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Responses of Fungal Hyphae to Soil Fungistasis. S. C. Hsu, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823; J. L. Lockwood, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823. Phytopathology 61:1355-1362. Accepted for publication 17 June 1971. DOI: 10.1094/Phyto-61-1355.

Hyphae of 20 different fungi were incubated for 4 hr on cellophane placed on natural soil amended with different amounts of alfalfa extract. Linear growth, expressed as per cent of growth occurring on alfalfa extract agar, was plotted against log concentration of alfalfa extract. The area under the curve, taken as per cent of the total area, was termed the “relative growth index”. Relative growth indices varied from 20.3 to 80.2. A high relative growth index indicated low sensitivity to soil fungistasis. Relative growth indices were directly correlated with hyphal growth rate on alfalfa extract agar (r = 0.91), and with hyphal diameter (r = 0.82). Hyphal diameter was also directly correlated with growth rate (r = 0.88) and spore volume (r = 0.84). Fungistasis sensitivity indices of spores and relative growth indices of hyphae were inversely correlated (r = 0.74). Lysis, as determined by loss of radioactivity from ¹⁴C-labeled hyphae on soil, was delayed when alfalfa extract was added to soil. Fewer than 10% of the hyphae of any of 10 fungi incubated on membrane filters on natural soil remained viable after 8 days. The conidial portion of germinated spores of five species with multicellular spores survived longer than the hyphae themselves, whereas conidia of five species with single-celled spores survived no longer than their hyphae. Some of the original, germinated conidia of Helminthosporium sativum and H. victoriae were able to regerminate at least 5 successive times on an acidified agar medium following complete lysis of germ tubes on soil.