Two Virginia isolates of soybean mosaic virus (SHV), VA and OCM, were classified as G1 and G3 strains, respectively, according to a soybean differential cultivar system. G1/VA and G3/OCM differed in the symptom severity induced on soybean (Glycine max [L.]Merr.) plants and by the production by G3/OCM of cytoplasmic strands, containing virus particles, within infected leaf cells. Two Illinois isolates, G1/IL and G3/IL, resembled G3/0CM in mild symptoms; induced in soybean plants and by the production of cytoplasmic strands ultrastructurally. G1/VA differed from the 3 mild strains by the severe symptoms induced on plants and by the absence of cytoplasmic strands ultrastructurally. All 4 strains were closely related serologically. The tolerant reaction of soybean to the 3 mild SMV strains was correlated with production of cytoplasmic strands.

Soybean plants were most susceptible to G1/VA and peanut stunt virus (PSV) when the primary leaves were inoculated at 50-75% of their full expansion. Plants developed more severe symptoms with either virus within a shorter incubation period than did plants which were inoculated when their primary leaves were less than or greater than 50-75% expansion.

The SMV strain, SMV-VA (G1/VA), was used to determine the cell types SMV moves into, using pinwheel inclusions as indicators of cell infection, at an early stage of infection when virus particles were first detectable in inoculated and noninoculated tissues of prilutry leaves. By using serologically specific electron microscopy (SSEM), virus particles were first detected in inoculated tissues at 6 days after inoculation. Pinwheels were detected within these areas in palisade, paraveinal, spongy, vascular parenchyma, phloem and immature xylem cells. The pinwheels were frequently opposed to the plasmalemma and were associated with plasmodesmata and plasmalemmasomes. Endoplasmic reticulum was associated with the base of those pinwheels found in the cytoplasm. The early stage of SMV pinwheel formation appeared to be similar to that reported for other potyviruses. The observation of pinwheels in the paraveineal mesophyll cells suggested the involvement of these cells in virus translocation within the soybean leaf.

A sinusoidal pattern of SMV particle concentration within the inoculated and noninoculated areas of the leaf was detected by SSEM. The appearance of necrotic lesions within the inoculated area was correlated with an increase in the number of detectable particles within the area. A similar correlation was made with an increase in particle numbers in the non-inoculated area and the appearance of vein-clearing on the first trifoliolate leaf. It was also noted that particle numbers increased in the midrib and petiole when particle numbers decreased in the inoculated and noninoculated areas. This pattern was hypothesized to be due to either an alteration in the viral replication rate or to translocation relationships within the leaf.