Over 60 spot infections located on the western advance line were
examined between August, 1913, and February, 1914. The spots were
located in 7 counties on the extreme western advance line of the
disease, and also some distance back of this line. The cutting out had
been done by practically as many owners as there were spots, under
supervision of various field men, so that the conditions were averaged
in every way. The point which was brought out most prominently by the
re-examination was the fact that where the stumps were well peeled and
thoroughly charred and where the tops and refuse were well cleaned up
and burned, and the merchantable material promptly removed from the
vicinity of the spot infection, there was no reinfection of the stumps
or sprouts of the treated trees. Where the work was carelessly done,
there was more or less reinfection. However, there were exceptions in
both cases. In some cases where the work was done only fairly well or
even poorly, there was less infection than might naturally be expected.
In some other cases where the work was done as well as it can be
expected under field conditions, there was a considerable reinfection.
This variation is probably explained by other factors which undoubtedly
enter into the effectiveness of sanitation cutting. Probably the age of
the original infection center is one factor governing the number of new
infections which appear after the first cutting out. If the original
infection is still so young that there is a comparatively small
canker, or if the condition of the growth has been unfavorable for
the production of ascospores, a small amount of new infection may be
expected, since the wind apparently distributes most of the infection
to the surrounding trees. On the other hand, if the diseased area of
bark at the center of an infection is large and has produced a great
number of perithecia, and the climatic conditions have been favorable
for the ejection of ascospores, a large number of incipient infections
are very apt to be left in the surrounding trees at the time of the
first removal cutting.
[Illustration: Healthy sprouts growing around a burned stump.]
Just how long after cutting it takes these incipient infections to
develop so that they can be detected in scouting depends on a number
of conditions, such as the location of the diseased area on the tree
and the height above ground where infection occurs, size of the tree,
season of the year and climatic conditions following the occurrence
of infection, location of the spot infection relative to topography,
etc. Probably the most important factor governing the number of new
infections after a removal cutting is the character and quality of
the man who scouted the area. Certain men have much better scouting
ability than others, and in some of the spots examined, at least, this
factor alone is sufficient to account largely for the conditions found
on reinspection. However, even the best scout cannot detect small twig
infections in the tops of tall trees before they have girdled the
twigs, and it is frequently very easy to miss well developed cankers
either at the base of large trees when no fruiting bodies have been
produced, or on the upper trunks of tall trees before the tops have
been girdled.
It was very noticeable that new infections appearing in a spot where
the original infection had been properly removed were almost always
within a short distance of the original infection. Probably half of the
new infections found, even after the second inspection, were on trees
that grew on the same stump or in the same tree group as an original
infected tree, and 90 per cent. of the newly infected trees were so
close that their tops interlocked or were directly exposed to the tops
of the previously infected trees. The accompanying diagram illustrates
the characteristic manner in which new infection appears. In several
cases the farthest infection as noted in the tabulated data was an old
infection which was missed at the time of the first inspection, and
which really constituted a separated spot infection.
Blight spots in northern Pennsylvania seem to be smaller, more widely
scattered, and to spread less rapidly from the center than spots in
the southern part of the State. One reason for this may be that there
is, as a rule, a much lower percentage of chestnut in the forest and
the chestnut appears to be sounder and in better health than much of
the chestnut in the southern part of the State. Further south along
the advance line, greater injury is noticed on young trees from the
bast miner; damage from ice storms and hail storms also appears to be
greater. Another possible factor is that the climate is warmer, and
favorable to the copious formation and ejection of ascospores over a
longer period than in the northern part of the State. Another possible
factor is differences in topography which favor the carrying of spores
long distances along regular “air lanes.” This may be the explanation
for long chains of spot infections which occur along the lower edges
of timber of the long, forested ridges, and on benches half way up
mountain sides. This is put forth merely as a suggestion and not as a
fact, although there is some evidence to warrant a hypothesis of this
kind.
The results of the investigation show clearly that the chestnut trees
immediately within and adjoining a spot infection (say 25 feet beyond
the outermost infected trees), should be cut out and the stumps
sterilized whether the trees appear to be infected at the time the
cutting is done, or not (Fig. 4). The investigation proves that
these trees in the majority of cases will become infected later on,
and it means extra expense and less effective control to wait until
the infection appears. In very small spot infections or even those of
considerable size, it is believed that such treatment will avoid a
recurrence of the blight in the majority of cases. However, to cut out
these apparently healthy trees is not sufficient; the sanitation work
must be done as thoroughly as if the trees were diseased. Even though
the merchantable portions are taken out of the woods and the tops
burned, the unpeeled stumps are very apt to become infected, especially
if nearby diseased trees have been ejecting ascospores. Four treated
spot infections were examined which proved this very conclusively. The
following facts relative to these spots are interesting:—
_Spot 1._ Five infected trees in Huntingdon County were treated in
April, 1912, by digging up the trees, stumps and all, and burning them
in an open field. In March, 1913, the spot was re-examined and three
infected trees found. The stumps were peeled and the tops burned, but
not over the stumps. At the same time all of the chestnut trees on a
half acre surrounding the spot that were large enough for fence posts
were cut out, the tops burned and the rest of the trees removed. The
stumps were left unpeeled and in January, 1914, 6 new infections were
found on small saplings that remained after the cutting, and all but 4
out of 75 stumps from which the bark was not peeled showed pycnidia on
the cut surface of the wood or bark, pustules in the dead bark on the
side of the stump, and usually, mycelium growing downward toward the
base of the stump through the live bark.
_Spot No. 2._ Seven infected trees cut March, 1913; stumps well peeled
but not burned over. In January, 1914, 9 new infections were found
on adjoining trees and 50 new infections were found on the stumps of
healthy trees cut in close proximity to the spot in March, 1913. These
stumps were not peeled and the pustules appeared in the bark on the
side of the stump, and in many cases showed mycelium running through
the live bark of the lower part of the stump.
_Spot No. 3._ Seven trees cut June, 1912; stumps peeled and well
burned. March, 1913, 7 infections were cut out, the stumps poorly
peeled and not burned. At this time 17 healthy trees were cut within
a radius of 30 yards and the bark was not peeled from the stumps. In
January, 1914, no new infections had appeared on any of the surrounding
trees, but 8 of the stumps were infected.
[Illustration: Healthy sprouts in spot infection where blight was cut
out by sanitary methods.]
[Illustration: Properly burned stump; stump on the left cut too high.]
_Spot No. 4._ One infection cut July, 1912. Stumps peeled and burned.
In April, 1913, 16 new infections were found on stumps cut at the
time the original infection was removed and immediately surrounding
the infected trees. These stumps were located as follows: One stump 3
yards west of center; 3 stumps northwest of center (farthest 35 yards);
3 stumps north of center (farthest 20 yards); 5 stumps northeast of
center (farthest 12 yards); 2 stumps east of center (farthest 3 yards);
2 stumps southeast of center (farthest 8 yards); these stumps were
peeled and not burned over. In February, 1914, 4 additional infected
stumps were found, the farthest being 12 yards from the center.
East of the advance line sanitation has proved effective in hindering
the progress of the disease, but not in eradicating it. Inspections
made of a tract of blighted chestnut at Haverford, Pa., cut in 1910 and
the stumps peeled, but not burned, showed both in 1912 and 1913, that
only about 20 per cent. of the stumps and sprouts were reinfected. On
a nearby tract where the trees were cut at the same time and stumps
left unpeeled, the reinfection was approximately 80 per cent. At
Hummelstown, Pa., on several acres of diseased chestnut, cut in the
winter in 1911-12, a portion of the stumps were peeled and lightly
burned. In the spring of 1913, 80 per cent. of the peeled stumps and 90
per cent. of the unpeeled stumps were reinfected. The reasons for the
high per cent. of reinfection was the fact that the peeled stumps were
not well burned, and the nearness of disease on trees in the adjoining
woods and on the adjoining unpeeled stumps. This is shown by the
location of the infection on the sprouts as follows: