Pink Root

The most striking symptom of pink root is, as the name indicates, pink roots. Infected roots first turn light pink, then darken through red and purple, shrivel, and later turn black and die. The pinkish-red discoloration may extend up into the scales of the bulb. New roots also may become infected. The vascular tissue in the center of the root (stele) can turn very dark red to purple.

Plants become stunted if infection continues, but the disease seldom results in plant death. Infection is confined to roots and outer scales of the bulb.

Some Fusarium species can also cause pink roots, particularly on dead or old roots. Field diagnosis of pink root can only be accurately accomplished by observing pink roots on actively growing plants. The dark-red-to-purple stele combined with a collapsed outermost layer of root tissue distinguishes pink root from root rot caused by Fusarium species and other fungi.

Pink root is primarily a problem on onion. It can infect garlic, but rarely causes economically significant damage.

The fungus is a common soil inhabitant that penetrates onion roots directly. It can persist in soil indefinitely and may spread via water or dirty equipment. Optimal temperatures for disease development are 75° to 85°F.

Wounds are not necessary for infection, and weak plants are more susceptible. The more years onions are grown in the field, the more destructive the disease becomes.

To prevent and control the disease:

• Avoid repeated cropping of onions on the same soil. Rotating to non-Allium crops for 3 to 6 years can reduce the incidence of pink root.
• Use resistant varieties.
• Maintain good soil tilth and fertility.
• Control insects and other diseases to maintain healthy plants.
• Fumigate the soil before planting.

Because some other plant species are hosts of this pathogen, rotation is only effective if the appropriate crops are grown in rotation with onion. Long-term rotations out of onion for 3 to 6 years are recommended, because the incidence of the disease increases with each crop of onions. Planting onions after cereals or sweet corn can also increase the risk of this disease because the inoculum potential generally becomes greater with these crops than with onions.

Disease-resistant varieties are available, but many popular varieties do not have this characteristic. Furthermore, depending on which strains of the fungus are present, many resistant varieties are resistant in some locations but not in others.

Solarization has proven effective in areas like the San Joaquin Valley, where onions are planted in fall after a summer fallow period.

Fumigation with metam sodium or chloropicrin can be effective against some strains of the fungus, but its effects can vary. Fumigation is also not always economical unless a high value seed crop is being grown.