1. A composition for controlling plant diseases, containing 4-oxo-4 - [(2-phenylethyl) amino] butyric acid and an external quinone inhibitor as active ingredients. 2. The composition according to claim 1, in which the quinone external inhibitor is at least one quinone external inhibitor selected from the group consisting of kresoxime methyl, azoxystrobin, trifloxystrobin, fluoxastrobin, picoxystrobin, pyraclostrobin, dimoxystrobin, pyribencarbine, orinominostrobin, metinostrobin, and a compound of formula (1): wherein X represents a methyl group, a difluoromethyl group or an ethyl group, X represents a methoxy group or a methyl amino group, and X represents a phenyl group, 2-methylphenyl group or 2,5-dimethylphenyl group. 3. The composition according to claim 1, in which the mass ratio of 4-oxo-4 - [(2-phenylethyl) amino] butyric acid and a quinone external inhibitor is in the range from 0.005: 1 to 1000: 1.4. Seed treatment agent containing 4-oxo-4 - [(2-phenylethyl) amino] butyric acid and an external quinone inhibitor as active ingredients. 5. Plant seed treated with effective amounts of 4-oxo-4 - [(2-phenylethyl) amino] butyric acid and an external quinone inhibitor. 6. A method for controlling plant diseases, which comprises applying effective amounts of 4-oxo-4 - [(2-phenylethyl) amino] butyric acid and a quinone external inhibitor to a plant or soil for a growing plant. 7. The combined use of 4-oxo-4 - [(2-phenylethyl) amino] butyric acid and a quinone external inhibitor for controlling plant diseases.1. Композиция для борьбы с болезнями растений, содержащая в качестве активных ингредиентов 4-оксо-4-[(2-фенилэтил)амино]масляную кислоту и хиноновый внешний ингибитор.2. Композиция по п.1, в которой хиноновый внешний ингибитор представляет собой по меньшей мере один хиноновый внешний ингибитор, выбранный из группы, состоящей из крезоксим-метила, азоксистробина, трифлоксистробина, флуоксастробина, пикоксистробина, пираклостробина, димоксистробина, пирибенкарба, м
