NEBRASKA FOREST FUNGI. II. 



SOME LEAF-INHABITING FORMS. 



By Raymond J. Pool. 



Each of the main groups of fungi furnishes a number of 

 species that attack the leaves of forest trees. Many such attacks 

 are so slight that they are hardly noticed while others assume 

 extremely destructive proportions. The ravages of fungous 

 pests of economic importance usually depend upon the virulence 

 of the diseases caused by parasitic forms and upon the rapidity 

 of their dissemination. Some are extremely virulent diseases 

 but infection travels so slowly that the disease is of little import- 

 ance. In some cases it seems that its spread is checked or finally 

 inhibited completely by its own ravenous nature. And again 

 there are numerous diseases not nearly so virulent that spread 

 with great rapidity and hence the summation of destruction be- 

 comes very great over a wide territory. Leaf fungi are apt to 

 behave in this manner. 



We have in reality obtained, at present, but the merest be- 

 ginning of a complete knowledge of the physiology of leaf fungi, 

 notwithstanding the seeming voluminous literature upon the sub- 

 ject. True, we know something of how the parasitic fungus 

 enters the host plant, and some of the general morphological 

 reactions of the latter to such an attack. But when we think of 

 the complications introduced by the heteroecious habit and by the 

 seeming ease of adjustment of parasitic fungi to differing host- 

 physiology without any apparent morphological response, we are 

 forced to confine our statements to generalities. At present we 

 merely classify certain external and internal evidences and call 

 these "symptoms of disease", which procedure are of course one 

 of the first steps in a logical investigation. Thus we generalize 

 and say that such a disease as the Tar Spot of the maple is 

 injurious because "it draws nourishment from the host that 

 would otherwise go to the upbuilding of host tissue", and "that 

 the photosynthetic area is considerably reduced." All this is 

 probably true, but is that all of the question? 



Very naturally the next thing considered is some suggestion 

 for treatment or eradication of the disease. The many mistakes 

 that have been made not only in diagnosis, but also in suggested 

 and attempted remedial measures, show quite well the present 

 conditions of affairs. No one contemplating entering this line of 

 investigation need be discouraged for, altho the field may seem 



