LAETISARIA ARVALIS (APHYLLOPHORALES, CORTICIACEAE): A POSSIBLE BIOLOGICAL CONTROL AGENT FOR RHIZOCTONIA SOLANI AND PYTHIUM SPECIES1

H. H. BURDSALL, JR.

Center for Forest Mycology Research, Forest Products Laboratory2 USDA, Forest Service, Madison, Wisconsin 53705

H. C. HOCH

Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva, New York 14456

M. G. BOOSALIS

Department of Plant Pathology, University of Nebraska, Lincoln, Nebraska 68583

AND

E. C. SETLIFF

State University of New York, College of Environmental Science and Forestry. School of Biology, Chemistry, and Forestry, Syracuse, New York 13210

SUMMARY

Laetisaria arvalis, a soil-inhabiting basidiomycete, is described from culture as a new species. Descriptions and illustrations of the basidiocarps and cultures are provided and the relationship of L. arvalis to Phanero­ chaete as well as its potential importance as a biological control agent are discussed.

About 1960, M. G. Boosalis isolated a fungus with clamp connections from soil planted to sugar beets (Beta vulgaris L.) for more than 50 yr near Scottsbluff, Scotts Bluff County, Neb. His early studies of this isolate indicated that it might be used as a biological control agent against Thanatephorus cucumerus (Frank) Donk (anamorph : Rhizo­ ctonia solani Kuhn) the cause of a root rot of sugar beets. Recently the

1This article was written arid prepared by U.S. Government employees on official time, and it is therefore in the public domain. 2Maintained at Madison, Wis., in cooperation with the University of Wisconsin.

728 729 BURDSALL ET AL. : LAETISARIA ARVALIS

isolate has been reported to be a hyperparasite of R. solani (Odvody et al., 1977) and a possible biological control agent of Pythium ultimum Trow (Hoch and Abawi, 1979). The fungus has a1so been isolated from sugar-beet fields in Ohio by L. R. Herr and, although isolates are not available for verification, from soil in Maryland by G. C. Papavisas (personal communication, 1979). To date, this fungus has remained unnamed and is referred to only as Corticium sensu lato (Hoch and Fuller, 1977; Odvody et al., 1977) and as Corticium sp. (Hock and Abawi, 1979; Hock et al., 1979). It is important for communication and for interpreting relationships to have a more biologically meaningful designation, especially when the organism has the potential importance that this one has. Consequently, the fungus is described here as a new species in spite of the fact that the teleomorph has not been found in nature. A study of the isolates fruiting in culture indicates it is a member of the genus Laetisaria Burds. (Burdsall, 1979).

Laetisaria arvalis Burdsall sp. nov. FIGS. 1-12

Basidiocarpis effusis; hyphis subiculis latis, fibulatis; hyphis fasciculis adsunt; hyphis aliis angustis, afibulatis; hyphis latis et hyphis fasciculis involutis; hyphidiis et cystidiis absens; basidiis (35-) 40-60 × 8-12 µm; basidiosporis 10.5-12.5 (-13.5) × 5.5-7 µm; scleratio adsunt, 400-600 µm diam. Habitat : In solo arvali; interdum mycoparasiticis.

Holotype: (Dried culture) Boosalis isolate, soil in sugar beet field, Scottsbluff, Scotts Bluff County, Nebraska, conserved in CFMR; isotype in BPI; type culture deposited in NRRL3 11375. Etymology: From arvalis (L., adj.) = pertaining to fields or culti­ vated land.

DESCRIPTION OF BASIDIOCARP Hymenium forming initially on cordons or on sclerotia, later some­ times growing and covering the areas between cordons, then becoming broadly effused, up to 200 µm thick, membranous; fertile area yellowish white (near 3A2)4 or Light Buff of Ridgway (1912), smooth to slightly farinaceous at 10 × magnification. Hyphal system monomitic; on agar medium subiculum a textura intricata; hyphae (FIGS. 1, 6) (3-)5-9(-13)µm wide, loosely dis­

3 Fermentation Laboratory, Northern Regional Research Laboratory, U.S. Department of Agriculture, Peoria, Ill. 61604. 4 Color designation code is that of Kornerup and Wanscher, 1967, indicating plate number, column, and row, respectively. MYCOLOGIA, VOL. 72. 1980 730

FIGS. 1-3. Laetisaria arvalis. Photomicrographs of hyphae and hymenial development, × 500. 1. Single hypha and cordon, not entwined by narrow hyphae. 2. Broad hypha with narrow hyphae beginning to entwine it. 3. Completely entwined cordon with metabasidium (arrow) produced from surface hyphae. 731 BURDSALL ET AL. : LAETISARIA ARVALIS

FIGS. 4, 5. Laetisario arvalis. Photomicrographs, × 200. 4. Mature basidium (only two of four sterigmata in focus) and two metabasidia (arrows) produced from subhymenial layers between cordons. 5. Cross section of sclerotium. MYCOLOGIA, VOL. 72. 1980 732

FIGS. 6-9. Laetisaria arvalis. Line drawings of some microscopic structures. 6. Thick-walled subicular hyphae. 7. Cordon. 8. Basidia. 9. Basidiospores. 733 BURDSALL ET AL. : LAETISARIA ARVALIS

FIGS. 10-12. Laetisaria arvalis. Macroscopic cultural characters on malt- extract, × 2. 10. Mat surface after 7 da growth. 11. Mat surface after 3 wk, note thickened cordons (these possess basidia) and early sclerotium development. 12. Mature sclerotia on mat surface. MYCOLOGIA, VOL. 72, 1980 734

tributed on agar, hyaline, thin walled or with walls up to 2 µm thick especially in broader hyphae ; septate, with occasional clamp connections on broadest hyphae, long celled, frequently branched ; cordons (FIGS. 1, 3, 7) up to 50 µm wide, formed from several (two to eight) parallel to entwined hyphae 7-10 µm wide, often enclosed in a layer of short-celled contorted hyphae 3-5 µm wide (FIG. 2), later in several layers, these cells becoming thick walled and giving rise to basidia (FIG. 3); sub­ hymenium developing between cordons as agglutinated and thickened tissue up to 150 µm thick, composed of hyphae 3-5 wide, with frequent septa, short celled, thick walled, lacking clamp connections, much branched, hymenium of basidia only; basidia (FIGS. 3, 4, 8) (35-)40­ 60(-70) × 8-12 µm, arising from probasidial cells enveloping cordons or on sclerotium, mostly broadly clavate, sometimes with slight pro­ basidial swelling, hyaline, thin walled above probasidial cell, 4-sterigmate, sterigmata up to 6 µm long, up to 3.5 µm broad at base; later similar basidia arising from subhymenium between cordons; basidiospores (FIG. 9) 9.5-12.5(-13.5) × 5.5-7 µm, ellipsoid to narrowly ellipsoid, with adaxial flattening and pronounced apiculus, hyaline, thin walled, smooth, no reaction in Melzer’s reagent, acyanophilous.

DESCRIPTION OF CULTURES Key codes: A-P-F-1-5-6-12-13-16 (using Davidson et al., 1942) 2.5.16.23.36.38.41.(48).50.56. (using Nobles, 1965). Growth on 1.5% malt-extract agar at 25 C rapid, covering 90-mm Petri dish in 2-3 da ; optimum growth temperature about 29 C; mat at first of woolly radiating aerial fibrils (FIG. 10), after 2-4 wk more appressed, with scattered radiating aerial cordons sometimes possessing basidia, fruiting patches developing later between cordons; margin dis­ tinct, finely fibrillose, even; sclerotia developing after 2-3 wk as light pink spheres (FIG. 11 ) on agar surface, these eventually reaching 400­ 600 µm wide, dark reddish brown at maturity (FIG. 12) ; medium not bleached or stained, odor of apricots. Oxidase reaction moderately strong with Bavendamm test (Davidson et al., 1942), slight growth on both gallic and tannic acid agars. Margin with leading edge of hyphae 8-lo(-12) µm wide, unbranched back at least 500 µm and often over 1 mm from tip, hyaline, thin walled, septate, with frequent clamp connections, lateral branches mostly 4-6 µm wide, hyaline, thin walled, frequently branched, smooth, septate, clamp connections lacking; aerial hyphae of two types: (a) 8-lo(-12) µm, hyaline, slightly thick walled, single or aggregated in groups of two to eight (FIG. 1) with frequent clamp connections, later becoming enveloped by second type (FIG. 2) ; (b) 3-5 µm wide, frequently branched, hyaline, contorted, short celled, septate, clamp connections lacking, entwining around the cordons, eventually becoming thick walled, 735 BURDSALL ET AL. : LAETISARIA ARVALIS and forming a subhymenium several cell layers thick (FIG. 3) ; after 2 wk or more basidia may form on cordons (FIG. 3) ; later hyphae of the smaller type sometimes spread onto agar surface to form thickening subhymenial layer up to 150 µm thick, forming tough membrane from which basidia arise (FIG. 4) ; sclerotia (FIG. 5) 400-600 µm wide, rind 20-40 µm thick, composed of several layers of intertwined hyphae 2-4 µm wide, thin walled, pale brown, smooth, lacking clamp connections, sometimes becoming thick walled and giving rise to typical basidia, medulla at first a loose textura intricata, hyphae 3-7 µm wide, thin walled or with slight wall thickening, hyaline, septate, lacking clamp connections, much branched at maturity, a textura globosa-textura angularis, cells (8-)10-25(-30)µm wide; cells of all sizes rather ran­ domly distributed but adjacent to rind a concentration of smaller cells or a textura angularis of large cells radially arranged and radially elongated, with slightly thickened walls, pale brown.

Cultures examined: Type culture, cited above; nos. ZH1, ZH2, ZH3, ZH4, ZH5, ZH6, ZH7, ZH8, and ZH9 all from soil debris in agricultural fields, Delaware County, Ohio.

REMARKS The description of L. arvalis on the basis of fruiting in culture was undertaken with reservation. However, the teleomorph from nature will probably be effused and look very similar to the fruiting bodies from culture. This consideration and the importance of having a specific epithet for the fungus prompted the designation. In addition, this species probably will be seen mostly in soil isolations and its cultural de­ scription may be the most useful. The generic placement of L. arvalis is still somewhat tentative. It possesses characters intermediate between Phanerochaete Karst. and Laetisaria and, after naturally formed basidiocarps are found, it may be necessary to reevaluate the generic disposition. The species is remi­ niscent of a species of Phanerochaete, but such characteristics as the production of sclerotia, the probasidial-type cells from which the basidia arise, the size of the basidia and basidiospores, and, very importantly, the habitat and mycoparasitic capability are most unusual for Phanerochaete species. Thus, Laetisaria arvalis is placed in Laetisaria because of the absence of clamps in the basidiocarp proper, the large basidia and basidiospores, the reduced probasidial cell, the parasitic capability (although it is not on higher plants as is L. fuciformis (McAlp.) Burds.), and the production of differentiated vegetative structures (sclerotia instead of the rhizomorph-like structures of L. fuciformis). However, it does not MYCOLOGIA, VOL. 72, 1980 736

possess a catahymenium as is found, though much reduced, in L. fuci­ formis. The cells on the cordons and sclerotia from which the basidia arise are usually undifferentiated hyphal cells with somewhat thickened walls. The metabasidium emerges to form sterigmata and basidiospores. The basal cell therefore appears to be a probasidial cell, although reduced and poorly differentiated. In culture, L. arvalis and L. fuciformis both possess clamp connections. It is of interest that the isolates of L. arvalis from Ohio have not produced basidia on the cordons or on a surface hymenia1 mat. All fruiting in these isolates has occurred on the surface of the sclerotia.

ACKNOWLEDGMENTS Dr. L. R. Herr, Delaware, Ohio, is thanked for providing isolates of L. arvalis from that state. Also thanked are Drs. P. L. Sanders and H. Cole, Jr., who provided cultures of L. fuciformis for comparison. Dr. J. Andrews and Mrs. F. F. Lombard are acknowledged for providing critical comments on the manuscript. Dr. M. J. Larsen provided the Latin diagnosis.

LITERATURE CITED

Accepted for publication December 3, 1979

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