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

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 oil 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 I 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 degrees C for 3 months on potato dextrose agar, all 14 isolates had beaked pyenidia, had single-celled conidia, and were identified morphologically as P. sclerotioides.