which was first recorded at Halifax, Nova Scotia, in 1920, now occurs throughout 
the Maritime Provinces of Canada, west to Quebec, and south through the beech- 
growing areas of New England to West Virginia (847, /080). Surveys indicate that 
the disease is moving south and west at a steady rate. 
F-520108 F-519951 
Figure 4.—Typical discoloration Figure 5.—Shade tree elms killed by elm 
of the phloem of an elm tree phloem necrosis mycoplasm. 
infected with elm phloem 
necrosis. 
The fungus may infect large areas on scale-infested trees, completely girdling 
and killing them. Trees only partially girdled may remain alive in a weakened state 
for many years unless broken by the wind. On some trees, the fungus 1s confined to 
strip or spot cankers on the bole. Parts of the crowns of these trees become chlorotic 
and die. 
In many stands, beech bark disease has killed more than half the beech, and the 
commerical value of many of the survivors has been seriously reduced. Disease 
control has not been attempted; the presence of disease-free trees in the midst of 
heavy infestations suggests resistance in the host population (/080). 
Oak wilt disease, caused by the fungus Ceratocystis fagacearum (Bretz) Hunt, 
occurs Over a wide area from Nebraska and Kansas to the Carolinas and Pennsylva- 
nia. The fungus attacks all native species of oak regardless of size, age, or vigor. 
Local spread from infected trees to adjacent healthy trees occurs through natural 
root grafts; insects, however, apparently are responsible for spread over longer 
distances. Oak bark beetles, Pseudopityophthorus spp., are recognized as being the 
most important vectors of the oak wilt fungus (/68, 506, 1021, 1022). In areas of 
the country where mycelial mats of the fungus develop on diseased trees, species of 
sap-feeding and mycophagus nitidulid beetles probably are vectors of the fungus 
(323, 811). 
Oak wilt is potentially a very serious disease (fig. 6). Given the proper conditions 
for an epiphytotic, it could cause heavy losses in the vast oak forests of the Eastern 
22 
