In the Mahoning Valley, all timber tracts on an area about 7 miles
square were mapped on a large scale topographic sheet, (Fig. 1). In
round numbers this investigation covered about 50 square miles of land
which varied widely as to elevation and geological formation. Spot
infections of blight were accurately located on the map, and each spot
was studied in detail as to the percentage of surrounding infection,
slope, exposure, soil, character of the stand of timber, and surface
features. Originally chestnut oak and yellow pine occupied the steeper
upper slopes, while the more gentle and fertile lower slopes were
covered with a stand consisting of 50 to 70 per cent. chestnut, with a
mixture of red oak, maple, and white pine. Very little chestnut grew in
the valleys where the forest consisted of heavy stands of white oak,
white pine, red oak, and maple. All of the flat bottom land and much
of that along the lower slopes has been cleared for farming, so that
part of the area studied consisted of woodlots with trees varying in
size from small coppice to 20 inches in diameter. The area is traversed
by several ridges extending northeast and southwest, and the poor
rocky soil of these ridges, particularly north of the Mahoning Valley,
is covered with young coppice of oak and chestnut, or with scrub oak
brush. Forest fires frequently burn over the ridges and the young
growth is therefore in poor condition.
At present no tract can be found on the area studied that is entirely
free from blight, but the chestnut trees south of Mahoning Valley are
diseased more than the stand north of the valley. The southern slopes
of the ridges, also the south and east portions of exposed woodlots,
are more seriously infected than the northern exposures. There are
thousands of cicada wounds in twigs of all species growing in these
woods. These wounds were made during the invasion of 1911. It is
very common on chestnut to find such wounds infected, and the cicada
has thus undoubtedly aided in the general distribution of the blight
throughout this region.
Every tract of chestnut timber showing the presence of blight, when
carefully examined, shows that the disease appears in spots. By careful
observation, the source of infection for the entire spot can be traced
to one or more badly infected trees which evidently bore the original
infection of that particular area. Such a tree or group of trees is
commonly referred to as an infection center, because from such centers
the disease advances in all directions. The age of these centers can
be determined quite accurately from the appearance of the original
infection, by the concentric rings of cankers and by the age of water
sprouts and shoots at base of cankers. Generally, the older the
infection, the further it has spread from the center.
Many of these infection centers have been carefully worked over, but
nothing definite can be said as to characteristic elevation, soil
conditions, exposure, or character of woods. Probably 90 per cent. of
these centers are found in the shallow depressions at the heads of
gullies; or, where a ridge slope forms a terrace-like flat. However,
it is evident from a large number of observations, that such centers
develop under any surface conditions favorable to the growth of
chestnut. They are found on well drained gravel slopes, dry knolls,
steep rock slopes, and in low fertile flats.
The spread of the blight seems more rapid in young coppice growth
of nearly pure chestnut, than in a chestnut stand of large trees.
In old stands the percentage of infected trees decreases abruptly
from the infection center outward. Often, a distance of twenty rods
will take one from an area of 40-50 per cent. infection to a zone of
one-fourth per cent. and beyond that no infection may be found. In
coppice growth the decrease is more gradual and a zone showing less
than 8-10 per cent. infection can seldom be found on a tract with an
infection center. The abundance of bast miner galleries in the bark
of young smooth-barked chestnuts probably explains the wide and even
distribution of the blight in such stands.
The importance of wind as an agent in disseminating blight cannot be
positively stated, but from observations made in this locality there
seems more evidence favoring wind distribution than any other factor.
The result of a large number of comparative observations show that:—
1. A large number of infections are in wounds made by cicadas and are
usually uniformly distributed around a blight center.
2. New infections are generally scattered through areas of young shoots
growing up after fire.
3. Freshly cut stumps with their new sprouts show a high per cent. of
infection even where the surrounding woodland is little affected.
4. Trees standing in exposed places, such as isolated trees in fields,
and trees along southern edges of timber tracts, show a high per cent.
of infection.
Very little can be said about birds as carriers of blight. Numerous
scattered spots of infection show signs of having been started by bird
distribution. However, the observations gave little reliable evidence
on this point. Many spots have a large, dead-topped tree standing near
the center. Often these trees have been infected on the lower branches,
longer than any of the surrounding trees. The dead, snaggy tops show no
evidence of death from blight. There is reason to believe that birds
were attracted by the open snag and carried the spores which later
started the infections in the lower branches.
This locality furnishes numerous opportunities for comparing the
percentage of infected trees on the north and south slopes. The stand
of chestnut is similar on the two slopes. The results of detailed
examinations show that there is more blight on the south slopes. Also,
many of the woodlots show a higher per cent. of infection on the
southern borders. To strengthen these observations several miles of
the Blue Ridge, (lying north of the Mahoning Valley, and not included
in the area studied), were also worked over, (Fig. 2.) This ridge is
higher than any other within the limits of area studied, and shows
the typical high percentage of blight on the south slopes, up to
the summit. Immediately across the summit, northward, the number of
blighted trees decreases. However, at the base of the north slope in
almost pure chestnut, it increases but does not average more than
60 per cent. of the amount of infection at the base along the south
side. There is a general decrease in the amount of infection on each
successive ridge to the north.
There are distinct differences in the moisture conditions in this
region. The stream valleys often have a clay loam soil too heavy and
moist to support chestnut. We find all variations in soil and moisture
from these valleys to the dry, rugged ridges where chestnut oak and
scrub oak form most of the stand. The amount of infection apparently
does not depend on soil moisture, as is shown by the percentages on the
infection map. Tracts lying in the valleys show similar percentages of
infection to those on higher ground. The theory that chestnut trees
growing on or near limestone soils are resistant to blight is not
supported by these observations. A belt of limestone borders Lizard
Creek Valley on the south, and the per cent. of infection is as high
in that region as elsewhere. Infection centers have been found near
limestone quarries, where the roots of the chestnut penetrated to bed
rock.