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 x 10(2), 16 x 10(3), 16 x 10(4), 16 x 10(5), and 16 x 10(6) teliospores of Tilletia controversa per row, or seed was inoculated with 0, 2 x 10(2), 2 x 10(3), 2 x 10(4), 2 x 10(5), and 2 x 10(6) teliospores per gram. To determine maximum possible infection, two 3.1-m rows of each cultivar were soil-surface inoculated at 10x the highest treatment rate. In the soil-inoculated plots, a minimum of 16 x 10(3) 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 x 10(5) rate. In the seed-inoculated plots, infection was rare and occurred only at inoculation rates of 2 x 10(5) teliospores/g or higher; the highest incidence was 0.4%.