Garlic Diseases: White Rot


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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Garlic Growth

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.

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