Title: Insect and disease conditions in the United States, 1979-83 : what else is growing in our forests?
Identifier: CAT87881361 (find matches)
Year: 1985 (1980s)
Authors: Loomis, Robert C; Tucker, Susan; Hofacker, Thomas H; United States. State and Private Forestry; United States. Forest Pest Management
Subjects: Forest insects United States; Trees Diseases and pests United States; Plants Disease and pest resistance
Publisher: (Washington, D. C. ) : The Service
Contributing Library: U.S. Department of Agriculture, National Agricultural Library
Digitizing Sponsor: U.S. Department of Agriculture, National Agricultural Library

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Dwarf Mistletoes mm Figure 4. Geographic distribution of dwarf mistletoes causing losses in commercially important tree species.
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L) Areas where commercial losses Resources Affected Timber. In Western North America, practically all members of the pine family—including the pines, true firs, spruces, Douglas- fir, larch, and hemlock—are parasitized. Commercially impor- tant principal hosts are listed in table 1; the areas where these significant hosts are infested with dwarf mistletoes are shown in figure 4. The vast southern pine forests of the United States are not affected; cedars, cypress, junipers, redwood, and giant sequoia are immune. The most important effect of dwarf mistletoes is volume reduc- tion: when trees are heavily in- fected, dwarf mistletoes reduce both height and diameter growth and increase mortality. The first symptom of infection is a swelling of host tissues. Later, the swellings enlarge and produce dense masses of distorted branches called witches' brooms (fig. 5). As the parasite spreads through the crown, the tree's growth is gradually reduced. Eventually, the top weakens and dies, diameter growth ceases, and the tree dies. Insects, particularly bark beetles, may cause an earlier death by attacking weakened trees. Other pests, such as decay fungi, enter wounds and swellings created by the mistletoes. The rate at which the tree dies largely depends upon its age when first infected and the amount of infection. Young trees with stem infections tend to die quickly. Older trees with well- developed, vigorous crowns may

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