What is quarantine in agriculture?

A crop or herd of livestock under quarantine means that an agronomist, pathologist, or veterinarian has found positive evidence of some kind of (usually) pathogenic infection or infestation in the crop or herd that requires absolute isolation from any other crop or herd. Absolute isolation means that the appropriate measure must, by law, be taken to prevent any further spread of the disease, pest, or condition.

A few examples of quarantineable diseases are: BSE ("mad cow") in cattle, scrapie in sheep, erysipelas in pigs, white rot in onions, rhizomania in sugar beet, and potato cyst nematode in potato.