If left unchecked, weeds will flourish.

如果不加遏制，杂草就会疯长。

《柯林斯英汉双解大词典》

The plant will soon choke ponds and waterways if left unchecked.

如不控制这种植物的生长，池塘和水道很快就要被阻塞。

《牛津词典》

Left unchecked, noncommunicable diseases devour the gains of economic growth.

若置之不理，非传染性疾病将会吞噬经济增长的成果。

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Left unchecked, we have every reason to believe that Qaddafi would commit atrocities against his people.

WHITEHOUSE: President Obama��s Statement on Libya

Within 20 years Medicare outlays, left unchecked, will crowd out all other federal budget items except defense.

FORBES: Magazine Article

But they will do two things that if left unchecked could result in more Democratic victories than otherwise expected.

WSJ: Karl Rove: Obama Gears Up to 'Flip the House' in 2014