Strains of Soybean Dwarf Virus

Abstract

Isolates of soybean dwarf virus (SDV) obtained from naturally infected plants were divided into two groups on the basis of their symptoms on soybean plants. One group was designated as a dwarfing strain, which caused a dwarfing of plants with shortened petioles and internodes. Another group was designated as a yellowing strain, which caused a slight chlorosis of leaflets and interveinal yellowing of older leaves. In the field, soybean, pea, and white (Ladino) clover plants were found to be infected with both strains, whereas bean plants were infected only with the yellowing strain and red clover only with the dwarfing strain.
Three isolates, SDV-DS (caused severe symptom) and SDV-DM (mild symptom) belonging to the dwarfing strain, and SDV-Y belonging to the yellowing strain, were used here as virus sources. Of 28 leguminous species inoculated by the aphid Aulacorthum solani (Kaltenbach), 13 species became infected with SDV-DS and 16 species with SDV-Y. In general, the symptoms caused by SDV-Y were more severe than those caused by SDV-DS on many infected plants: stunting, chlorosis, and interveinal yellowing or marginal reddening of older leaves. The mode of transmission of SDV-Y was quite similar to that of SDV-DS.
When soybean plants were inoculated simultaneously with SDV-Y and SDV-DS or SDV-DM, the infected plants usually developed the symptoms more severe than those caused by either virus isolate alone, showing a marked rugosity of leaflets and a severely stunting of plants. Plants infected doubly with SDV-DS and SDV-DM showed the intermediate symptoms between these. Mixtures of these isolates could be separated by single aphid transfer to soybean plants. Soybean plants infected already with SDV-Y were not protected against infection with SDV-DS or SDV-DM, although the symptom development of SDV-DS or SDV-DM was delayed. Similarly, SDV-DS or SDV-DM also gave no protection against SDV-Y. However, SDV-DS and SDV-DM could protect against each other in soybean plants.