Rhizoctonia: Avoid And Control Beets

The famous beet rot is just one species of Rhizoctonia. We show how to recognize the Rhizoctonia fungus and what to do about it. Rhizoctonia not only affects our beets but also damages many other plants. It is a dreaded emergence disease, which is also often called the root killer.


The fungus Rhizoctonia is on everyone’s lips when it comes to root and emergent diseases. We explain below what the fungus is and how you can fight and avoid it.

Rhizoctonia: characteristics and distribution

The correct name of this mushroom is Thanatephorus cucumeris – only its asexual or anamorphic form is called Rhizoctonia. Among other things, the fungus can cause late beet rot, white hatred, or potato pox. It is widespread worldwide and is native to the world. There are a few subgroups of Rhizoctonia solani that are only found in certain parts of the world, such as the Philippines, Japan, or the USA.

Rhizoctonia can survive on organic matter in the soil for up to three years. When the soil temperature rises above 15 ° C, the fungus becomes active and begins to grow. The mycelium then overgrows the beet in the ground, for example, and the hyphae penetrate the beet through cracks and injuries. Rhizoctonia can also penetrate the beet directly because it produces enzymes that can break down cell walls. Once the fungus has penetrated, it spreads throughout the plant.

The outbreak of the disease is triggered by external factors. The fungus is present in all soils, but warm temperatures and waterlogging promote the infestation. Rhizoctonia also very often causes seedling disease and casserole damage. Conditions conducive to infestation with Rhizoctonia are:

  • Waterlogging and high soil moisture
  • High temperatures
  • Heavy soils
  • Bad soil structure
  • Lack of oxygen
  • Low pH

Tomato seedlings can also be infected by Rhizoctonia

Rhizoctonia species and host plants

There is not just one type of Rhizoctonia, but many different, each of which differs in its preferred host plant and its harmful effects. Here is a brief overview of the various Rhizoctonia species:

  • Rhizoctonia solani: Often occurs on beets, legumes, cereals, tomatoes, salads, and potatoes; can survive as mycelium or form so-called sclerotia, which serve to persist the fungus in the soil; In contrast to other mushrooms, it does not form spores
  • Rhizoctonia cerealis: Also called a pointed eye spot and occurs on grain
  • Rhizoctonia crocorum: Also called the violet root killer and occurs on carrots
  • Rhizoctonia carotae: Is a storage rot pathogen in carrots

Recognizing Rhizoctonia: damage and symptoms

Rhizoctonia can cause various symptoms, which we briefly describe below. The symptoms differ depending on which culture the fungus occurs in.

Rhizoctonia on beets

Late beet rot due to Rhizoctonia solani often occurs on beets such as beetroot, Swiss chard, or sugar beet and causes the beet plants to wither from the outside in. The leaves then turn yellow and lie in a wreath around the beet on the ground. On the beets, dark brown, dry rot spots can also be seen that extend far into the beet. Even if the beet has already died, it continues to shrink.

Rhizoctonia causes rot in beets

Rhizoctonia on potatoes

Some symptoms can also develop on potatoes if they are infected with Rhizoctonia solani. For example, dark brown to black potato poxes form on the skin, which you can easily scrape off and which, luckily, do not penetrate the potatoes. The tubers can also remain small and deformed in the event of an infestation. In the case of an infestation by Rhizoctonia, there are also problems at the beginning of the vegetation of the potato, as this can lead to damage. In this case, you will find missing spots on the tuber, dark spots on the seedlings, and dead shoot tips.

A very well-known symptom is called “dry core”: This shows up with sunken round areas on the shell, whereby the tissue underneath is mostly destroyed and sharply demarcated from healthy tissue. These rotten spots can even fall out and resemble a wireworm infestation.

In particularly humid summers, whiteness is also evident, in which a grayish-white fungal lawn forms at the base of the stem. This is then the sexual stage of the fungus.

Damage to potato plants is similar to that of a wireworm infestation

Rhizoctonia on cereals

Among other things, Rhizoctonia cerealis occurs on grain and causes so-called eyespots on the grain stalks. A pointed spot with a brown edge and a light inner surface forms on the base of the stalk. If you cut open the stalk, you can find a cotton wool-like mushroom mycelium. In Germany, the damage caused by this fungus is rather minor, the risk is only slightly higher in cool and humid weather. Rhizoctonia solani can also occur as an emerging disease in cereals. The seedlings are already attacked in the ground and there is little emergence.

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Rhizoctonia on carrot

Violet root killer or Rhizoctonia crocorum is a disease on carrots that covers the beet with a purple fungal mesh starting from the tip so that it begins to sink in and rot easily.

However, Rhizoctonia carotae can also occur on the carrot. Here, a white network of mushrooms forms on the carrots, which resembles a spider’s web. Later it turns yellowish-brown and yellow drops form. This Rhizoctonia species is a dreaded storage rot pathogen in carrots.

Rhizoctonia on legumes

Rhizoctonia solani also occurs on legumes such as haricot beans or peas. Growth is delayed, the leaves turn yellow, and the plants wilt. Often the base of the stem also begins to turn black and rotten.

Yellow leaves on legumes are a sign of an infestation by Rhizoctonia solani

Rhizoctonia on tomatoes

In tomatoes, Rhizoctonia solani leads to seedling diseases, as the seedlings die before or shortly after emergence. Watery spots that appear constricted can form on the stems. This also causes the tomato plants to tip over.

Rhizoctonia on lettuce

The black rot in various salads is caused by Rhizoctonia solani and causes the outermost leaves of the salads, which lie on the ground, to either turn brown and wet rotten or dry and wafer-thin. This rot then extends into the heart of the head of lettuce. This disease is becoming more and more important in salads and occurs particularly at high temperatures.

Rhizoctonia on onion

In onion plants, Rhizoctonia solani occurs as a seedling disease and causes damage to the germinating onions. The onions are particularly badly damaged if they emerge slowly because they are set too deeply or the weather is cool.

Prevent and Avoid Rhizoctonia

Rhizoctonia prefers moist soil conditions. Therefore, you should make sure that no waterlogging forms in your garden. Avoid compaction and try to have well-ventilated healthy soil through thorough tillage. It is also best to bring in organic matter to keep the soil life active.

A healthy floor prevents infestation
The crop rotation in the garden is also particularly important: If you have ever had problems with a root rot pathogen such as Rhizoctonia, you should absolutely avoid placing plants in this area in the next few years that can also be attacked by Rhizoctonia. Since the perennial organs of the fungus can survive in the soil for up to three years, there is a risk of renewed infection.

To protect your plants, you can also use plant-strengthening liquid manure, which increases the resistance of your plants in the garden. These include, for example, nettle or field horsetail manure. Field horsetail manure is particularly suitable for combating Rhizoctonia, as it shows action against fungi. You can find out more about this plant manure here.

In addition, only use healthy plants and high-quality seeds, because sick plants can bring many pathogens into the garden and may not get rid of them. In the beginning, you should also make sure that the seeds germinate quickly, because the longer it takes to germinate, the more time the fungus has to attack the seedlings. Of course, you can also pre-germinate the seedlings to give your plants a head start in terms of development.

Fight Rhizoctonia

Unfortunately, for soil-borne fungi like Rhizoctonia, there aren’t many options for combating acute infestation. Rely on plant strengthening and optimal soil conditions to prevent fungal diseases from becoming too easy. In an emergency, you can attack the fungus with chemical pesticides: For allotment gardens, for example, some pesticides are effective against Rhizoctonia solani.

At the moment we are working on a seed treatment with which the seed can be treated before sowing and thus an infestation can be prevented. In addition, there are already resistance breeds so that the plants independently develop a higher tolerance or resistance to fungi such as Rhizoctonia.

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