Invasion of red clover (Trifolium pratense) roots by soilborne fungi

Abstract

Red clover plants were grown in the glasshouse in pots containing soil (at 20°C) from breeders’ evaluation plots with a history of poor persistence of red clover. Fungal invasion of plants grown from seed treated with a mixture of metalaxyl and benomyl (each at 1.5 g(kg seed), of plants grown from untreated seed in sOIl drenched WIth a mixture of metalaxyl (0.15 g/m2) and benomyl (1.0 g/m2), and of untreated plants, was investigated using plate culture isolation techniques. Seed- and soil-drench fungicide treatments increased seedling emergence and prevented post-emergence damping-off. Pythium ultimum Trow and an unidentified sphaerosporangiate Pythium sp. were Isolated from roots of untreated seedlings which dampedoff. Fungi commonly isolated from surface-sterilised tap roots of 3- and 6-week-old treated and untreated plants were (in order of decreasing frequency): Fusarium oxysporum Schlecht., sterIle dark forms, F. solani (Mart.) Sacc., Chalara elegans Nag Raj & Kendrick (= Thielaviopsis basicola (Berk. & Br.) Ferr.), Cylindrocarpon sp., C. destructans (Zins.) Scholten, Verticillium dahliae Kleb., sterile hyaline forms, Acremonium sp., Humicola sp., Chrysosporium sp., and Cylindrocladium scoparium Morgan. Ch. elegans caused dark necrotic lesions. V. dahliae was the predommant fungal isolate from stele cylinders of tap roots of 12-weekold plants and was associated with vascular browning of steles although F. solani, F. oxysporum, Ch. elegans, Cy. scoparium, Chrysosporium sp., and Phoma sp. were also often present. Fewer fungal colonies grew on isolation media from roots of plants treated with fungicide than from roots of untreated plants at 3 and 6 weeks, but by 12 weeks the incidences were similar. In invasiveness tests using red clover seedlings grown on water agar, 43 of 66 fungal isolates, including examples of all common species, formed necrotic lesions containing fungal hyphae on roots. A further II isolates, including V. dahliae, penetrated roots but caused no necrosis. In pathogenicity tests using 5-week-old plants inoculated by dipping roots in conidial suspensions, Ch. elegans, Cy. scoparium, V. dahliae, and F. solani, but not F. oxysporum, reduced shoot dry matter production; whereas Cy. scoparium killed all test plants. These results suggest that Ch. elegans, Cy. scoparium, and V. dahliae should be considered as potentially important pathogens of red clover in New Zealqnd in addition to the Fusarium spp. normally associated with root rot diseases in other parts of the world. © 1986 Taylor & Francis Group, LLC.