8o Minnesota Plant Diseases, 



drawn before the death of the host-plant part. In the second 

 case the fungus produces on the surface of its invading threads 

 a chemical substance which kills the host plant tissues and the 

 fungus absorbs its nourishment later from this killed area. It 

 is only fungi which know the first method that can stimulate 

 their host to extraordinary growth and over production of food 

 material, as in the witches'-brooms. The second method is 

 common among half saprophytes. 



The destroyers of organs. Many leaf mildews attack so 

 vigorously that the whole leaf is unfit to perform its natural 

 function. In this case the leaf is impoverished and usually 

 turns yellow or brown and falls. In other cases the fungus, 

 while not withdrawing much nourishment, may cause leaves 

 to fall prematurely. Such are often known as leaf-casting dis- 

 eases. The blight of potatoes may extend over all of the foli- 

 age leaves, blighting them and causing death. The death or 

 fall of leaves before the normal period of fall is a serious injury 

 to the host, as every leaf lost is a fractional loss in the manufac- 

 ture of starchy material. The destructive attacks of smuts on 

 the fruit of grasses and upon the stamen of pinks have already 

 been mentioned. Rusts of grasses never, of course, cause the 

 fall of leaves but they may very seriously affect the starch- 

 making power of the plant, and so very materially injure, the 

 crop. Branches are sometimes killed off by fungus parasites. 

 Such may be the wound parasites which attack the trunks and 

 branches of trees. The tax which a witches'-broom levies on 

 a branch may kill off, by indirect action, the branch beyond its 

 insertion. Whatever the attacked organs may be, if invaded 

 in sufficient numbers their loss may cause the death of the 

 whole plant. 



The destroyers of whole plants. There are many fungi 

 whose usual effect of parasitism is the death of the whole host 

 plant. Conspicuous among these is the damping-off fungus 

 which attacks seedlings. Wound-parasites of trees, when they 

 have successfully invaded the trunk of the tree, or root-wound 

 parasite may cause the death of the whole tree. Powdery mil- 

 dews, rusts, and especially the downy mildews, such as the po- 

 tato blight, very often kill the whole plants. Death is here the 

 accumulative effect of the attack on the various organs. Sev- 



