STEVENS: PREVALENCE OF ENDOTHIA GYROSA 137 
rubra, and Q. velutina, extend well beyond the known range of 
E. gyrosa. 
In considering the prevalence of a fungus, however, not only 
the occurrence of its hosts, but their relative abundance must be 
‘considered. In general, a fungus will be more abundant in a given 
locality the more numerous are the favorable hosts. The fact is 
generally recognized in horticultural practice that an isolated 
plantation is more likely to escape disease than one close to other 
similar plantations. It is to be expected then that E. gyrosa 
would be found most abundantly where its favorite hosts are most 
numerous, and this is actually the case. 
FOREST REGIONS 
A glance at the map of the forest regions of the United States 
issued by the Forest Service shows that the region in which £. 
gyrosa is most abundant comprises the southern forest and portions 
of the central forest. The following brief descriptions of the three 
forest regions under consideration are taken from the Forest 
Service map of the “‘Forest Region of the United States,’’ issued 
in 1910: 
Northern forest (northern portion): White, red and jack pines, 
spruces, firs (balsam), poplars and aspens, birches, tamarack. 
(Southeastern portion): Maples, beech, birches, aspen, chest- 
nut, white, red, and scrub pines, spruces, fir (balsam), hemlocks, 
tamarack, arbor-vitae. 
Central forest. White, black and red oaks, hickories, chestnut, 
walnut, and butternut, yellow poplar, cherry, ashes, elms, maples, 
beech, locust, buckeyes, cottonwood. 
Southern forest. Yellow pines, white, live, red, and black oaks, 
hickories, cypresses, white cedar, juniper, red and tupelo gums, 
magnolias, bays, elms, hollies, ashes. 
In both the forest regions in which E. gyrosa is abundant, its 
favorite hosts, white, black and red oaks (Quercus spp.), together 
with beech (Fagus), in the central forest, and red gum (Liguidam- 
bar) in the southern, are included among the characteristic species. 
In the northern forest beech is the only host of EZ. gyrosa 
mentioned among the characteristic trees, and even beech only 
in the southern portion. It is evident then that though several 
