6 Wood Discoloration

112 5 Damages by Viruses and Bacteria
and in the phloem or xylem. They cause e.g., leaf necrosis in oak and planes
and distorted growth of larch (Nienhaus 1985b; Linn 1990; Butin 1995).
Mycoplasmas (genus Spiroplasma) and phytoplasmas (in former times:
MLOs; genus Phytoplasma) are the smallest (100–750 nm) independently
growing bacteria. They are pleomorphic, sporeless, immovable, and filterable.
Spiroplasma grows on nutrient medium, Phytoplasma does not. Plant
pathogens are transferred by grafting, root grafts, vegetative propagation of
infected material, Cuscuta species, and sucking insects, in which they multiply,
into the phloem (Nienhaus and Kiewnick 1998). They cause a great
number of yellow-type diseases, necroses, growth disturbances, or dying of
rice, maize and sesame, vegetables, sugar cane, fruit trees, coconut palm,
whitethorn, alder and ash, witches’-brooms on poplar, and sandal spike (Tattar
1978; Nienhaus 1985a, 1985b; Linn 1990; Sinclair et al. 1990; Lindner 1991;
Lederer and Seemüller 1991; Raychaudhuri and Mitra 1993; Raychaudhuri and
Maramorosch 1996).
Bacteria appear in trees and wood as both primary and secondary colonizers
often in the context of succession together with fungi. They live on easily
accessible nutrients and may prepare the substrate for fungi (Shigo 1967;
Cosenza et al. 1970; Shigo and Hillis 1973; Shortle and Cowling 1978; Rayner
and Boddy 1988). Soil bacteria may increase vitamin content (thiamine) of
wood in ground contact, which promotes subsequent decay Basidiomycetes
(Cartwright and Findlay 1958; Henningsson 1967).
Bacteria penetrate into the sapwood of a tree via wounds. In hardwood
vessels that are not closed by tyloses or other wound reactions, they might
spread with the capillary water over larger distances. In softwood samples,
however, only a few tracheids were passed due to the small free spaces within
the pit membrane (Liese and Schmidt 1986).
The wet heartwood (wetwood) of several tree species, particularly fir, hemlock,
poplar, elm, also beech and oak, means any water-saturated and dead
wood in living trees. Characteristics are the unpleasant smell of butyric acid
and other acids, dark discolorations and gas escape from the heartwood if an
increment borer has been used. The exact cause of wetwood formation, whether
being due to bacteria or necrotic changes in the parenchyma cells, is not clarified.
Wetwood develops in connection with mechanical wounds, branch breaking,
decay, stem cracks, and insect attack. So-called acid wetwood, predominantly
in conifers, contains several organic acids (butyric, acetic, propionic
acid) produced by (facultative) anaerobe bacteria. Alkaline wetwood, mostly
in hardwoods, develops with participation of obligate anaerobe methanogenic
Archaea. These Prokaryotes attack the pits or cause their incrustation, give rise
to discolorations, their metabolites may stress the tree, and the unpermeable
wetwood tends to crack during drying (Carter 1945; Hartley et al. 1961; Wilcox
and Oldham 1972; Bauch 1973; Knutson 1973; Bauch et al. 1975; Tiedemann
et al. 1977; Ward and Pong 1980; Ward and Zeikus 1980; Schink et al. 1981; Mur-
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