David W. Koch

Competitive effects of cool-season grasses on re-establishment of three weed species.

The competitive ability of five cool-season grasses relative to Dalmatian toadflax, musk thistle, and downy brome was assessed in two field studies. In 1994, Bozoisky Russian wildrye and four wheatgrass varieties (Critana thickspike, Hycrest crested, Luna pubescent, and Sodar streambank wheatgrass) were seeded into populations of downy brome and musk thistle at Riverside, WY. The same grasses were seeded into populations of Dalmatian toadflax at Cheyenne, WY, in 1995. In 1997 and 1998, weed populations at both study sites were reduced in areas seeded with the five grasses relative to unseeded controls. Hycrest crested and Luna pubescent wheatgrasses were the most competitive against the three weed species. Bozoisky Russian wildrye was more competitive against Dalmatian toadflax than against the other weeds. Sodar streambank wheatgrass suppressed musk thistle and downy brome but was not competitive against Dalmatian toadflax. Seeded grasses, such as Hycrest crested and Luna pubescent wheatgrass, appeared to limit the re-establishment of these weeds. Economic model predictions of the net present values and the internal rates of return suggest that Hycrest crested and Luna pubescent wheatgrass can provide financially feasible long-term weed control only if desired grass yields are maintained for more than 15 yr. Nomenclature: Crested wheatgrass, Agropyron cristatum (L.) Gaertn. × Agropyron desertorum Gaertn. var. ‘Hycrest’; Dalmatian toadflax, Linaria genistifolia spp. dalmatica (L.) Maire and Petitmengi #3 LINDA; downy brome, Bromus tectorum L. # BROTE; musk thistle, Carduus nutans L. # CRUNU; pubescent wheatgrass, Thinopyrum intermedium (Host. Barkworth & Dewey) Nevski var. ‘Luna’; Russian wildrye, Psathyrostachys juncea (Fisch) Nevski var. ‘Bozoisky’; streambank wheatgrass, Elymus lanceolatus (Scribn. & J. G. Smith) Gould var. ‘Sodar’; thickspike wheatgrass, Elymus macrourus (Turcz.) Tzvelev var. ‘Critana’. Additional index words: Competition, economic analysis, integrated controls, perennial grass. Abbreviations: ANOVA, analysis of variance; IRR, internal rate of return; LSD, least significant difference; NPV, net present value.

Distribution of Phoma sclerotioides and incidence of brown root rot of alfalfa in Wyoming, U.S.A.

Brown root rot of alfalfa, caused by Phoma sclerotioides, has been reported from Alberta and Saskatchewan, Canada, and was the cause of widespread “winterkill” of alfalfa in southwestern Wyoming, U.S.A., in 1996. Affected plants exhibit characteristic dark, sunken, necrotic lesions on upper tap and lateral roots. Surveys of irrigated fields were conducted in nine counties to determine distribution of the pathogen within Wyoming. Symptomatic as well as asymptomatic root-tissue isolations demonstrated that P. sclerotioides was present in most counties surveyed, indicating its distribution may be statewide. Disease incidence was assessed in two fields where P. sclerotioides occurred. Results indicate that the pathogen is uniformly distributed within infested fields.

Pathogenicity of 14 isolates of Phoma sclerotioides, causing brown root rot of alfalfa

Abstract Fourteen isolates of Phoma sclerotioides were compared for pathogenicity to alfalfa (Medicago sativa). Thirteen isolates obtained from roots of diseased alfalfa plants collected in Wyoming were compared with one isolate from the American Type Culture Collection (ATCC 56515) previously obtained from a diseased alfalfa root collected in Canada. Barley (Hordeum vulgare) grain inoculum of all 14 isolates was prepared and placed adjacent to the upper taproot of 4- to 6-month-old ‘Multi-plier’ alfalfa plants grown in Tall One Treepots (Stuewe & Sons, Corvallis, Ore.) containing a pasteurized soil-sand mixture. After inoculation, plants were placed outdoors in Laramie, Wyoming, USA, to undergo exposure to winter conditions previously reported to be essential for fungal infection. Bales of straw were placed around potted plants and loose straw was placed on top of plants for additional protection against winter injury. Each spring, plants were returned to the greenhouse and roots were given a disease rating for brown root rot on a scale of 1–5 (1, no disease symptoms, healthy-appearing root; 5, acute root symptoms, plant dead). Plants were maintained in the greenhouse during the summer and, again, placed outside in the early fall for a second winter exposure period. All 14 isolates were pathogenic to alfalfa. Mean disease severity rating and plant mortality of isolates in test 1 were 4.0 and 34.5%, respectively, after the first winter and 4.8 and 56.5%, respectively, (cumulative mortality of 91.0%) after the second winter. Final disease severity ratings and plant mortality for test 2 were 3.9 and 51.6%, respectively, after the first winter and 4.5 and 7.9%, respectively, (cumulative mortality of 59.5%) after the second winter. Although differing somewhat in morphology when maintained in the dark at 10 °C for 3 months on potato dextrose agar, all 14 isolates had beaked pycnidia, had single-celled conidia, and were identified morphologically as P. sclerotioides.