Pink Root on Onions

Pink root on onions caused by
Phoma terrestris
(Photo: NMSU-PDC)

Featured Diagnosis: Pink Root on Onions. Onion samples from Southern New Mexico recently submitted to the NMSU - Plant Diagnostic Clinic were infected with a disease known as pink root. Pink root is caused by the soil-borne fungus, Phoma terrestris. This pathogen is common in New Mexico and is problematic worldwide wherever onions are grown. The disease is especially devastating in warmer climates. Although many isolates of this fungus are specific to onion, some have the ability to infect other hosts including tomatoes, soybean, eggplant, pepper, spinach, carrots, small grains, cucurbits, corn and ryegrass.

Pycnidium and spores of Phoma terrestris
(Photo: NMSU-PDC)

The most noticeable symptom of the disease is the reddish-purple discoloration that occurs on infected roots. In the early stages of the disease, the color change in subtle, but the discoloration darkens as the disease progresses eventually leaving the roots stained dark purple. By the end of the season, infected roots have become dry and brittle and often disintegrate. Roots of infected plants become dysfunctional and plants will suffer from nutrient deficiency and drought. When young seedlings are infected, they may die; however, death is not the end result when more mature plants are infected. In this case, plants are stunted, exhibit leaf tip dieback and bulb size is reduced affecting overall yield and marketability.

Early symptoms (left) and late symptoms (right) of
pink root on onions (Photo: NMSU-PDC)