The pathological investigations of the Commission have shown that wind,
water (rain), and birds are the principal agencies in disseminating
the blight. A single spore thread may produce from 100,000,000 to
200,000,000 pycnospores, and even a small canker produces dozens of
spore threads in a season. A single perithecium has been observed to
eject ascospores almost continuously for a period of 26 days, at the
rate of 4.7 spores per second. Insects assist by making wounds through
which the spores of the fungus enter the bark, and also, to some
extent, by distributing the spores locally. The ejection of ascospores
into the air following rain, and the washing of pycnospores down the
trunks and into the soil during rain, appear to be the principal
agencies in spreading the disease. Birds have been proved to carry
spores in great numbers, and undoubtedly are responsible for a certain
proportion of infections, at least, of advance infections.
The planting of diseased nursery stock in regions free from the blight
appears to be one of the principal agencies in spreading the disease to
great distances. The disease was probably introduced into this country
on nursery stock, and in the early years, nursery stock apparently
played the most important role in getting the disease quickly and
firmly established. This point is well illustrated by a shipment of
three chestnut trees sent from a New Jersey nursery into Western
Pennsylvania in 1912. Through a misunderstanding, these trees were not
held at the State line for inspection, but were carried direct to their
destination. When the inspection was made, the disease was found at two
places on one of the trees, although the nurserymen claimed to have
carefully examined the trees before shipment. At Warren, Warren county,
Pennsylvania, 11 out of a shipment of 12 nursery trees purchased in
1910 were found affected with the blight in 1912. In Elk County, 34
diseased nursery trees were found in a young chestnut orchard, and
the disease had already reached adjoining native chestnut trees. In
Somerset County, there is evidence to support the belief that an
infected area covering about one-third of the county spread originally
from diseased scions grafted on native trees. There are many similar
occurrences outside of Pennsylvania.
All observers have noted that the blight advances by attacking widely
separated trees far ahead of the generally infected territory.
In Pennsylvania, the main spread of the blight has been from the
southeastern corner of the State. During rains and immediately
following, when the spores are being ejected, the wind is usually from
the south or east, thus tending to carry the spores north and west.
At least, it is a matter of common observation that the southern and
eastern edges of woodlots very frequently show the first infections.
In order to learn more about the spread of the blight, two areas in
the region of general infection, one in the Mahoning Valley in Carbon
County, and the other in the vicinity of Topton Mountain, in Berks
county, were studied in the spring of 1913 by Mr. J. Wesley Sitler, a
field agent of the Chestnut Tree Blight Commission.
[Illustration: Sketch map showing blight centers and percentage of
infection in a portion of the Mahoning Valley, Carbon and Schuylkill
Counties, Pennsylvania.