4 Department Circular 370, U. S. Dept. of Agriculture 
posed of myriads of spores about the size of bacteria. These spores 
are borne within the pustule in such numbers that when moist they 
are forced out in a slender serpentine mass, much as paste 1s squeezed 
from a tube. Spores of this type are sticky. They adhere to birds 
and insects coming in contact with them and thus may be carried 
long distances. Rain dissolves the spore horns and so spreads in- 
fection on trees already diseased. ; 
Pustules producing the second type of spore differ from_those 
producing the first in that during wet weather they are dotted with 
very small openings rather than surmounted by spore horns. The 
openings, which are often at the end of small protuberances (Pl. 
IX, B), are the mouths of flask-shaped structures. Within the 
flasks the spores are borne, definitely arranged in groups of eight 
2-celled spores. Each group is inclosed in a thin transparent club- 
shaped sac. Under suitable conditions the sacs escape through the 
neck of the flask. When free the sacs burst in miniature explosions, 
throwing the spores into the air. Thus freed the spores are carried 
by air currents, often to great distances. 
The enormous numbers in which both types of blight spores are 
roduced, together with the fact that they are adapted for different 
methods of dissemination, give insight into one aspect of the diffi- 
culty of halting such a foreign invader once it became widely estab- 
lished and began its march. 
SPREAD OF THE BLIGHT 
The original infection spread rapidly from its center at New York 
City, and later from other centers. It soon spread over New Eng- 
land, but made its most rapid advance from New York City in a 
southwesterly direction along the southeastern slopes of the Blue 
‘Ridge and Allegheny Mountains. 
Even in a given direction advance is not uniform. Not only does 
the blight fungus spread from infected trees to healthy ones near by 
but it is often carried for many miles—sometimes hundreds of 
miles—on shipments of chestnut lumber or nursery trees, by migrat- 
ing birds, wind-blown insects, or the air itself. Under favorable 
conditions such spores may produce new centers of spread known as 
spot infections or advance infections. The rapid progress of the 
blight over the chestnut area is largely due to the starting of these 
advance spot infections many miles ahead of the main infection 
area. These advance spots rapidly enlarge and run together, form- 
ing a continuous infected area. The method of the delay and control 
work in Pennsylvania and other States was to locate and eradicate 
these advance infections. For instance, the eradication of the large 
Georgia-Tennessee spot infection when it was still small would have 
resulted in delaying the time of the death of the chestnut growth of 
that part of the southern Appalachians for many years, as the main 
infection area is still hundreds of miles north of this large infected 
area. The work in Pennsylvania resulted in marked delay in the 
spread of the disease westward across that State. Valleys or other 
areas with limited chestnut growth also delay the spread. Con- 
versely, certain tracts of timber located within the infected area 
rh or a time be disease escaping and be little infected by the 
ungus. 
