APS Abstracts of Presentations

Variation for Phymatotrichopsis omnivora tolerance in alfalfa (Medicago sativa L.)
H. LEE (2), S. Marek (1), C. Young (2), M. Sledge (2)
(1) Oklahoma State University, Stillwater, OK; (2) The Samuel Roberts Noble Foundation, Ardmore, OK
Phytopathology 96:S65

Alfalfa (Medicago sativa L.) is the most widely grown forage crop because of its high protein, vitamins, energy, and digestibility. However, alfalfa production in southern Oklahoma and Texas is difficult due to the soil-borne fungus, Phymatotrichopsis omnivora (Duggar) Hennebert, which causes Phymatotrichum root rot. Currently, no practical disease management methods are available and no alfalfa varieties are known to be resistant or tolerant to this fungus. We have established a growth chamber screening procedure to identify tolerance to Phymatotrichum root rot in alfalfa. Twenty alfalfa cultivars and Medicago truncatula Jemalong A17 were planted in 96-cell trays containing Houston black clay and maintained in a growth chamber at 28°C. A strain of P. omnivora isolated from alfalfa (Courtney, OK) was cultured on sterilized sorghum grains and 5 g used to inoculate each plant. Four weeks after inoculation the plants were scored for resistance based on the following scale: (1) dead, (2) stunted with yellowing or some dead leaves, (3) stunted but green, (4) tall with some yellowing of leaves, (5) tall with green leaves. A group of alfalfa cultivars, Enhancer, Magnum IV, OK 49, and Pioneer 5681, exhibited a low but consistent level of tolerance to Phymatotrichum root rot. Several tolerant alfalfa genotypes from these cultivars were selected for futher studies.

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