12 MICHIGAN CIRCULAR BULLETIN NO. 135 
branches broken in wind storms have the same appearance as those which 
have been killed by the chestnut blight organism. Closer inspection, however, 
shows these branches to be hanging down from the break rather than in the 
natural position of an unbroken but blight-killed branch. In Michigan, where 
most of the chestnuts have been planted, many have been observed with 
broken twigs and larger branches broken by wind storms. 
The undersized burrs which are borne on infected branches are also an 
important and conspicuous sign of the presence of this disease. Close ex- 
amination of dying twigs and branches or larger parts of the tree shows 
the presence of cankers. These cankers are more or less elliptical areas of 
discolored portions of the stem which may be either sunken below or swollen 
above the surrounding bark. The areas invaded by the causal organism be- 
come yellowish-brown in color and gradually spread and enlarge until the 
branch is completely girdled. The cankered areas may crack open and 
form longitudinal fissures or checks into the inner bark. (Plate V, A and 
B.) If the bark is laid back from the cambium region in these cankered 
areas, layers of the mycelium of the fungous plant are visible as radiating 
masses, buff-yellow in color. This is a characteristic diagnostic symptom 
of the disease. Such radiating mats are called mycelial fans. An excellent 
example of such a fan is shown in Plate VI. 
Water sprouts arise below the cankered areas soon after the fungus has 
made a good growth in the bark. These water sprouts arise due to the 
stimulus brought about by the interference with the food transfer in the 
inner cortical region of the bark where the fungus has invaded the food 
conducting tissues. Although water sprouts are not an infallible sign of 
this disease, chestnut trees bearing them may be suspected and should be 
examined more closely especially if these sprouts are being produced in the 
upper parts of the tree. Another symptom often associated with this disease 
when the fungus has first attacked the tree in the uppermost branches is 
“staghead,”’ a condition in which dead leafless branches stand up above the 
green foliage of the living parts. This symptom is especially noticeable in 
regions where the chestnut blight fungus has been present for several years. 
Again, this is not an infallible sign of the disease because other things, such 
as root rot or change in water level, can bring about this condition. 
Causal Organism 
The chestnut blight is caused by the fungus Endothia parasitica (Murr.) 
A. and A. As the mycelium penetrates the branch it forms in closely ap- 
pressed plates of fungous growth already referred to as “mycelial fans.” 
When the fungus has grown for a time there are produced in the bark 
pycnidial pustules which appear as minute raised papillae. These pustules, 
which are rarely larger than the head of a pin, are fruiting bodies. Each 
pycnidium contains enormous numbers of spores known as pycnidiospores. 
In damp weather sticky spore masses embedded in mucilaginous material 
ooze ‘out from these bodies in the form of yellowish-orange “spore-horns” 
or tendrils. (Plate VII.) The presence of these spore horns together 
with the mycelial fans under the bark furnish infallible macroscopic evidence 
of this disease, especially when found on living branches. As in many fungi, 
two forms of spores are produced. Pustules which appear on the surface of 
the bark as small black dots with minute openings contain the second type 
of spores produced. (Plate VIII A.) These openings are the mouths of 
