Hyphal Interactions BetweenRhizoctonia Solaniand SomeVerticilliumSpecies

Abstract

The biological control of soil-borne plant pathogens with mycoparasitic fungi (e.g., Trichoderma and Gliocladium spp.) has received considerable attention (Ayers and Adams, 1981; Lumsden, 1981; Papavizas and Lumsden, 1980). Species of Verticillium have been often reported in the literature as mycoparasites but only a few taxa have been cited as parasites of soil-borne plant pathogens. For most common Verticillium species host range studies have not been conducted and their ability to attack soil-borne plant pathogens has not been determined. Hyphal interactions between selected species of Verticillium and the plant pathogen R. solani were examined in this study. Species and isolates of Verticil? lium were chosen to represent the common soil-borne members of the genus. Although most taxa are known as mycoparasites, isolates were not obtained di? rectly from fungal substrates (Table I). A measure ofthe variability within a single species was obtained by including six isolates of V. lecanii from different sources. Identification of the isolates of V. psalliotae and V. lamellicola was verified by determining growth rates on 2% malt extract agar at 20 to 30 C (Gams and Van Zaayen, 1982). All cultures were maintained on Difco potato dextrose agar slants (PDA) and stored at 4 C. Two isolates of R. solani (R-19 and R-33) were tested in combination with each isolate of Verticillium. Both were highly virulent members of anastomosis group AG-4 and isolated from diseased bedding plants (Stephens et al, 1982). Hyphal interactions between Verticillium and R. solani isolates were observed by growing paired cultures from agar discs (5 mm diam) placed at opposite sides of agar plates (9 cm). All results were based on minimum of two plates prepared on two different occasions for each Verticillium/Rhizoctonia combination on at least two media: 2% water agar and Difco potato dextrose agar. In preliminary tests with some isolates, hyphal interactions were also examined on Difco cornmeal agar and 2% agar in an aqueous extract of composted hardwood bark container medium. The extract was prepared by soaking one liter of container media in one liter of distilled water overnight followed by filtering the mixture through cheesecloth. This medium had a final pH of 7.1 and was used to approximate the nutritional conditions of the environment from which two species were isolated. All plates were incubated at 24 C under continuous light and were examined after 5-14 da by direct examination with a compound microscope using either brightfield or Nomarski interference illumination. A coverslip was placed over the zone of contact for examination at high magnification and for photography. In some cases, a small drop of cotton blue in lactophenol was used to stain the hyphae. Replicate plates were prepared with a layer of cellulose dialysis membrane on the