甘草抽出物のキュウリべと病および炭疽病に対する発病抑制機作に関する研究

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  • Studies on The Mechanism of Cucumber Downy Mildew and Anthracnose Disease Suppression by Licorice Extract
  • カンゾウ チュウシュツブツ ノ キュウリベ ト ヤマイ オヨビ タンソビョウ ニ タイスル ハツビョウ ヨクセイ キサ ニ カンスル ケンキュウ

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Licorice is widely used as a raw material of pharmaceutical, cosmetic, and food additives. It is known that the constituents of licorice have both antioxidant and antimicrobial activities. In our previous study, we reported the disease control effect of some foliar fungal diseases of vegetables through the application of the purified licorice extract, previously called fravolicorice. In this study, we aimed to elucidate the control mechanism of the licorice extract against cucumber downy mildew and anthracnose disease. The extract, commercially named MZ-1, is a yellowish powder manufactured by including a large amount of flavonoids during the extraction process of glycyrrhizic acid from the licorice root. 1. MZ-1 was fractionated using column chromatography (stepwise elution with ethanol [EtOH]) into 5 fractions. The 99% EtOH-soluble fractions showed high disease suppressive effects against cucumber downy mildew and anthracnose, as assessed using a potted cucumber plant. Among the major flavonoids included in the 99% EtOH-soluble fraction, isoliquiritigenin and licochalcone A mediated the high disease suppression effect against cucumber downy mildew. Conversely, these flavonoids showed no inhibitory effect against anthracnose at the same test concentrations, but strongly inhibited the fungal spore germination. 2. Glycyrrhetinic acid included in the above-mentioned fraction showed high control effect against downy mildew and anthracnose and also strongly inhibited fungal spore germination in anthracnose. However, the control effect of glycyrrhizic acid, which is a glycoside of glycyrrhetinic acid, was low. 3. MZ-1 inhibited zoospore release from the conidia of downy mildew, stopped zoospore movements, and caused the collapse of zoospores. However, MZ-1 showed no disease suppressive effect when used after 6 h since the conidia of the downy fungus was inoculated into the cucumber plant. 4. The 50% EtOH-soluble extract of herbal licorice showed suppressive effects against downy mildew and anthracnose. MZ-1 contained a high concentration of licochalcone A, isoliquiritigenin and glycyrrhetinic acid compared with the above extracts. 5. Up to 3 days after the spraying of MZ-1, no change in the level of flavonoids was observed in the cucumber leaves. Moreover, the flavonoids were not detected in the leaves above the treated leaves 2 days after the treatment. 6. An ethanol mixture of isoliquiritigenin and glycyrrhetinic acid diluted with water was physically stable, showing no coagulation and precipitation. Similarly, an ethanol mixture of glycyrrhizic acid and glycyrrhetinic acid was physically stable after dilution with water. 7. The preventive value of MZ-1 against downy mildew was 86 when cucumber plants in a field were sprayed using a 100-fold dilution of MZ-1. A spreading agent slightly inhibited the effect of MZ-1. 8. The results were obtained using MZ-1 extracted from Glycyrrhiza inflata. When the raw material was changed from G. inflata to G. glabra, the extracted made by the same process as MZ-1 did not include licochalcone A. However, such extracts include glycyrrhetinic acid and other flavonoids such as isoliquiritigenin. Therefore, the control effect would not be less.

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