Hedgcock and Hunt : Dothichiza populea in the U. S. 303 
rapid growth of the cambium in the callus that is now formed 
{pi. ip4, f. 2). As the canker becomes older, the bark sloughs 
off, leaving an open wound on the trunk, surrounded by appar- 
ently healthy bark. Meanwhile, the fungus attacks the limb or 
twig around the base of which the canker was formed, spreading 
very rapidly through it, forming pustules over a large portion of 
the bark. 
Cankers much like those on the trunk are frequently formed on 
the lower limbs and twigs of diseased trees. On the larger limbs, 
their formation and appearance are much like those on the trunk. 
When the fungus attacks smaller limbs and twigs the canker 
usually extends around each limb or twig and very soon kills it 
{pl.i94,f.i). 
In trees recently transplanted and in trees heeled in for early 
planting in the spring, the fungus spreads most rapidly. Trans- 
planted trees may become badly diseased by cankers formed from 
fresh infections between October and the following May {pi. 
195, f. 1,2). 
Dothichiza populea is the most rapidly growing canker-produc- 
ing fungus known to the writers. One canker selected from a 
number produced on black poplars between October, 1915, and 
May, 1916, was twelve inches long and encircled the trunk of 
the tree for nearly two thirds of the length of the canker. In 
the same lot of trees, out of 100 examined 90 were diseased either 
in limbs or trunk, 27 were girdled at the trunk by cankers which 
had killed the tops of the trees, and 13 more had distinct trunk 
cankers which had not yet encircled the trunks. 
Dothichiza populea causes but slight outward discoloration of 
the bark in the cankers it produces, the color being slightly darker. 
The fruiting bodies (pycnidia) of the fungus are formed be- 
neath the surface of the bark and cause marked elevations or 
pustules to appear on the cankers soon after they develop {pi. 
195, /• T ^)- These pustules are smooth on the surface at first 
and have the same color as the adjacent bark. At maturity they 
rupture near the center of the top and small cream-colored ten- 
drils are exuded, which gradually assume a tawny olive to a 
walnut-brown color. These are composed of millions of pycno- 
spores, which spread the disease to adjacent trees. According to 
