Seasonal pattern of tomato mosaic tobamovirus infection and concentration in red spruce seedlings

Tomato mosaic tobamovirus (ToMV) infects red spruce (Picea rubens) and causes significant changes in its growth and physiology. The mechanism of infection and the pattern of virus concentration in seedling roots and needles were investigated. One-year-old red spruce seedlings were obtained from the nursery in April and June 1995 and August 1996 and tested for ToMV using enzyme-linked immunosorbent assay (ELISA). Virus-free seedlings were divided into three treatments: control, root inoculated, and needle inoculated. Two control, five root-inoculated, and five needle-inoculated seedlings were sampled destructively at biweekly intervals for 3 months and then tested for ToMV by ELISA. ToMV was transmitted to seedlings by root but not by needle inoculation. The virus was detected in 67 to 100% of roots but in less than 7% of needles of root-inoculated seedlings. The percent infection of root-inoculated seedlings differed significantly between the April and June and between the April and August inoculation periods. Virus concentration in infected seedling roots increased initially, peaked within 4 weeks postinoculation, and steadily declined thereafter. Significant differences in ToMV concentrations in roots also were detected among inoculation periods and sampling dates. Early spring may represent the optimal time for infection of seedlings, as well as for assaying roots for ToMV.