SUMMARY: 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 neighbouring virus-free plants.


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