238 AGRONOMY 



ruin the crop. There are numerous species of rust, each re- 

 stricted for the most part to a limited number of hosts. One 

 of the most remarkable features of their life history is the 

 fact that two different species of plants are usually required 

 to complete their round of existence. The wheat rust is found 

 in spring upon the barberry and not until later does it infect 

 the wheat plant. The corn rust grows first upon a species of 

 oxalis ; the apple rust upon coniferous trees. Late in the 

 season the rusts produce spores which last through the whiter 



FIG. 176. Apples affected by apple scab 

 From Duggar's " Fungous Diseases of Plants " 



and set up the infection upon the first host plant again. It 

 occasionally happens that the first of the two plants necessary 

 for a complete life cycle of a rust is absent from the locality. 

 In this event most species are able to omit this part of the 

 cycle and begin at once upon the second host. Many rusts 

 produce no less than four different kinds of spores. 



Wound parasites. Many enemies of the woody plants are 

 fungi that gain entrance through wounds, as where a branch 

 has been torn off by the wind, or through a break in the bark 

 caused by insects or mammals. These parasites live upon the 

 old parts of the tree until well established, but ultimately 

 extend to the living parts and cause their death. Often their 



