Blight
Blight is a disease that infects domesticated plants with a growing period shorter than 15 days.
Blighted crops will not grow or yield anything when harvested, and can spread the blight to other nearby plants while losing health points and dying.
Progression
Blight starts on one species of plant, at 10% severity, affecting 20% of your plants[Fact Check] (1 in 5). It will progress slowly once plants are infected, and once it reaches 28% it can spread to nearby plants within a 4-tile radius.
Unaffected Plants
Trees, wild, decorative plants, and devilstrand mushrooms are immune to blight.
Removal
As of version 1.4, blighted plants can be removed en masse by selecting a blighted plant and clicking the "Cut blighted plants" button. Colonists with the Plant Cut work enabled will then cut the blighted plants. Consider temporarily setting the Plant Cut priority to 1 in the Work tab if colonists do not respond immediately.
Prior to version 1.4, the affected plants needed to be manually selected for cutting (using the right-click plant cut command, or in the Orders tab of the Architect menu), or burned with Molotov cocktails or Incendiary launchers, though this risks igniting healthy plants, colony structures, or colonists.
Mitigation strategy
To mitigate the effects of blight, it is recommended to leave 4-square wide[Fact Check] strips between crops, left empty or planted with species not affected by blight.