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One bad apple spoils the whole barrel.
Literal
Rotten apple [subject-が] one exist [then-と] barrel whole [subject-が] ruined become.
A barrel-metaphor version of the proverb about one rotten apple ruining the whole batch. ~と here is the conditional 'if/when' — 'if there is one rotten apple, the whole barrel goes bad.' たる (barrel/cask) is an older word, evocative of European wooden casks.