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. This year, a garlic sample from the GROW Community Garden on the grounds of the Trinity Lutheran Church tested positive for white rot (Stromatinia cepivora). White rot affects all Allium species, including onions, shallots and garlic. It is also highly contagious and easily spread through the small, black sclerotia the disease produces. Sclerotia (plural = sclerotia, individual = sclerotium) are tough survival structures of the white rot fungus that can potentially survive in the soil for decades. Sclerotia are very small, about a quarter of the size of the ball point of a pen or the size of a poppy seed. An infected bulb can produce hundreds of sclerotia meaning over just a few seasons the soil can become severely infested with the pathogen.
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. U of I Extension offices can usually confirm the presence of the disease with the aid of a microscope.
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 dont 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.
By Kathleen Painter, UI Extension Educator, Boundary County, and James Woodhall, UI Extension Plant Pathologist, Twin Falls