Influence of site, soil, and inoculum density on dwarf bunt (Tilletia controversa Kühn) of winter wheat Public Deposited


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  • Field microplot studies conducted over two seasons (1982-1984) evaluated the influence of four physically and chemically diverse soils collected from Pacific Northwest wheat fields on the incidence of dwarf bunt of winter wheat caused by Tilletia controversa Kuhn. These soils were placed at Flora and Pendleton, Oregon to determine the effects of environment (e.g. amount of snow cover) and/or the use of vermiculite to simulate snow cover. In 1982-1983, growing site had a significant (P ≤ 0.001) affect on the amount of dwarf bunt which developed in these test soils after they were mixed with 5 rates of inoculum (0.0 - 20 g. T. controversa teliospores/45 kg of soil). Percent dwarf bunt in these soils ranged from a high of 45% at Flora, with greater than 80 days of snow cover, to 15% at Pendleton which had 2 days of snow cover. In 1983-1984, an additional experiment established at Flora, used test soils placed in 61 cm diameter microplots infested with three concentrations of inoculum (0.0 - 1.5 g T. controversa teliospores/0.5 1 water) sprayed on the soil surfaces. The modified infestation method in addition to more snow cover days (133) than in the 1982- 1983 season resulted in a higher incidence of disease. Percent dwarf bunt reached a high of 89%. An application of vermiculite was used to simulate snow cover. Vermiculite applied at sowing, delayed plant emergence and reduced disease incidence at Flora when compared to the corresponding no vermiculite treatments. In the absence of vermiculite, a snow cover always in creased the incidence of dwarf bunt. Although a persistent snow cover enhanced infection by dwarf bunt, it was not necessary for ensuring teliospore survival and some infection in the near absence of a snow cover. In 1984, dwarf bunt infected plants were observed at Pendleton in the test soils which had been infested with teliospores in 1982. Also teliospores, buried in the test soils at each site and retrieved at periodic intervals for in vitro germination assessment, remained viable over an 18 month period. The number of soils with teliospores which germinated upon retrieval reflected a seasonal cycle with more teliospores germinating in March - April than when retrieved in November. Soil characteristics, either physical or chemical, also influenced the amount of disease which developed. The high clay content (54%) of the Banida, Idaho test soil hindered water infiltration and excess surface water was observed accumulating on the soil surface. In 1984, this soil had less dwarf bunt than in the other three test soils and disease was significantly (P ≤ 0.05) reduced in this soil as compared to the disease which developed in the silt/clay test soil from Logan, Utah. When the test soils were fumigated with methyl bromide, before infesting them with T. controversa teliospores, no significant (P ≤ 0.05) differences in percent dwarf bunt were detected among the four soils. The incidence of dwarf bunt in creased in all fumigated soils as compared to the nonfumigated treatments. In vitro germination tests evaluated the affect of chemical and biological soil properties on T. controversa teliospore germination. Teliospore germination on media made from soil extracts of the four test soils was significantly (P ≤ 0.05) reduced on media with a pH above 7.0 as compared to media with a pH of 5 to 6. An alkaline pH of 8.2 combined with the physical properties of the Banida soil may have contributed to the reduced amount of dwarf bunt observed in this soil. Additional in vitro studies used teliospores placed on test soils which had been steamed with moist air at 60° C/30 min. Germination significantly (P ≤ 0.05) increased on two of the steamed soils as compared to their nonsteamed treatments. Fumigation of the soils with methyl bromide significantly (P ≤ 0.05) increased teliospore germination on three of the four soils. The addition of 1% nonsterile soil to 3 of the 4 fumigated soils reduced but never significantly (P ≤ 0.05) decreased germination when compared to germination on the respective fumigated soil. This lack of a successful transfer of a suppressive biological factor indicated that the soil sterilization process may have resulted in the release of a compound stimulatory to germination or that increased increments of nonsterile soil need to be added before a significant response is detected. This study has shown that although snow cover favors infection by dwarf bunt, certain soil characteristics either physical, chemical, and/or biological also influence the final incidence of dwarf bunt.
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