Graduate Thesis Or Dissertation
 

Inactivation of Verticillium dahliae in peppermint stems by propane gas flaming

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  • The fungus Verticillium dahliae Kleb. causes a wilt disease of peppermint, Mentha pipertita L. Propane flaming of peppermint stubble is widely used to control Verticillium wilt in the Willamette Valley of Oregon, but little is known about the temperature required for effective control of the fungus. The purposes of this study were: (1) to determine the internal stem temperatures required to greatly reduce the population of V. dahliae; (Z) to determine the tractor speeds and propane gas pressure required to obtain these internal stem temperatures; (3) to discover what happens to the surviving fungal population after flaming; and (4) to measure temperatures under the soil surface during propane flaming and determine whether these might affect survival of the peppermint rhizomes. In preliminary laboratory experiments, exposure of stems to a flame until the internal temperature reached 50C killed an average of 83.1% of the Verticillium propagules; percent killed ranged from 58.3 to 99.9 %. Though the upper limit of this range was considered adequate to control Verticillium, the average kill and lower limit of the range were considered inadequate. Average mortality at 60C was 97.6% with a range from 94.3 to 100%. This and higher temperatures should be adequate to control the disease. Field experiments showed that tractor speeds of 2.0 and 2.5 mph at 35 psi gas pressure resulted in an average of 99.5 and 99.2% kill of the fungus, respectively, and adequately eliminated infected plant debris on the ground. The slower speed should be used on cool mornings or days, and the higher speed should be used on warmer days. A multiple regression analysis of data obtained from field tests showed that speed was the best indicator of percentage kill of the fungus. Peak temperature was correlated with both speed and percentage kill of the fungus at the 1% level. The results of assays to determine the fate of the surviving population of Verticillium immediately after flaming showed that little or none of this population remained after one week in the field. Flamed stems were placed in the field and periodically assayed over a 14 week period. Three of 38 stems contained Verticillium, and only one showed an increase. In every case where the fungus survived flaming, a high population (80,000 propagules per gram tissue or more) of Verticillium was present immediately after flaming. Soil temperature recordings showed that the temperature decreased with depth. At 2.0 mph, there was an 11. 8C increase at a depth of 0.1 cm and only a 0.1C change at a depth of 1.0 cm. At 2.5 mph, the temperature changes at these depths were even less. At depths around 1.0 cm, where rhizomes important for the regrowth of the next year's crop exist, the temperature changes were not great enough to injure them.
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