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Lists the recorded hosts (coniferous and broadleaved species) and localities of O. mixta in Kenya; describes its life history; estimates the relation between degree of defoliation and population density; and discusses natural control and laboratory tests of three insecticides against late-instar larvae.


Examination of freshly cut Norway spruce (Picea abies) during all seasons of the year revealed that an alkaline, light green to olive-colored reaction zone was present between sapwood and the brownish central core of wood decayed by Fomes annosus. The reaction zone, which contained dead, almost starch-free parenchyma, was formed in advance of the pathogen. Bacteria and nonhymenomycetous fungi were isolated occasionally from reaction zones, particularly in lower parts of stems where moisture content of this tissue frequently was high. Reaction zones darkened in color shortly after trees were felled. This color change was dependent upon oxygen, and it appeared to be mediated by oxidative enzymes produced by the host. Reaction zone formation was attributed to a nonspecific response to injury or infection during necrosis of parenchyma. The reaction zone contained more extractives than adjacent tissues, particularly phenols with ultraviolet absorption spectra typical of lignans. Reaction zone and incipiently decayed wood contained more potassium, calcium, and magnesium than sound sapwood or heartwood. Expressed sap from the reaction zone had a fungistatic effect on F. annosus; this could not be wholly explained by the high pH of this extract (about pH 8.0). Reaction zone and incipiently decayed wood were significantly more resistant to decay than sound heartwood and sapwood. Spruce oleoresin and one of its components, abietic acid, inhibited linear growth of F. annosus in vitro. Results support the hypothesis that insufficient quantities of inhibitory substances in heartwood contribute to the susceptibility of spruce to extensive central stem rot, whereas limited invasion of sapwood is due to accumulation of inhibitory substances in a reaction zone.