Invasion of wood by basidiomycetous fungi III. Mycelial spread in heavily wounded understory trees as determined by the interaction of kratovirulence and pathovirulence properties

Abstract

AbstractForty species of wood‐destroying basidiomycetous fungi (WBF) were sterilely transferred to wood dowels to be inserted into stem drill holes of vital and moribund understory trees in a manner that allowed common microbial wood contaminants to take part in the infection. The WBV are members of the 7 ecological categories comprising pathogenic WBF (pathogens) with high pathovirulence (PV) and thus low kratovirulence (KV, competitive saprophytic ability), as well as increasingly saprophytic WBF (saprophytes) with declining PV and increasing KV. It was recently shown that harvested timber devoid of vital defence responses is subject to random microbial contamination. These contaminants afford the antagonism to exclude all WBF, in particular pathogens, from entering this substrate that do not muster a basal level of KV. The wounded tree widely resists uncontrolled contamination with both xylophilous microorganisms and WBF, thus lowering microbial antagonism to potential WBF invaders. The heartwood region believed dead and nonresponsive is included in the tree's defence concept. In most tree species the outer conductive sapwood of 2 to 4 growth rings is the most resistant part of the stem. This resistance is lost when in declining trees the vital defence responses abate. On these standing‐timber substrates, the PV of WBF is superimposed over KV while the reverse is true for harvested timber. Consequently, no infections formed most white‐rot and plant‐resideue saprophytes (high KV) in nonhost trees as well as white‐rot parasites (diminutive KV) on host and nonhost trees, while the white‐rot sapro‐phytes inserted in host trees incited perthophytic decay pockets. Heartrot‐like compatible host‐pathogen combinations developed between host trees and brown‐rot pathogens (high to low PV, diminutive KV), and between host trees and certrain white‐rot saprophytes (high KV, no to occasional PV). Heartrot‐like infections avoided to extend decay to the outer conductive sapwood. In contrast, the white‐rot pathogen, Stereum rugosum (diminutive KV), inflicted devastating damage on heartwood, outer sapwood, and cambium to prevent wounds from overhealing. Apart from the behaviour of some WBF with wide host range the preferred host spectrum of the decay fungi examined coincided with the spectrum reported. Host preference was frequently expressed in assiction to, or rejection of, gymnosperm or angiosperm wood. Saprophytes that dominate harvested timber owing to superior KV properties remain the exception of standing timber, not due to a lack in infection mechanisms but to the intolerable constraint they suffer in view of the tree's chemical defence. When the defence reactions abate in the dying host, saprophytes will rapidly expand. The conventional term „heartrot fungus”︁ was redefined. It may describe a group of WBF which on their dominant host trees may permanently prefer the inner stem wood while on other hosts they may sometimes prefer outer sapwood, too. This may prevent heartrot WBF from displaying unique physiological traits that should explain their exclusive growth in inner stem parts. In this connection, the physiological traits that may contribute to pathosism in WBF were also outlined. In standing timber, certain pathogenic brown‐rot WBF incited a dramatic acceleration of wood deeay. Osmoporus odoratus caused up to 85% weight loss on spruce within 15 months, while the comparable in vitro weight loss did never exceed 2 to 5%.