Onions, chives, and garlic plants with pink roots are not happy.
This disease rarely causes significant problems in garlic, but it can shrink your onion and chive harvest by quite a bit.
The pink root pathogen
Pink root is caused by a fungi called Phoma terrestris. Phoma terrestris is nearly always present in the soil and it pretty much lasts forever. Normally, it causes no serious problems. If your onion plants are weakened by drought, insufficient or excessive fertilizer, water stress, insect feeding, compacted soil, or any number of other less than ideal circumstances, your onions may become susceptible. This pathogen thrives in temperatures between 75° and 85°F and can be moved around the garden by splashing rain or water, and on tools.
Symptoms of pink root
Aside from the obviously pink roots, plants infected with this fungal disease also exhibit roots that darken to red, purple, and, eventually, black. These roots shrivel up and die. These discolorations may move up into the bulb. This infection leads to stunting, but it rarely kills the plant. This disease looks a lot like fusarium wilt.
Preventing and controlling pink root
Keeping plants healthy and employing crop rotation are the two best ways to avoid pink root from causing too many problems. A note on crop rotation: do not follow a cereal crop with onions, as it creates conditions that promote this particular pathogen. Severe infestations can be eliminated with soil solarization, but that’s a pretty drastic measure for the home gardener.
If you see pink, purple, or black shriveled roots on your onions, try growing them in a different area, in fresh soil, and be sure to feed, weed, and water them properly, and protect them from insects, to ensure that they stay healthy enough to protect themselves.
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