Effect of Clover Phyllody Virus on Nodulation of White Clover (Trifolium repens) by Rhizobium trifolii in Soil

Abstract

When grown in sterilized soil inoculated with an effective Rhizobium strain which produced many large pigmented nodules on virus-free plants, plants infected with clover phyllody virus (CPV) produced mainly small white nodules characteristic of an ineffective reaction. Mainly small nodules were also given by virus-free plants exposed to mixtures of effective and naturally ineffective Rhizobium strains, which separately gave predominantly large and predominantly small nodules, respectively, indicating that the ineffective strain had a high competitive ability for invasion sites on the roots. When the CPV-infected plants were removed and fresh seed sown in the soil, germination was poor and the seedlings produced predominantly small nodules. The rhizobia seemed modified to a less effective form which produced mainly small nodules and competed successfully with unmodified bacteria. Yields of clover in swards may therefore be decreased by CPV infection, not only through a direct effect on plant growth but also through effects on the soil Rhizobium population, and hence on the growth of infected plants and neighboring virus-free plants.