Garlic White Rot

Garlic

Garlic is a valuable crop for many farm direct markets on Vancouver Island.  Unfortunately there is a disease, Allium White Root Rot, that can decimate production.   This fungus destroys the root structure of the garlic ( and onions and leeks as well ) and covers the bulb with a sooty grey / black mold. This fungus has come up from California / Oregon / Washington and appeared in the Saanich area about 7 or 8 years ago and in the  Cowichan Valley shortly thereafter.  Please let us know if you have experienced it in your neighbourhood.

Information from UC Davis: White Rot  Pathogen: Sclerotium cepivorum

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. 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.