Rhizoctonia solani

Rhizoctonia solani is a species of fungus that belongs to the order Cantharellales. This fungus is usually found in its simple, thread-like form rather than as the more complex fruit bodies it can sometimes produce. It is part of a group of closely related species that scientists are still studying.

This fungus is known around the world because it can make many different plants sick. It causes diseases like root rot, damping off, and wire stem, which can affect the health and growth of plants. Despite this, Rhizoctonia solani can also help some plants, like orchids, by forming special partnerships called mycorrhizal associations that support the plants' growth.

Taxonomy

In 1858, a scientist named Julius Kühn discovered a fungus on sick potato tubers and named it Rhizoctonia solani, after the potato plant Solanum tuberosum. Later, in 1956, another scientist named M.A. Donk gave it a different name, Thanatephorus cucumeris, for its spore-bearing form.

Researchers found that Rhizoctonia solani is actually a group of related fungi. They studied the fungi by looking at their tiny threads, called hyphae, and found that they could be divided into at least 25 different groups based on how these threads connected. Modern science, using DNA, supports this grouping. Today, Rhizoctonia solani includes both simple and complex forms of the fungus.

Hosts and symptoms

Rhizoctonia solani sensu lato causes many important plant diseases. It can make grass patchy (called brown patch), prevent seeds from sprouting (damping off), create black spots on potatoes, and cause rot in roots of crops like sugar beets and cucumbers. This fungus can infect many different plants, and its behavior can vary depending on the specific strain.

Rhizoctonia solani mainly attacks seeds and young plants hidden in the soil. One common problem it causes is damping off, where seeds either fail to grow or seedlings die just after they appear above the soil. When it attacks plants above ground, it can create brown spots and lesions on leaves, stems, and roots. The fungus prefers warm, wet conditions to grow and spread. It can cause crops like cereals, tomatoes, potatoes, and cabbages to become weak and unhealthy.

Disease cycle

Rhizoctonia solani can live in the soil for many years in the form of small, hard structures called sclerotia. These sclerotia help the fungus survive cold winters. The fungus is attracted to plants by special chemicals that plants release.

The fungus can enter a plant in different ways. It might go through the plant's skin directly or use natural openings. Once inside, the fungus can break down the plant's cells and use them for food. This process creates new sclerotia, and the cycle starts again when new plants grow. The disease can cause problems like rot in the soil, stem, or young plants.

Environment

This fungus prefers warm, wet weather, and problems with it usually show up in late summer. Farmers often notice sick plants when they harvest their crops. Things that help the fungus grow include having host plants nearby, lots of rain or irrigation, and warmer temperatures during spring and summer. Soil that doesn’t drain well, either because of its natural texture or because it’s compacted, also makes it easier for the fungus to thrive. The fungus spreads through small, tough structures called sclerotia, which can move by wind, water, or when soil is moved from one place to another.

Identification

Basidiocarps (fruit bodies) of Rhizoctonia solani are thin, web-like, and smooth. Under a microscope, they have wide hyphae without special connections called clamp connections. The fungus can also grow in a form called anamorphs, which includes hyphae and small structures called sclerotia. When grown in a lab, the fungus produces white to brown threads called mycelium, and its hyphae often form chains and branch at right angles.

Management

Controlling Rhizoctonia solani is challenging, but there are ways to reduce its effects. Farmers can use methods like rotating crops, planting in warmer soil, and choosing resistant varieties to help protect plants. Keeping the soil well-drained and avoiding areas that stay wet can also prevent the disease from becoming worse.

Some chemicals, like pentachloronitrobenzene (PCNB), can help reduce infections, and certified seed without harmful structures can be used. In special cases, scientists have created rice plants through genetically engineered methods that are better at fighting off the fungus. The disease is usually worse in soils that are moist and stay around 15–18 °C (59–64 °F).

Economic importance

Rhizoctonia solani is a harmful fungus that can affect many crops in the United States. It can cause big problems for farmers, sometimes reducing crop yields by a lot — from 25% to even 100%. The fungus also makes the soil stick to the crops, which can lower the quality of products made from them. One of the biggest problems it causes is a disease called sheath blight, which is the second-worst disease for rice plants after rice blast.

Mycorrhizal association with orchids

Rhizoctonia solani is one of several Rhizoctonia species that form special partnerships with orchids, called mycorrhizal associations. These partnerships can involve both harmful strains of the fungus that might cause disease in plants, as well as harmless strains that do not cause any harm. This relationship helps orchids grow and thrive.

Genome

Scientists have studied the genetic makeup of a specific type of the fungus Rhizoctonia solani. They found that its DNA amounts to about 51.7 million base pairs, though the actual total is likely larger due to repeated sections and extra DNA that made sequencing difficult. This fungus is thought to have around 12,726 genes that help it grow and survive. Researchers are also looking at other types of this fungus to learn more.