Nature of resistance to Verticillium dahliae Kleb. in strains of peppermint (Mentha piperita L.) developed by irradiation Public Deposited


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  • Verticillium dahliae Kleb. causes a wilt disease of peppermint. Attempts to control the disease by soil fumigation, crop rotation, flaming of stubble, and deep plowing have met with only limited success. Development of resistant varieties through conventional breeding procedures is difficult because commercial peppermint is male-sterile. Several strains of commercial peppermint that show field tolerance to Verticillium wilt have been developed by irradiating mint stolons, planting them under wilt conditions in the field, and selecting for resistance. The purposes of this study were (1) to test these irradiated strains of mint for their reaction to Verticillium wilt in Oregon (2) to compare these strains with commercial peppermint for their reaction to root penetration and subsequent infection by V. dahliae (3) to determine sites and mechanisms of resistance that might function in the resistant strains. Field experiments showed that the irradiated peppermint strains had a significantly lower incidence of wilt than did the non-irradiated commercial 'Mitcham'. Disease severity in individual resistant plants was sometimes as great as in susceptible plants. Therefore, disease incidence is more important than disease severity in selecting resistant strains. Dead stems of resistant strains had fewer microsclerotia of the fungus than did susceptible Mitcham. This shows that irradiated strains were less extensively invaded and would return less inoculum to the soil. Oil yield data from field plots showed that yield of the irradiated strains was not depressed in relation to Mitcham. Growth of Verticillium on stem pieces and sap-extract media of the resistant strains and on Mitcham control suggested that nutritional differences were not related to resistance and that inhibitory substances, possibly phenolic compounds, were present. The polyphenoloxidase (PPO) level of healthy, field grown plants fluctuated throughout the growing season and dropped to a low point at the time of flowering. The level of this enzyme did not seem to be associated directly with resistance but may be related to the types of phenolic compounds in the plants. The low level of PPO at the time of flowering suggested that plants with flowers might be more susceptible to wilt than non-flowering cuttings. Inoculation experiments showed, however, that differential resistance was maintained after plants had flowered. Flowering had no influence on wilt susceptibility and selection of a resistant variety could be made without regard to flowering. Resistant, moderately resistant, and susceptible strains of mint were invaded by conidia of Verticillium within 30 minutes after inoculation. Thus, resistance is not related to ability of the fungus to initially invade plants. Experiments using a cotton and mint isolate of V. dahliae showed that the cotton isolate is only weakly virulent to mint. The cotton isolate is, therefore, a different physiological strain of the fungus from the mint isolate. Roots of resistant, moderately resistant, and susceptible mint plants were penetrated nearly equally by V. dahliae, suggesting that resistance is not wholly dependent on resistance to root penetration. Resistance to Verticillium is present in stems as well as roots of mint plants. When the root system was bypassed by direct stem inoculation or by inoculation of cut shoots, resistance was maintained. Cross-protection was demonstrated when mint strains were inoculated with a cotton isolate of Verticillium and challenged one week later by a mint isolate. This suggests that active resistance mechanisms in peppermint are present and can be initiated by an avirulent strain of the fungus.
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