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Oak Pests - A Guide to Major Insects, Diseases, Air Pollution and Chemical Injury

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Various fungi

Importance. -- Top rot results from the invasion of the heartwood by many of the same fungi which cause butt rot. The incidence of infection is similar to butt rot, but typically less. However, the associated volume losses are much less since this disease occurs in that part of the tree which is not usually used for timber. Top rot results in limb breakage and thus becomes important as a safety hazard in high use and urban areas.

Identifying the Fungi. -- Three common top rot fungi, Stereum gausapatum, Inonotus andersoni and Xypobolus subpileatum must usually be identified by cultural characteristics because conks rarely are seen on dead branches. Others, such as Hericium erinaceus, Tyromyces fissilis, Pleurotus ostreatus and Laetiporus sulphureus were described previously with butt rot fungi.

Identifying the Injury. -- The presence of broken limbs often indicates decay (figure 58). Examination of the broken surface will confirm its existence. Presence of branch scars alone indicates top rot. The incidence and amount of decay rise sharply with increased size and/or age of scars. The decay can be estimated by judging the scar size and age in broad classes (table 1).

Table 1. Expected length of decay in the main stem behind branch scars of oaks.

Diameter class of branch scar
-------------------------------------- (cm) --------------------------------------

less than 3.5(8.9)

3.5 - 6.5 (8.9-16.6)
greater than 6.5 (16.6)
less than 15 years
more than 15 years

Biology. -- The life cycle of heart rot fungi, as top rots, is about the same as was discussed for butt rots. The environment differs, thus some different fungi are involved. Those butt rot fungi which invade the roots are not encountered and others, such as Stereum spp., become more prevalent.

Control. -- There are no measures for direct control of top rots. Recognizing top decay and early harvesting of infected trees is the most useful control method. Prevention of injuries inflicted during logging could help reduce top rot in growing stock. In urban or high-use areas, early detection and prompt removal of hazardous limbs should be carried out.

Figure 58

Figure 58. -- Fungus conk indicates top rot.
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