Titer Variation in Infected Sorghum Differing in Resistance to Maize Dwarf Mosaic Virus Strain-B

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Abstract
Variation associated with sampling for enzyme-linked immunosorbent assay (ELISA) and conditions necessary to make meaningful titer determinations to access resistance to maize dwarf mosaic virus strain-B (MDMV-B) in sorghum (Sorghum bicolor) were investigated. ELISA and infectivity assays were highly correlated. In ELISA, maximum sample variation was associated with leaf 3 and the least sample variation with leaves 5 and 6. ELISA values at a given leaf position increased with time after infection at all temperatures except 35.degree. C, at which they decreased. ELISA values progressively decreased in each leaf above leaf 4, except at 35.degree. C, where the opposite was true. ELISA values were different for equivalent leaves with various lengths from plants of the same age. A gradient in ELISA values were present in leaves, with values increasing from the leaf base to the tip. The resistant cultivar (Colt) had lower ELISA values at 7 days postinoculation (DPI), but not at 14, 21, and 28 DPI for leaves 4, 5, and 6. At 28 DPI, the resistant cultivar showed attenuation of symptoms and a large decrease in ELISA values for leaves 7 and 8. At 35.degree. C, MDMV-V was not detected in the resistant cultivar.