Insects and Tree Diseases 
Insects are involved directly or indirectly in the transmission of many serious 
diseases of forest and shade trees in the Eastern United States. Some of these 
diseases, such as Dutch elm disease and elm phloem necrosis, are among the most 
devastating tree diseases known. The majority of tree diseases are caused by various 
species of fungi. However, phanerogams, bacteria viruses, mycoplasmas, and 
rickettsiae also cause plant diseases. Some disease organisms, such as those 
causing Dutch elm disease and chestnut blight, attack living, healthy trees and kill 
them quickly. Other diseases, such as stains and heartrots, occur in living trees 
weakened or injured by environmental conditions. 
Fungal diseases can be transmitted by insects through the direct transfer of spores 
from infested to healthy trees. The bodies of insects emerging from diseased hosts 
become contaminated with the spores and when the insect attacks healthy trees, 
spores are introduced into the wounds. Viral, mycoplasmal, and rickettsial organ- 
isms are most often directly transmitted by species of sucking insects, such as 
leafhoppers. Wounds made by insects can indirectly serve as courts of entry for 
wind- and rain-borne spores. 
Dutch elm disease is caused by the introduced fungus, Ceratocystis ulmi 
(Buisman) C. Moreau. The disease was first recorded in North America in Ohio in 
1930, and by 1975 had been reported in all of the contiguous 48 States except 
Arizona, Florida, Louisiana, Nevada, New Mexico, and Utah (/069). Both native 
and exotic species of elm are hosts for the disease, with the common American elm 
as its most susceptible host. Principal insect vectors are the smaller European elm 
bark beetle and the native elm bark beetle (224, 274). Other insects carry spores of 
the fungus, but the life cycle of the elm bark beetle is ideal for disease spread. Adult 
beetles emerging from diseased trees carry spores to healthy parts of the same tree 
and to adjacent healthy trees where the beetles feed and lay eggs. The spores 
germinate in the feeding wounds and initiate new infections, which weakens the 
tree and aids the larval feeding (/09/). Much of the effort to control Dutch elm 
disease has involved insect control. Early efforts were made by using insecticides. 
Sanitation efforts have been more long lasting by reducing the opportunities for 
beetles to move the fungus. However, the common occurrence of elms has made 
this a costly method (970, 1069, 1097). 
The first symptoms of Dutch elm disease are wilting and yellowing or drying of 
foliage. Defoliation and death of the affected branches soon occurs. Diseased trees 
commonly die gradually, branch by branch, over a period of several months or even 
years. A brown discoloration in the water-conducting vessels develops in affected 
trees. In early spring, this occurs as brown streaks in the wood just under the bark of 
diseased branches (fig. 3). Later in the growing season, it appears as brown spots or 
as a partial or complete brown circle in one or more outer rings of the wood. 
Elm phloem necrosis is a disease of American, slippery, cedar, winged, and 
September elms. It occurs in parts of Ohio, Indiana, Hlinois, Missouri, Lowa, 
Nebraska, Kansas, Oklahoma, Arkansas, Mississippi, Alabama, Georgia, Ten- 
nessee, Kentucky, West Virginia, Pennsylvania, New York, and New Jersey. Evi- 
dence indicates the disease is not caused by a virus, as earlier suspected, but rather 
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