Survival of Verticillium albo-atrum in alfalfa seeds

Hard seeds of alfalfa (Medicago sativa L.) were artificially inoculated with Verticillium albo-atrum Reinke & Berthold and assessed for survival of the pathogen under laboratory and field conditions. When infected seeds were stored air-dry, V. albo-atrum survival was low (less than 10% after 10 months) at above-freezing temperatures (20 and 30 °C) but high (more than 90% at 10 months) at subfreezing temperatures (−20 and −10 °C). Survival of the pathogen was also low (10% at 3 months) in seeds buried in soil at 20 °C but high (90% at 10 months) at −5 °C. More than 75% of the hard seeds remained viable after 18 months of air-dry storage at −20, −10, 4, 20, or 30 °C. The incidence of verticillium wilt in alfalfa plants due to seedborne V. albo-atrum decreased with seed storage period, averaging 40% after seed was stored for 1 month and zero when seed was stored for 12 or 18 months. In field experiments, survival of seedborne V. albo-atrum decreased with increasing period of burial but at different rates in each year. The percentage of seeds with viable V. albo-atrum was greater when the seeds were buried at 10 cm compared with 5 and 2 cm (48 vs. 39 and 38%, respectively). The complete loss of viability of V. albo-atrum in infected seeds stored at 30 °C for 6 months indicates that seed-borne V. albo-atrum could be eliminated by storing alfalfa seeds at 30 °C for 1 year. Key words: Verticillium wilt, Verticillium albo-atrum, Medicago sativa, alfalfa, survival, seedborne.