Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2010
Publication Date: 3/10/2011
Citation: Goates, B., Peterson, G.L., Bowden, R.L., Maddux, L.D. 2011. Analysis of induction and establishment of dwarf bunt of wheat under marginal climatic conditions.. Plant Disease. 95:478-484. Interpretive Summary: Dwarf bunt caused by Tilletia contraversa is a fungal disease of winter wheat that, in the U.S., is limited in distribution to regions of Pacific Northwest (PNW) where mild winter temperatures and long periods of continuous snow cover provide climatic conditions required for fungal spore germination and infection of wheat during winter. The Peoples Republic of China was concerned that the importation of U.S. wheat would pose a threat to China’s wheat production and therefore placed a 35-year embargo on wheat from PNW ports. Fifteen other countries also imposed restrictions on the importation of wheat containing spores of T. contraversa. In 1999, the USDA developed a quantitative risk assessment model for the importation of U.S. wheat containing spores of T. contraversa. This model, when applied to China, concluded that there were few regions that meet the environmental requirements of the disease and that the low levels of spores in U.S. wheat were unlikely to support disease establishment. These findings lead to the 1999 Agreement on U.S.-China Agricultural Cooperation that established a tolerance level for spores in U.S. wheat from PNW ports. Research that supports the USDA dwarf bunt risk model conclusions was conducted in a U.S. winter wheat field that is climatically analogous to China’s major winter wheat production area. The results demonstrated that the introduction of large numbers of T. contraversa spores would not lead to the establishment of dwarf bunt. In this study, a dwarf bunt susceptible wheat cultivar was inoculated with different levels of spores. A duplicate study conducted in a disease conducive area of the PNW demonstrated that the highest rate of inoculum used in the experiments was sufficient to cause almost 100% infection. At maturity, wheat spikes in all plots where examined for disease. If disease was found, infected spikes were crushed and the spores were placed back in the same plot. This experiment was repeated for three seasons in three different fields and each field was replanted with a susceptible cultivar for the next 4 to 6 seasons and examined for disease each year. One field had no disease during all of six seasons. In two fields, the disease was induced at trace levels at the three highest inoculation rates. Disease carryover to the second year occurred during one year in one field at the highest inoculation rate, but no disease occurred during the following three seasons. This study indicated the introduction of T. contraversa teliospores into either the central U.S. winter wheat growing area or the analogous North China Plain, would not likely lead to disease loss or the establishment of dwarf bunt.
Technical Abstract: Dwarf bunt caused by Tilletia contraversa is a disease of winter wheat that has a limited geographic distribution due to specific winter climate requirements. The pathogen is listed as a quarantine organism by several countries that may have wheat production areas with inadequate or marginal climate for the disease; in particular the Peoples Republic of China. To evaluate the risk of disease introduction into such areas, field experiments were conducted in the US in an area of Kansas that is a climatic analog to the northern winter wheat areas of China. The soil surface of four replicate 2.8 X 9.75 m plots, planted with a highly susceptible cultivar managed to be in the most susceptible condition, was inoculated with six teliospore concentrations ranging from 0.88 to 88,400 teliospores/cm2. A single initial inoculation was done in each of three nurseries planted during separate seasons followed by examination for disease for 4 to 6 years afterward. Any diseased spikes produced were crushed and returned to the plots where they were produced. One nursery had no disease during all of six seasons. In two nurseries the disease was induced at trace levels at the three highest inoculation rates. Disease carry over to the second year occurred during one year in one nursery in plots at the highest inoculation rate, but no disease occurred the following three seasons. A duplicate nursery planted in a disease conducive area in Utah demonstrated that the highest rate of inoculum used in the experiments was sufficient to cause almost 100% infection. This study demonstrated that in an area with marginal climatic conditions it was possible to induce transient trace levels of dwarf bunt, but the disease was not established even with a highly susceptible cultivar and high levels of inoculum. Our results support the conclusions of the 1999 Agreement on U.S.-China Agricultural Cooperation and the Risk Assessment Model for Importation of United States Milling Wheat Containing T. contraversa.