Influence of cultural factors on alfalfa seedling infection by Pythium debaryanum Hesse
It is well recognized that considerable care must be exercised on some soils to establish stands of small-seeded legumes, notably alfalfa and red clover and sometimes sweet clover. Farmers generally are aware of the necessity of sowing a good grade of seed of a hardy variety on a well-prepared seedbed under a light nurse crop, after the soil, if acid, has been limed. The common rate of seeding alfalfa is 10-15 pounds per acre, which, with good-quality seed, provides for 50-75 seeds per square foot. Second year stands in Iowa, however, rarely contain more than 10 plants per square foot. Obviously a reduction of any part of an alfalfa stand loss, with its accompanying hazard of complete failure, would be of real benefit to the Iowa farmer.
The role of Pythium in destroying alfalfa seedling stands on three acid Iowa soils was first pointed out by the author (1, 2) in two short preliminary papers in 1934 and 1935. Pythium was observed to produce necrotic lesions on germinating seed and alfalfa seedlings more commonly on slightly acid Clarion loam, Webster loam and Tama silt loam than on two similar areas of neutral Webster loam and Webster silty clay loam. At 9°C. fewer alfalfa and alsike clover seedlings became infected than at 20° -25 ° C. Plantings of alfalfa and alsike clover on steamed and formaldehyde-treated acid soil yielded as many or more healthy alfalfa seedlings than similar plantings on neutral field soil. Alsike seedlings seemed more resistant to Pythium than alfalfa seedlings at 20°-25° C. The prevalence of infection of alfalfa seedlings in acid soils at moderate (15°-30° C.) temperatures is comparable to the situation recently reported (3) from sugar beet seedling damping-off studies in northern Iowa.