Relationship Between Soilborne and Seedborne Inoculum Density and the Incidence of Dwarf Bunt of Wheat

The incidence of dwarf bunt of wheat as a function of inoculum density was studied in a susceptible and a partially resistant cultivar at three disease-conducive locations for three seasons. Prior to seeding, plots were fumigated with methyl bromide to eliminate residual inoculum. Each cultivar was seeded into two 1.2-m rows in four replicates. The soil surface was inoculated with 0, 16 × 10², 16 × 10³, 16 × 10⁴, 16 × 10⁵, and 16 × 10⁶ teliospores of Tilletia controversa per row, or seed was inoculated with 0, 2 × 10², 2 × 10³, 2 × 10⁴, 2 × 10⁵, and 2 × 10⁶ teliospores per gram. To determine maximum possible infection, two 3.1-m rows of each cultivar were soil-surface inoculated at 10× the highest treatment rate. In the soil-inoculated plots, a minimum of 16 × 10³ teliospores/row was needed to cause trace amounts of disease (0.6% maximum), even when the positive indicator treatment had up to 88% incidence. Only trace amounts or no disease occurred below the 16 × 10⁵ rate. In the seed-inoculated plots, infection was rare and occurred only at inoculation rates of 2 × 10⁵ teliospores/g or higher; the highest incidence was 0.4%.