Compatibility of Rhizobium japonicum with Chemical Seed Protectants

The literature on the compatibility of Rhizobium sp. with seed protectant chemicals is controversial because of variation in methods and the lack of quantitative data. The present study was planned to develop a reliable quantitative method of measuring compatibility of Rhizobium japonicum with seed-applied fungicides. The effect of four seed protectant chemicals on survival of R. japonicum on soybean (Glycine max L. Merr. cv. ‘Hark’) seed was studied in laboratory growth chamber experiments. The chemicals tested and levels used were tetramethyl-thiuram-disulfide (thiram), 0.6 g/kg seed; N-trichloromethylthio-4-cyclohexene-l,2-dicarboximide (captan), 0.8 g/kg seed; 5,6-Dihydro-2-methyl-l,4-oxathiin-3-carboxanilide (carboxin), 1.1 g/kg seed; and pentachloronitrobenzene (PCNB), 0.9 g/kg seed. Quantitative measurements for compatibility included plate counts for survival of R. japonicum on inoculated chemically treated seed and counts of taproot nodules on plants at 2 weeks age. R. japonicum were compatible with Thiram and Carboxin but not with PCNB and Captan. PCNB reduced Rhizobium survival on seed and decreased taproot nodulation. Captan was less toxic to rhizobia than was PCNB, but taproot nodulation of plants was greatly reduced by captan. Carboxin had little effect on viable rhizobia or taproot nodulation when seeds were planted within 4 hours of inoculation. Nodulation was poor when these seeds were planted 24 hours after inoculation despite the presence of what would normally be considered adequate viable rhizobia. Thiram had no adverse effect on viable rhizobia or taproot nodulation even when seeds were held 24 hours before planting. The results also indicate that compatibility must be measured quantitatively in relation to time and that nodulating ability of the surviving bacteria is the prime factor in determining compatibility.