How to Manage Pests
UC Pest Management Guidelines
Onion and Garlic
Pathogen: Phoma terrestris
(Reviewed 1/07, updated 6/08, pesticides updated 6/16)
In this Guideline:
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, turn black, and die. The pinkish red discoloration may extend up into the scales of the bulb. New roots also may become infected. If infection continues, plants become stunted. The disease seldom results in plant death. Infection is confined to roots and outer scales of the bulb. Many weak Fusarium species can also cause pink roots, particularly on old roots; diagnosis of pink root can be accurately accomplished only on actively growing plants.
COMMENTS ON THE DISEASE
Pink root is primarily a problem on onion; garlic is infected by the pink root organism, but the disease rarely occurs at an economically important level. The fungus is a common soil inhabitant that penetrates onion roots directly; wounds are not necessary for infection, but weak plants are more susceptible. The pathogen can persist in soil indefinitely; the longer onions are grown in the field, the more destructive the disease becomes. The fungus can be spread in water and on dirty equipment. Optimum temperatures for disease development are 75° to 85°F.
Prevention and control include avoiding repeated cropping of onion on the same soil, use of resistant varieties, good soil tilth and fertility, control of insects and other diseases to maintain healthy plants, and preplant soil fumigation. Because so many crops are hosts of the pathogen, rotation is not an effective control, but long-term rotations out of onion for 5 years or more are recommended because each crop of onions increases disease incidence. Planting onions after cereals can also be hazardous because the inoculum potential generally becomes greater with cereals than with onions.
Disease-resistant varieties are available, but many popular varieties do not have this characteristic; furthermore, many resistant varieties are resistant in some locations but not in others, depending on which strains of the fungus are present. Fumigation with metam sodium or chloropicrin can be effective against some strains of the fungus but is not effective against many of the more virulent strains. It is also not always economical unless a high value seed crop is being grown. Solarization has proven effective in areas like the San Joaquin Valley where onions are planted in fall after a summer fallow period.
UC IPM Pest Management Guidelines: Onion and Garlic
R. M. Davis, Plant Pathology, UC Davis
Acknowledgment for contributions to Diseases:F. F. Laemmlen, UC Cooperative Extension, Santa Barbara and San Luis Obispo counties
R. E. Voss, Vegetable Crops, UC Davis