REACTION ZONE RELICS AND THE DYNAMICS OF FUNGAL SPREAD IN THE XYLEM OF WOODY ANGIOSPERMS

Relics of reaction zones, typical of those found at the margin between fungally colonized and healthy sapwood, have been identified in the decayed xylem of three woody angiosperms. In Aesculus hippocastanum L., decayed by Ustulina deusta (Fr.) Petrak the locations of these relics were indicated both by residual traces of the characteristic induced compounds that accumulate in reaction zones, and by the dark pseudosclerotial plates that are formed by this fungus at reaction zone margins. In Fagus sylvatica L., decayed by a Ganoderma sp., the sites of former reaction zone margins were marked by bands of wood in which a high proportion of the vessels had suberized linings or occluding tyloses. In Fraxinus excelsior L., decayed by Inonotus hispidus Bull. ex Fr.) P. Karst., the sites of reaction zones were indicated by the presence of brown, insoluble, deposits in the cells and vessels of the xylem. In all cases these relics formed narrow, clearly defined bands, typically separated from neighbouring reaction zones (functional or otherwise) by intervening decayed regions largely devoid of characteristic reaction zone features. These results, obtained from three species all somewhat different in the nature of their reaction zone responses, indicate that the invasion of functional xylem tissues by decay-causing pathogens in these trees is discontinuous. Reaction zones form persistent static boundaries to the lesion. When these fail the invasion of previously uncolonized sapwood tissues may be rapid, and is accompanied by little or no dynamic expression of defence or repair responses ahead of the infection front. Ultimately this invasive phase ends, and locally at least, the lesion becomes bounded by a fully developed reaction zone once again. The mechanisms that may limit fungal colonization of these host tissues are discussed. © 1991.