In applying sanitation measures for the control of the blight, it is
not practicable to use tree surgery methods and spraying, (except
possibly in orchards), but only to cut out bodily every infected tree
and to sterilize the stumps. When the blight is generally distributed
through a region, as is the case in southeastern Pennsylvania, it is
manifestly impossible to eradicate the disease by sanitation methods
without also practically eradicating the host. A detailed study of spot
infections as they occur on the western advance line of the disease
is therefore of more interest than the conditions which exist in the
generally infected territory.
On the advance line, as in the eastern part of the State, there is no
rule for the location of an infection center, nor is there any rule as
to the part of the tree which is attacked first by the disease. It is
true, however, that on the western advance line more infections occur
on isolated trees and on the edges of timber tracts than elsewhere,
and that the majority of infections first appear in the tops of
trees. Likewise, in its spread from tree to tree around a center,
the blight shows no general rule, except that the trees immediately
adjoining a primary infected tree are most apt to show the first
secondary infection. The following tabulation gives the details of 175
infected trees in a spot infection of 271 trees, located at Orbisonia,
Huntingdon County, Pennsylvania, studied in 1911 by Mr. R. C. Walton.