Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/14/1999
Publication Date: N/A
Citation: Interpretive Summary: Dwarf bunt is a disease of winter wheat caused by the fungus Tilletia controversa that occurs in the U.S. primarily in isolated areas of the Northwest, and is highly dependent on favorable and somewhat rare environmental conditions. The disease is currently under excellent control in the U.S., but is of concern because of regulations that influence international markets. In particular, the People's Republic of China, a major importer of U.S. wheat, does not have dwarf bunt and prohibits the import of wheat and other grains from the northwestern U.S. where grain is commonly contaminated with spores of the fungus in at least trace amounts. In addition, the fungus has caused problems with grain shipments from U.S. ports outside the Northwest. To allow better assessment of the risk of introducing dwarf bunt into new areas, research was conducted for three seasons, at three locations that commonly have disease conducive conditions, to determine the relationship between levels of spore inoculum and disease incidence. It was determined that in highly favorable environments, a minimum of 16,000 spores per four-foot row of wheat was required to cause trace amounts of disease, and no disease occurred at two inoculum levels less than this. Ten to 100 times this number of spores were required to cause disease when environmental conditions were less than ideal, and little or no disease resulted in marginal environments even when 16 million spores were applied to a four-foot row. This information will aid assessment of the risk of introducing dwarf bunt into new areas via international grain shipments and could help lessen restrictions on U.S. grain exports.
Technical Abstract: 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, 1,600, 16,000, 160,000, 1,600,000, and 16,000,000 teliospores of Tilletia controversa per row, or seed was inoculated with 0, 200, 2,000, 20,000, 200,000, and 2,000,000 telispores per gram. To indicate 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 16,000 teliospores per 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 1,600,000 rate. In the seed inoculated plots, infection was rare and occurred only at inoculation rates of 200,000 telispores per gram or higher; the highest incidence was 0.4%.