Little is known about the microscopical symptoms of roots from apple trees suffering from replant disease. Roots were sampled from healthy trees and from such diseased trees in four orchards from May until October. Roots were stained for the detection of mycorrhizal infection and other roots were fixed, dehydrated, embedded in paraffin, sectioned, and stained in safranin and fast green in order to elucidate morphological details. Healthy tree roots possessed considerably higher frequencies of mycorrhizal infection than diseased trees during the entire growing season. Arbuscules and hyphae were very common, vesicles were sometimes present, and possible chlamydospores of Glomus radiatum (Thaxter) Gerd. & Trappe were found in several samples. Whereas roots from healthy trees were structurally intact, roots from declined trees had extensive sloughing away of the epidermal and cortical layers and the cortical cells possessed significant amounts of densely stained material. Nematodes and possible hyphae of Rhizoctonia, Phytophthora, and Pythium were found in roots of declined trees. Althugh the stele of these roots appeared unaltered, hyphae were sometimes observed in the vascular elements.

The button mushroom, Agaricus bisporus, is a commercially important cultivated filamentous fungus. During the last decade, the button mushroom industry has depended mainly on two strains (or derivatives of these two strains). Using one of these highly successful strains (strain UI) we examined the phenomenon of strain instability, specifically, the production of irreversible sectors. Three "stromatal" and three "fluffy" sectors were compared with a healthy type Ul strain and with a wild-collected isolate. Compost colonization and fruit body morphology were examined. The main objective of this study, however, was to examine the meiotic stability of the sectored phenotype. Single basidiospores were isolated and subjected to a grain bioassay in which the ability to produce sectors was measured. Our results were as follows: (i) basidiospore cultures obtained from a wild-collected isolate showed no tendency to produce sectors; (ii) approximately 5% of the basidiospore cultures obtained from healthy type Ul strains produced irreversible sectors in the grain bioassay; (iii) the five primary sectors examined produced basidiospore cultures, half of which produced normal-looking growth in the grain bioassay and half of which produced some degree of sectoring; and (iv) the one sectored isolate that represented the F2 generation gave ratios similar to the 1:1 ratio observed for the Fl cultures.