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Phytophthora. Root Rot in Seeding-year Alfalfa as Affected by Management Practices Inducing Stress¹

Root rot (Phytophthora megasperma Drechs.) on alfalfa (Medicago sativa L.) in the seeding year is associated with soil excessively wet for ten days or longer. The objective of this experiment was to determine the incidence of Phytophthora root rot (PRR) in October of the seeding year on tiled, well-drained loam soil at East Lansing, Michigan. Samples were obtained on all combinations of the following: early (April 27) and late (May 19) seeding, irrigation (two 5-cm applications) and no irrigation, Vernal and Saranac cultivars, two and three cuttings, and seven seeding rates — 2.2, 4.5, 9.0, 13.5, 18.0, 22.5, and 36.0 kg/ha.

In this test, irrigation of 5 cm on July 29 and on August 15 increased Phytophthora root rot in alfalfa roots. The natural precipitation between April 1 and October 31 was 56 cm, 4.5 cm above normal. The cultivars Vernal and Saranac were similar in susceptibility to root rot under all treatments. Increasing cutting frequency from two to three cuttings in the seeding year resulted in more PRR under all conditions of seeding date or irrigation levels. Alfalfa seeded at below-normal seeding rates of 2.2 and 4.5 kg/ha was significantly less diseased than at normal (9.0 or 13.5 kg/ha) or higher seeding rates. Most comparisons indicated a trend of more PRR in alfalfa seeded late on May 19 than earlier on April 27. Incidence and severity of Phytophthora root rot was increased by a combination of stress factors including high soil moisture levels, frequent cutting, high seeding rates, and late spring seeding.

Key Words: Medicago sativa L. • Seeding rates • Cutting frequency • Irrigation • Dates of seeding

² Formerly Kellogg Foundation fellow, now assistant professor, Dep. of Plant Husbandry, Univ. of Helsinki, Helsinki, Finland, and professor of crop and soil sciences, Michigan State Univ., East Lansing, MI 48824.

Received for publication March 4, 1975.