200 miles of 

 New York City, where the disease was first seen in 1905. 



apples occur usually only in wet seasons. Dry weather is 

 favorable to the spread of the spores of the loose smuts of cereals 

 from an infected plant to the blossoms of another. When the 

 seed is planted, untreated, the following year an abundance of 

 smut results. 



Low temperatures at the time of planting favor certain diseases, 

 and high temperatures favor others. For this reason the preva- 

 lent diseases of one season may be very different from those of the 

 following season. 



Soil influences. The most usual effect of soil is in harboring 

 bacteria and fungi from one season to another and in this way 

 transferring the infective agent from one crop to the next. This 

 is particularly important in causing infection with the fungi that 

 cause the damping off of seedlings ^nd the various forms of root 

 rot. 



The character of the soil, whether acid or alkaline, may also 

 influence the growth of the fungi or the host in such a way as to 



