Root Disease
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Root disease spreads from roots of diseased trees to those of healthy ones. The result is usually several to hundreds of trees dying or dead in groups called root disease pockets. Trees of all sizes, ages, and species are killed by root disease. Detection of this disease is of critical importance in stand management.
Susceptibility varies among tree species, age groups, individual trees, and pathogens present. These aspects of root disease are discussed in the sections dealing with each pathogen as they relate to identification of pathogens.
There are general stand and tree symptoms which can be used to detect most root diseases. Root disease occurs in two patterns in stands. The first is in root disease pockets, and the second is scattered individual tree and small group mortality. Both types may occur in the same stand creating a mosaic effect.
Stand symptoms of root disease
Root disease pockets range in size from a fraction of an acre to hundreds of acres. They usually have abundant regeneration or dense brush growth in the center. This is ringed with dead and dying trees intermixed with apparently unaffected trees along the margin of the pocket (fig. 57). Root disease pockets have various shapes. They range from essentially round to long, narrow strips, to irregular patches. They are often restricted to particular aspects, drainages, and timber types within a given area. Less susceptible tree species abundant in infested stands sometimes mask the presence of a root disease pocket because only the most susceptible species are killed. Such stands simply appear to be understocked or irregularly stocked. On aerial photographs, root disease centers sometimes resemble doughnuts or ringworm-like patches within otherwise uniform forest canopies (fig. 58). Generally only the largest root disease centers will be detected from aerial photographs. Where a mosaic of scattered and pocket mortality is occurring, the stand will have a coarse texture using aerial photographs with occasional large snags visible if the scale of the photograph is large enough.
Scattered root disease often goes undetected because of the subtle nature of expression. There may be only a few trees per acre dying at any one time and these are scattered among the apparently unaffected trees (fig. 59). The eventual toll of this type of root disease can be even greater than that of root disease pockets because it is usually more extensive throughout a stand, drainage or timber type. It takes a trained eye to detect scattered root disease using aerial photographs. The texture of the canopy is a little coarser and more large snags are visible in the photographs.
Tree crown symptoms of root disease
Tree crown symptoms vary according to rapidity of death, involvement of bark beetles, and season of death (fig. 60). As a general rule, trees with root disease lose their needles beginning with the oldest and progressing to the youngest. The appearance is that trees are thinning from the lowest part of the crown up, and the innermost part of the crown (nearest the stem) out, toward the branch tips (fig. 61, 62).
If bark beetles attack dying trees they often hasten mortality. If mortality is hastened, the trees may not have time to shed many needles before they die. In this case, the crown may turn uniformly yellow or red (fig. 61). Small trees which are killed rapidly by root disease may turn uniformly red without having been attacked by bark beetles.
Stress cone crops are sometimes produced by dying trees. In this case a heavy crop of cones may be seen in chlorotic or dead trees even if it is not generally a good cone-producing year for that species. (These are poor cones for seed production.)
Shortened terminal growth and short leaves are often symptoms of root disease infection. These symptoms are especially apparent in seedlings and saplings a year or two before death (fig. 63).
References
Field Guide to Diseases and Insect Pests of Idaho and Montana Forests, USDA Forest Service Northern Region, Publication Number R1-89-54
