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Disease Cycle and Epidemiology

Figure 9. (Click on figure to see an animation of disease development and spread)

Epidemiology

Armillaria species have two means of dispersal and there are two corresponding parts of the disease cycle (Fig. 9; click on image to see animation). The first means of dispersal is by airborne, sexual basidiospores, and this may occasionally result in establishment of a new infection center. The second means of dispersal is growth of the pathogen from tree to neighboring tree, either as mycelium that is transferred where diseased roots contact one another, or as rhizomorphs that grow through soil until they contact and infect a nearby, susceptible host.

Role of basidiospores

The role of basidiospores has long been an enigma in the disease cycle. We know that Armillaria species commonly produce mushrooms and large numbers of spores, but attempts to demonstrate infection or saprobic establishment by basidiospores in nature have usually been unsuccessful. However, there is abundant, if mostly indirect, evidence from studies of population structure and occurrence of the disease in plantations on previously unforested sites, indicating that establishment of the fungus from basidiospores does happen on occasion. Variation in population structure suggests that variation in the frequency of this occurrence may be due to climate, especially the moisture regime.

Local transmission and disease centers

Local dispersal by fungal growth is certainly the most common source of infection. It occurs by transfer of mycelium at root contacts and grafts or by growth of rhizomorphs through soil to nearby trees. The source of inoculum may be a root system of a tree recently infected or killed many years before. If the disease progresses this way, an expanding group of dead trees, often called a root-disease center, can develop (Figs. 10-12). Typically the oldest mortality is in the interior and recent mortality and symptomatic trees are at the edge. Isolates from trees within a root disease center typically belong to the same genet, or clone, of the fungus. Although infection may not be spatially and temporally organized in this way, it is important to recognize disease centers when they occur to make a full diagnosis and understand the local epidemiology of the disease.


Figure 10	Figure 11

Figure 12

Survival and inoculum

Mycelium in colonized roots and the rhizomorphs produced from them serve as the most common inoculum. In general, larger pieces of woody inocula are more likely to cause infection than smaller pieces. Armillaria species may survive for up to 50 years or more in stumps, depending on the climate, size of stump, and other factors.

Stress

In some cases, Armillaria root disease causes most damage on trees that are stressed by one or more abiotic or biotic factors. These may include drought, defoliation, other diseases and insects, soil compaction and other soil problems, and numerous other factors. The role of stress in the disease is sometimes generalized too much, and the disease is described as generally being secondary to some other problem. In fact, the disease is sometimes primary, attacking otherwise healthy hosts, and in other cases secondary, attacking stressed hosts.

An example of a secondary, stress-related disease is in oak (Quercus) forests of the eastern United States. Armillaria gallica is normally a pathogen in such forests, but it tends to cause butt rot (slow decay of the inner, older wood of major roots and the base of the stem) and rarely kills trees. It is also an efficient saprobe, colonizing root systems of dying trees and stumps. However, when trees are defoliated over several years by gypsy moth (Lymantria dispar, a nonnative insect), trees become more susceptible, the more vital tissues of the root and stem base are attacked by A. gallica, and trees are killed. Although the disease is stress-related and secondary here, it is the difference between life and death for these trees: those that are not attacked by Armillaria root disease can usually recover from defoliation and again grow normally in the absence of further defoliation.

In contrast, A. mellea causes a lethal, primary disease in many conifer forests and plantations, and in many orchards and vineyards. In some cases, inoculum for such disease has been traced to root systems of previous forest trees, but in some cases spread occurs among crop trees and vines also.