Pink Root
Setophoma terrestris
A soilborne fungal disease of onions and other alliums that turns the roots pink to red and then kills them, so plants are starved, stunted, and yield small bulbs. The fungus persists in the soil for years and is most damaging where alliums are grown on the same ground repeatedly.
🔎 How to spot it
The diagnostic sign is the roots: infected roots turn light pink, then deepen to red and purplish as they shrivel, dry up, and die, and the plant keeps replacing them with new roots that are infected in turn. Above ground the plants look stunted, the leaves may yellow, die back at the tips, and wilt as if drought-stressed, and bulbs stay small. Pulling a stunted plant to find pink, rotted roots confirms it.
🥀 Damage it causes
By killing the roots over and over, the fungus cripples water and nutrient uptake, so plants are stunted, leaves die back, and bulbs end up undersized and poor in quality with reduced storage life. The disease rarely kills plants outright but steadily robs yield, and losses are heaviest on continuously cropped or infested ground. Young, vigorous plants tolerate it better than stressed ones.
🔬 What causes it
Pink root is caused by the soil fungus Setophoma terrestris, which infects allium roots and survives in soil and on debris for years. It is favored by warm soils and builds up where onions, garlic, and their relatives are grown repeatedly on the same land, and it hits hardest when plants are already stressed by other pests, poor fertility, or drought, which weaken their ability to outgrow the root loss.
🛡️ Prevent it
Rotate alliums onto fresh ground on a long cycle of several years to keep soil populations down, and avoid planting into fields with a known pink-root history. Grow tolerant or partially resistant varieties, such as some Sweet Spanish types, where the disease is established. Keep plants vigorous with adequate water and fertility so they outgrow root infection, and control other root pests and diseases that compound the stress.
🧯 If it is already here
There is no practical cure once roots are infected, so management leans on rotation, resistant varieties, and keeping plants vigorous to push growth ahead of the root loss. Remove and do not compost heavily infected plant debris, and plant clean transplants and sets. Soil fungicides give only limited help in a home garden, where rotation and tolerant varieties do most of the work.
💡 Good to know
Pink root is named for its unmistakable sign, so finding pink-to-red shriveled roots on a stunted onion is the quickest diagnosis. Because the fungus lingers in soil for years and worsens with continuous cropping, a long allium rotation is the single most important defense. Anything that keeps onions growing strongly helps them tolerate the disease.
🌱 Plants it affects
18 plants in the library can be affected by this problem
For educational and informational purposes only. Disease management advice is general guidance drawn from university cooperative extension sources; always identify a problem positively and read and follow the label on any product before use, especially around food crops, children, and pets.