White rot major garlic issue

Growers, researchers scramble to replace methyl bromide Probably the most serious issue the California garlic industry has to contend with today, other than competition from off-shore sources, is white rot.

Tough as it is, some practices and products are being developed to better manage this soilborne disease, also known as Sclerotium cepivorum.

The persistent fungus is one reason, along with the high cost of land, the garlic industry moved from the Gilroy area to western Fresno County. Even though the disease is not widespread on the west side, about 6,000 acres are identified as having some degree of infection.

Garlic and onions, in the ground for 7 to 13 months to allow more efficient use of processing facilities, are necessarily exposed to more disease pressure in general.

Finding new uninfected land might seem like a solution, but that's not the answer. Although soils to the northern San Joaquin Valley are conducive to growing garlic and onions, they also get more rainfall, and that makes it much more difficult to schedule planting and harvesting and to manage disease.

The location of processing plants roughly in a belt from Gilroy to Hanford relatively close to production fields makes finding new growing areas more difficult. There might be ideal growing areas outside California, but the costs of transporting the crop to processors would be prohibitive. All in all, the added risk or expense doesn't give much justification for looking for new growing districts.

Processors contract with west side growers to grow the crop, and individual processors plant and harvest the crop with their own equipment.

The growers like garlic and onions because they are great rotational, cash crops. In short they help with the bottom line and fit well into the management program of larger operations in that part of the San Joaquin Valley.

White rot infection can range from no more than a few plants to an area the size of a residential garage in a field. The larger infection sites can be traced back to contamination by seed, equipment, or soil movement.

That drew attention early on to the need for better field sanitation and closer monitoring of planting and harvesting equipment, bins, and seed.

It is also found in other production areas around the world, Tasmania, New Zealand, and China. China and many other countries have been exporting powdered garlic and fresh garlic to the U.S., and processed garlic is not suspected as being a source of infection.

Our industry goes to great lengths to use only seed that is certified clean of the disease. State inspectors walk seed fields in Nevada, Northern California, Oregon, and Washington, and if a single plant is found to be infected with white rot, the entire field is prohibited from use for seed.

Production fields are also monitored but not with as much intensity as seed fields, and ground having a white-rot history is identified within the industry. Once a field is identified as infected, alliums are not grown on it again. The risk is simply too great.

The disease frequently occurs along roads, distributed by contaminated equipment, and it is readily spread across fields by land planes and any earth moving equipment.

Infection sites, or strikes, are visible by withered plants.

Dormant indefinitely The fungus remains dormant in the soil indefinitely, and infection does not occur until the roots of onions or garlic reach it. Allium species actually produce a chemical signal that stimulates the disease to germinate from overwintering sclerotia. There is no sign of infection until it reaches a bulb.

White rot seems to avoid soil temperatures greater than about 70 degrees, which may explain why we don't see it in alliums in the Imperial Valley and Arizona.

So what can growers do to manage white rot? Likely a solution will take a two-pronged attack by selecting a couple of cultural or chemical alternatives.

Fumigation with methyl bromide is done now in seed production fields, but for field crops its cost is prohibitive and the material is being phased out of use.

Rotations of alliums at four- or five-year intervals, even where no white rot is suspected, is common to avoid other diseases. I don't know of anything along biological control lines.

One thing that can be done the year after a garlic or onion crop is for the grower to make certain any volunteers are removed. These volunteers can trigger the fungus to germinate and perpetuate itself.

I don't think we can avoid using some kind of chemical control to try to mimic the aldehydes the allium plants give off to stimulate white rot to germinate. A petroleum-based product that will do that is registered in Australia and there's some interest in getting it registered here. It is shanked in 12 to 18 inches deep like a fumigant.

Then there's garlic powder that can be broadcast across the field and deep-disked in. It is listed by the California Department of Pesticide Regulation among products having pesticidal properties - like cedar blocks or cloves used for repel moths from clothing - but known not to be harmful to humans and not requiring labeling.

It may be difficult for most growers to find equipment to place garlic powder deep enough in the soil. In the original trials, it would only give control while roots were at a shallow depth and not when they penetrated lower.

The main trouble with garlic powder is, unlike the synthetic product's known and consistent proportion of the attractant, the important ingredient may vary from one batch to the next.

This might work as part of a system of practices. I doubt we can use the synthetic material or garlic powder alone. We will probably use it after volunteers are removed and rotation crops are grown. It would have to be used more than one season.

Ideally, after four or five years, you could then plant garlic or onions and use a commercial fungicide as a seed treatment or applied in-furrow. Some new compounds look quite promising for this purpose.

Flooding will kill off white rot. One possibility being considered is for land near Tulelake to be flooded as wetlands for a period and then planted to a crop of onions.

Some experimental work is being done with solarization, which may also kill the fungus.

Another possibility might be using drip irrigation to deliver a certain amount of toxic material to protect the garlic.

It will be a long, expensive process to find a system to manage white rot. We may never find one, since white rot has survived for decades.

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