How to Manage Pests

UC Pest Management Guidelines

Onion and Garlic

White Rot

Pathogen: Sclerotium cepivorum

(Reviewed 1/07, updated 6/09, pesticides updated 6/16)

In this Guideline:


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. A semi-watery decay of the bulb scales results. Roots also rot, and the plant can be easily pulled from the ground. Associated with the rot is a fluffy white growth, the fungal mycelium, which develops around the base of the bulb. As the disease progresses, the mycelium becomes more compacted, less conspicuous, with numerous small spherical black bodies (sclerotia) forming on this mycelial mat. These sclerotia, the resting bodies of the pathogen, 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.


The pathogen persists as small, dormant structures, called sclerotia, in soil. Sclerotia can survive for over 20 years, even in the absence of a host plant. Disease severity depends on sclerotia levels 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 disease loss, and 10 to 20 sclerotia per kilogram result in infection of essentially all plants.

Sclerotia can be spread throughout a field or from field to field by flood water, equipment, or on plant material, including wind blown scales. Sclerotia remain dormant in the absence of onion or other Allium crops. Their germination is stimulated by Allium root extracts and exudates that extend into the soil about 0.5 inch from the root.

Disease development is favored by cool, moist soil conditions. The soil temperature range for infection is 50° to 75°F, with optimum being 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.


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

Cultural Control

Do not move cull bulbs, litter, and soil from infested to noninfested fields. Always clean equipment before moving from one field to another. Onion seed is not likely to carry sclerotia, but transplants and sets can. On garlic, the disease is commonly introduced into the field on seed cloves. The most effective way to avoid introducing the disease this way is to plant only clean stock from known origins that have no history of white rot. However, the fungus is vulnerable at temperatures above 115°F, thus dipping seed garlic in hot water will greatly reduce the amount of pathogen and is a good preventative measure, although it may not completely eradicate the fungus. Also, temperatures above 120°F may kill the garlic, so careful temperature control is essential.

If disease is observed, cessation of irrigation will minimize damage but not stop the disease. 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 sclerotiacan survive more than 20 years in soil, but it does help prevent buildup of the pathogen.

Organically Acceptable Methods

Cultural control.

Management Decisions

The white rot fungus produces no functional spores. Instead, it propagates only by the production of round, poppyseed-sized sclerotia produced on the roots of decayed host plants. The sclerotia germinate only in response to root exudation peculiar to the genus Allium. The specific reaction between sclerotia and exudates suggests a possible use of sclerotial germination stimulants for controlling white rot disease. If products containing the root exudates are applied to the ground in the absence of an Allium crop, the sclerotia may be "tricked" into germinating. In the absence of a host, the mycelium from germinating sclerotia persist for a few days to several weeks depending on the soil temperature, then die after exhausting nutrient reserves. Natural Allium products, or certain artificial products of petroleum cracking (e.g., diallyl disulfide) applied to the soil also stimulate sclerotia to germinate. In the absence of an Allium crop, these compounds result in high mortality of the fungus, which allows a subsequent successful onion or garlic crop. To use garlic extract, apply it at least one year after all Allium crops, including volunteer Alliums, have been removed from the field. The optimum conditions for germination of sclerotiaoccur when soil temperatures are between 59° to 64°F; this is also the best time to apply the garlic extract.

Common name Amount per acre REI‡ PHI‡
(Example trade name)   (hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees (PDF), and environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
  (Orius 3.6F) 20.5 fl oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: Demethylation Inhibitor (3)
  COMMENTS: Apply in 4- to 6-inch band over/into each furrow.
  (Fontelis) See label 12 3
  MODE-OF-ACTION GROUP NUMBER1: Succinate dyhydrogenase inhibitor (7)
  COMMENTS: Apply in 4- to 6-inch band over/into each furrow. Use allowed under a Supplemental Label.
  (Cannonball WG) 7 oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: Phenylpyrroles (12)
  COMMENTS: Apply in 4- to 6-inch band over/into each furrow. Use allowed under a Supplemental Label.
  (Endura) 6.8 oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: Succinate dyhydrogenase inhibitor (7)
  COMMENTS: Apply in 4- to 6-inch band over/into each furrow.
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 actions. 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, 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.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453


R. M. Davis, Plant Pathology, UC Davis
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
T. Turini, UC Cooperative Extension, Fresno County
A. Ferry-Abee, UC Cooperative Extension, Tulare and Kings counties

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

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