Ectomycorrhizae as biological deterrents to pathogenic root infections.

The infection of feeder roots of most flowering plants by symbiotic fungi and the transformation of these roots into unique morphological structures called mycorrhizae (fungus-roots) undoubtedly constitute one of nature's most widespread, persistent, and interesting examples of parasitism. Most plants of economic importance to man are actually dual organisms-part plant and part symbiotic root-inhabiting fungi. Two major classes of mycorrhizae are recognized-ectomycorrhizae and endomycorrhizae.1 A third class, ectendomycorrhizae, an intermediate type, is present on roots of certain tree species under specific ecological situations. The fungal symbionts of ectomycorrhizae penetrate intercellularly and par­ tially replace the middle lamellae between cortical cells of the feeder roots. This hyphal arrangement around the cortical cells is called the Hartig net. Ectomycorrhizal fungi also form a dense, usually continuous, hyphal network --called the fungal mantle-over the feeder root surface. Thickness of the fungal mantle varies from 1-2 hyphal diameters to as many as 30-40, de­ pending on fungal associate, host, and environmental conditions. Most tree species in the Pinaceae, Salicaceae, Betulaceae, Fagaceae, and certain other families, normally form ectomycorrhizae. The majority of ectomycorrhizal fungi are Basidiomycetes, primarily in the Amanitaceae, Boletaceae, Corti­ nariceae, Russulaceae, Tricholomataceae, Rhizopogonaceae, and Scleroder­ mataceae. Several orders of ascomycetes, primarily Eurotiales, Tuberales, Pezizales, and Helotiales, contain species suspected of being ectomycorrhizal with trees. The fungal symbiont in endomycorrhizae intracellularly penetrates corti­ cal cells of the feeder root. Endomycorrhizal fungi may form large vesicules and arbuscules in cortical tissues, called vesicular-arbuscular (VA) mycorrhi­ zae. The endomycorrhizal fungi do not form a dense fungal mantle, but de­ velop on the root surface a loose, intermittent arrangement of mycelium,