STEVENS: PREVALENCE OF ENDOTHIA GYROSA 135 
greater opportunity for infection in that region, aided by more 
favorable temperature and more abundant rainfall. Other things 
being equal, the more frequent the occurrence of places favorable 
for its growth the more abundant a fungus will become. This 
process is cumulative, for the more prevalent a fungus the more 
completely it will occupy the available places of infection. 
‘Greater opportunity for infection in the south is due to the 
greater number of host species and to the greater importance of 
these species relative to other trees. And especially to greater 
opportunity for the infection of individual hosts from injuries due 
to different climatic, soil and cultural conditions. 
Host RELATIONS 
Endothia gyrosa is found commonly in the United. States on 
hosts of four genera: Castanea, Fagus, Liquidambar, and Quercus 
—least abundantly on Castanea, most abundantly on Quercus— 
the collections made in the work already referred to (8) being in 
the following numbers: Castanea, 18; Fagus, 47; Ligquidambar, 
49; Quercus, 158. Actually the fungus is much less frequent on 
Castanea than is indicated by these figures, since the eighteen 
collections represent all the cases in which E. gyrosa was found on 
Castanea, while the number of collections on the other hosts 
represent simply a few specimens from each locality visited and 
could have been increased many times. 
Endothia gyrosa has been collected by Dr. Shear and the writer 
on the following species of Quercus: Q. alba L., Q. coccinea Muench., 
Q. digitata (Marsh.) Sudworth, Q. georgiana Curtis, Q. imbricaria 
Michx., Q. marilandica Muench., Q. nigra L., Q. phellos L., Q. 
Prinus L., Q. rubra L., Q. velutina Lam., Q. virginiana Mill. 
From the first, however, it was obvious that the fungus was more 
abundant on the various species of black oaks than on the species 
belonging to the white oak section of the genus. Of the 158 
specimens from all parts of the United States which were collected, 
eighty are known to be on species of black oak, nineteen on species 
of white oak. The specimens, for which the host species is not 
given, are apparently divided in about the same ratio. These 
unidentified specimens are chiefly from the southern states where 
the collecting was done largely in midwinter. 
