In my next series of posts, I will discuss various diseases that infect garlic, and different ways to treat them. In this post, I will consider white rot, perhaps the most severe allium disease. White rot is an insidious fungus that can render soil unusable for garlic for decades. Also known as Sclerotica cepivorum, it occurs in many parts of the world, affecting alliums such as onions and garlic. This disease is a particular nightmare for organic growers, since the fungus is nearly impossible to remove effectively once a field has been infected.
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White rot is generally introduced into a field through contaminated plant material or soil, hence why it is so important both to purchase seed stock confirmed to be disease-free, and to quarantine new seed away from known healthy stock. The same rules apply when you are buying soil or compost. Once even a small area of a field is infected, white rot is easily spread to healthy soil and plants from physical contact with contaminated ones. It can also be spread by machinery and flood water that were previously in contact with diseased material.
The spread of white rot is accomplished not by spores, but through the sclerotic — hard, black beads that live both in the soil and on infected plants. White rot sclerotica persist in the soil for decades, surviving through cold winter temperatures. They are at their most active in cooler temperatures, ideally below 20-24 degrees Celsius (68-76 degrees Fahrenheit). Higher temperatures will inhibit growth of the fungus, and heat over 45 degrees Celsius (113 degrees Fahrenheit) will kill it. White rot sclerotic will remain dormant in the soil until they come within less than a centimeter of an allium, wherein the exudate from the plant will encourage the fungus to germinate.
Symptoms of White Rot
White rot can be difficult to differentiate from other diseases above ground. It usually affects patches of plants, rather than individuals. Growers may first notice stunted plant growth, followed by the early yellowing and death first of the outer leaves, then the rest of the leaves and the central stem. If allowed to progress, there will also be an obvious rotting of the stem above the bulb.
The disease is much more apparent on the bulb itself. White rot fungus manifests as a fluffy white growth on the roots and root plate, eventually spreading upwards over the outer skin of the bulb. Infected plants must be immediately removed, along with the surrounding soil, and burned.
Photo by Andrea Cross
White Rot Treatment
White rot is feared by many garlic growers because it is so difficult to control. Although there are a number of methods to treat white rot once it has occurred, their effectiveness is not reliable. Since any remaining sclerotica can remain dormant in soil for decades, the fungus can render a field uninhabitable for alliums, effectively ending the growth of garlic in that field.
This being the case, prevention is the best way to avoid a white rot infection. Besides carefully monitoring seed and soil introduced to a field, crop rotation of three to four years is essential. It is also advantageous to remove harvest waste from the field and disposing of it by either burning or segregated composting.
Photo by Andrea Cross
Some growers prevent the fungus being introduced by decontaminating their seed stock. Alcohol, bleach and hot water are all used to bathe garlic seed prior to planting. Although this method is usually effective, if the treatment is too prolonged or performed at too high a temperature, the garlic can die.
Once infected, there are a number of organic options for minimizing the he spread of the fungus through a field, although these methods are not always successful. For infections that occur in only small patches, one method is to simply remove the affected plants and the surrounding soil. The diseased material is then disposed of, usually through burning.
If the infection is more widespread, ceasing irrigation of the field during the growing season will dry the fungus out. This method will only minimize the infection, and the remaining sclerotica make the chance of reinfection likely. Conversely, the affected field can also be flooded with water for a number of months, although the practicality and effectiveness of this method are often negligible.
Solarizing is another option for organic growers. Transparent polyethylene is laid over the infected soil during the hottest months, increasing the temperature and killing the white rot. This method is problematic for those growers with larger fields, however, as the soil must be turned over repeatedly down through at least two feet, to ensure as much of the fungus as possible is exposed to the heat. The process is time-consuming and tedious, and often not adequately successful.
Since the fungus is stimulated by allium exudate, a last organic method is to seed a field with allium (preferably garlic) juice solution or powder. The presence of allium material will cause the sclerotica to germinate, but because there is no garlic actually present, the fungus will starve and die. This process can be repeated several times if necessary. Like the previous methods used, this route is time-consuming, and renders the field unusable for at least a season. Even then, there is no guarantee the fungus has been completely eliminated.
Non-organic growers have only a slightly easier time destroying the disease. There are currently three fungicides used to treat white rot: tebuconazole, fludioxonil and boscalid. These chemicals can be tilled into the soil in which the garlic is going to be planted, and also applied into the furrows at the time of planting. Unfortunately, even these fungicides are not 100 percent effective, making prevention the best defense against a fungus as intrepid as white rot.
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