How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

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.

COMMENTS ON THE DISEASE

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.

MANAGEMENT

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 sclerotia can survive more than 20 years in soil, but it does help prevent buildup of the pathogen.

Organically Acceptable Methods
Cultural control and garlic extract treatments.

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 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 sclerotia occur when soil temperatures are between 59 to 64°F; this is also the best time to apply the garlic extract.

Soil fumigation with methyl bromide is effective, but it will not totally eradicate the fungus and it may not be economical. Metam sodium reduces sclerotia levels but its performance in the control of white rot is sporadic, and therefore is less reliable then methyl bromide. Chloropicrin is not effective.

Application of iprodione at planting will offer some protection in a contaminated field.

PUBLICATION

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453
Diseases
R. M. Davis, Plant Pathology, UC Davis
B. J. Aegerter, UC Cooperative Extension, San Joaquin Co.
Acknowledgment for contributions to the diseases section:
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara/San Luis Obispo counties
R. E. Voss, Vegetable Crops, UC Davis

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