Agriculture: Onion and Garlic Pest Management Guidelines

Pink Root

  • Phoma terrestris
  • Symptoms and Signs

    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.

    Comments on the Disease

    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.

    Management

    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.

    Common name Amount per acre REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used
     
    A. METAM SODIUM*§
      (Vapam HL) Label rates See label NA
      COMMENTS: Fumigants such as metam sodium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone.
     
    B. CHLOROPICRIN*§
      (Chloropicrin 100 Fumigant) Label rates See label NA
      COMMENTS: Fumigants such as chloropicrin are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 7, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment until the harvest may take place. In some cases, the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest.
    * Permit required from county agricultural commissioner for purchase or use.
    § Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Use Materials Requirements, which may be lower than maximum label rates.
    NA Not applicable.
    Text Updated: 02/19
    Treatment Table Updated: 02/19