White rot in garlic: Diagnosis, identification, management

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Courtesy photo White rot is a soil-borne illness that affects garlic.

While garlic is easy and fun to grow here in Boundary County, there are some serious soil-borne diseases that can affect your ability to grow this crop. Throughout the county, we have had reports of white rot in garlic this year. White rot affects all Allium species, including onions, shallots and garlic. It is highly contagious and easily spread through the small, black sclerotia the disease produces.

On Monday, Aug. 12, at 3 p.m., Dr. James Woodhall, University of Idaho Plant Pathologist and director of the diagnostic lab at the UI Parma Research & Extension Center, will discuss detection and diagnostic procedures for this disease. He will explain the biology of the fungus and various management options. Call 208-267-3235 for more information.

The initial symptoms of white rot are yellowing and eventual dieback of the leaves. Sometimes white fungal growth is present at the bulb base. As the disease progresses, the plant is easy to pull out of the ground. The sclerotia can be seen on the decaying bulb. The initial symptoms can be confused with Fusarium basal rot, another soil-borne garlic disease, but the presence of sclerotia usually indicates white rot. We can usually confirm the presence of the disease with the aid of a microscope at your local UI Extension office.

White rot is a very serious disease due to the length of time it can survive in soil, its aggressiveness and the fact that it is highly contagious. It can spread in infested soil and bulbs. Just one individual sclerotium for every two pounds of soil is enough to initiate infection. Infested soil attached to garden tools and shoes as you work in your garden is a very effective means of transmission. The disease is favored by cool, wet conditions. Once infected, plants may wither and die, or the garlic bulbs may rot later in storage.

Managing white rot consists of avoidance and sanitation, as there are no cost-effective strategies for the home gardener. Make sure your seed garlic is free of disease. Inspect your plants for any evidence of white rot or small black sclerotia. If you notice yellow, stunted and wilting plants like those in the photo, you should inspect your garlic for potential disease, including white rot, pink rot, soft rot bacterial infection, and onion maggot. If you do have white rot on your plants, you will need to pull them and any neighboring plants with symptoms. Dispose of the diseased plants, taking care to contain all soil and plant material without spreading sclerotia. For example, you could bag the diseased plants in a black plastic trash bag and take them to the landfill.

If you find white rot in your garden, you will need to avoid planting any Allium plants, including garlic, onion, and shallots, in that area. You will need to sanitize any garden tools after working in an affected area to ensure that you don’t spread the disease. If possible, avoid planting any root crops in an area affected with white rot, and move your planting area for any Allium species to another area. In areas of the world where this disease is common, such as the United Kingdom, soil is routinely tested for the presence of white rot before a commercial planting of an Allium crop.

Location: UI Extension Office, 6447 Kootenai Street

Phone: 208-267-3235

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