Fred A. Gray

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.

Evidence for the heritability of resistance to brown root rot of alfalfa, caused by Phoma sclerotioides

Brown root rot (BRR) of alfalfa, caused by Phoma sclerotioides, a low-temperature pathogen, is associated with winterkill in the contiguous United States. Identified in Canada during the 1920s, BRR has been identified from legume roots in four western Canadian provinces. Some Canadian cultivars exhibit BRR resistance, although only one was consciously selected for the trait. Heritability of BRR resistance was evaluated in this study in which containergrown alfalfa plants were inoculated with P. sclerotioides. Plants that were evaluated are progeny (cycle 1) of one generation of selection and intermating of healthy plants drawn from six diverse North American alfalfa populations (five commercial cultivars, one experimental population) that survived exposure to P. sclerotioides soil infestation for several winters. These were evaluated together with plants grown from seed of the original unselected populations (cycle 0), and putative check cultivars ‘Multi-plier’ (BRR susceptible) and ‘Peace’ (BRR resistant). Plants were rated for disease severity for a total of 4 test years. Cycle 1 plants exhibited a reduced level of disease severity compared with cycle 0 plants (P < 0.05), indicating heritability of BRR resistance in alfalfa. Also, especially important for future alfalfa breeding efforts, this study demonstrates that ‘Multi-plier’ and ‘Peace’ are useful checks for screening BRR resistance.

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.