Agriculture: Onion and Garlic Pest Management Guidelines

White Rot

Symptoms and Signs

Leaves of plants infected with the white rot pathogen show yellowing, leaf dieback, and wilting. Leaf decay begins at the base, with older leaves being the first to collapse. This results in a semi-watery decay of the bulb scales.

Roots also rot, and as a result, the plant can be pulled from the ground easily. At early stages of disease development, a fluffy white growth (the fungal mycelium) is associated with the rot, which develops around the base of the bulb. As the disease progresses, the mycelium becomes more compacted and less conspicuous, with numerous small, spherical black bodies (sclerotia) forming on this mycelial mat. These sclerotia are the resting bodies of the pathogen and are approximately the size of a pin head or poppy seed.

Plants can become infected at any stage of growth, but in California, symptoms usually appear from mid-season to harvest.

Comments on the Disease

The pathogen persists as small, dormant structures (sclerotia) in the soil. Sclerotia remain dormant in the absence of a suitable host (garlic, onion, or other Allium crops), and can survive in the soil for over 20 years.

Sclerotia can spread throughout a field, or from field to field, via flood water, equipment, or plant material, such as wind-blown scales. Allium root exudates, which may seep ½ inch into the soil from the root, stimulates the germination of the sclerotia. Exudates from non-Allium plants do not stimulate or otherwise affect the germination of white rot sclerotia.

Disease severity depends on soil temperature and the amount of sclerotia in the soil at planting. As few as one sclerotium per 10 kilograms of soil can initiate disease. Only one sclerotium per kilogram of soil can cause measurable yield loss, and 10 to 20 sclerotia per kilogram cause essentially all plants to become infected.

Disease development is favored by cool, moist soil conditions. The soil temperature range for infection is 50° to 75°F, with an optimum of 60° to 65°F. At soil temperatures above 78°F, the disease is markedly inhibited. Soil moisture conditions that are favorable for onion and garlic growth are also ideal for white rot development.

Management

The most effective controls for white rot are avoidance and sanitation. Once a field is infested, fungicide applications are necessary to produce onion or garlic crops.

Because onion and garlic exudates stimulate the germination of white rot sclerotia, the use of such products to germinate and kill sclerotia are currently being investigated. At this time, however, this practice remains a challenge because most natural products (such as garlic juice) do not contain enough active ingredient to be effective and vary widely in the amount of the active ingredient they contain, and synthetic diallyl disulfide (or DADS, the main exudate that germinates sclerotia) products are not readily available on the market. Efforts are underway to increase the efficacy of natural onion and garlic products, but at this time they are not a reliable alternative.

Cultural Control

The most effective way to avoid introducing the white rot pathogen is to plant only clean stock from known origins that have no history of white rot. Onion seed is not likely to carry sclerotia since the pathogen is not seedborne in onion, but infected transplants and sets can carry sclerotia. On garlic, the disease is commonly introduced into the field on seed cloves. Therefore, do not move cull bulbs, litter, and soil from infested to noninfested fields. Always clean the soil off of equipment before moving from one field to another.

In addition, follow a long-term rotation schedule, and do not follow Allium crops with other Allium crops. Rotation alone will not control white rot because sclerotia can survive in soil for more than two decades. However, rotation does help prevent an increase in the soilborne inoculum of the pathogen.

Because the fungus is vulnerable at temperatures above 115°F, dipping seed garlic in hot water is a good preventive measure that will greatly reduce the amount of pathogen, although it may not completely eradicate the fungus. Careful temperature control is essential when using this method, since temperatures above 120°F may kill the garlic.

If the disease is observed, ceasing irrigation will minimize damage but will not stop the spread of the disease.

Organically Acceptable Methods

Use cultural control in an organically certified crop.

Chemical Control

If white rot has been a recent problem in the field, apply a fungicide in a 6-inch band over the planting trench immediately before planting. This will provide effective control of white rot.

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. TEBUCONAZOLE
(Orius 3.6F, Tebu-crop 3.6F) Bulb onion, garlic, shallot:
20.5 fl oz
12 7
Green onion and leek:
4–6 fl oz
  MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Must have two to four hours drying time before the product is resistant to weathering. In green onions and leeks, tebuconazole provides white rot suppression.
 
B. PENTHIOPYRAD
  (Fontelis) Label rates 12 3
  MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7)
  ...OR...
  BOSCALID
  (Endura) 6.8 oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7)
  ...OR...
  PYRACLOSTROBIN/FLUXAPYROXAD
  (Merivon Xemium) 5.5–11 fl oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone-outside inhibitor (11)/succinate dehydrogenase inhibitor (7)
  COMMENTS: Do not make more than two applications without rotating to a fungicide with a different mode-of-action group number.
 
C. FLUDIOXONIL
  (Cannonball WG) 7 oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylpyrroles (12)
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 to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
# Acceptable for use on organically grown produce.
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.
Text Updated: 02/19
Treatment Table Updated: 02/19