Rhizoctonia solani is a widespread soil-borne fungal pathogen that causes various diseases in a broad spectrum of plants worldwide. It is a complex organism with multiple genetically distinct groups, presenting challenges for both home gardeners and large-scale agriculture. The fungus is highly adaptable and can persist in diverse environments, posing a persistent threat to plant health and productivity.
Identifying Rhizoctonia Damage
Rhizoctonia solani manifests in several ways, causing distinct symptoms depending on the plant part affected and the plant’s developmental stage. One of the most common and damaging diseases caused by this pathogen is damping-off, which affects seedlings. This can occur as pre-emergence damping-off, where seeds or sprouts rot before they emerge from the soil, leading to poor stands. Post-emergence damping-off happens after seedlings have emerged, causing a soft decay of the taproot or rootlets, leading to the collapse and death of young plants. Reddish-brown, sunken lesions often appear on the hypocotyl (the part of the stem below the cotyledons) near the soil line, which can girdle the seedling.
Root rot symptoms include discolored roots, ranging from yellow to black, with sections of decay and a failure to develop new roots. Infected plants may exhibit stunted growth, wilting, and yellowing foliage, sometimes resembling a nitrogen deficiency. In more mature plants, Rhizoctonia solani can cause stem rot or wirestem, where lesions develop on the stem near the soil line. These lesions are reddish-brown, sunken, and can enlarge to girdle the stem, restricting water and nutrient movement, and making the stem tough and wiry.
Web blight, also known as aerial blight or foliar blight, occurs when the fungus spreads to the upper parts of the plant, particularly under humid conditions. Initial symptoms on leaves appear as water-soaked, grayish-green lesions that turn tan to brown or black as they mature. A characteristic web-like fungal growth, or mycelium, may be visible on infected foliage, stems, and pods, especially in dense canopies. This can lead to leaves matting together and widespread blight.
Crown rot affects the plant’s crown at or near the soil line. It results in a dry, brown rot with deep fissures, leading to sudden yellowing, wilting, and collapse of foliage.
On potato tubers, Rhizoctonia solani causes black scurf, which appears as raised, hard, black patches or incrustations on the surface. These sclerotia, or resting bodies of the fungus, are superficial and can be scratched off, but they can make the tubers unsightly. The same fungus also causes stem canker on potato sprouts and stems, characterized by sunken, reddish-brown lesions. Severe cankers can girdle the stem, leading to stunted plants, delayed emergence, and in some cases, the formation of small, green aerial tubers.
How Rhizoctonia Persists and Spreads
Rhizoctonia solani exhibits remarkable persistence in the environment, primarily surviving in the soil and on plant debris. The fungus forms specialized resting structures called sclerotia, which are compact masses of hardened fungal mycelium. These sclerotia are resilient, with thick outer layers that allow them to endure unfavorable conditions like extreme temperatures or drought for extended periods. The fungus can also persist as mycelium (thread-like fungal growth) within infected plant material or freely in the soil.
The pathogen has an exceptionally broad host range, capable of infecting over 250 different plant species across various families. These include economically important crops like wheat, rice, corn, potatoes, soybeans, and sugar beets, as well as many ornamental plants. This wide host range contributes significantly to its ubiquity and makes crop rotation a complex management strategy.
Rhizoctonia solani spreads through several mechanisms. Movement of contaminated soil, plant material, and tools can easily disseminate the fungus. Water, including irrigation water and rain splash, also plays a role in spreading sclerotia and mycelial fragments between plants and across fields. While the fungus can produce airborne spores, this form of transmission is not common in agricultural settings.
The development and spread of Rhizoctonia solani are highly influenced by environmental conditions. The fungus thrives in warm, moist soils, with optimal growth occurring in temperatures ranging from 20°C to 30°C (68°F to 86°F). High humidity, poor soil drainage, and compacted soil create favorable conditions for disease development. High nitrogen levels can also increase plant susceptibility. Seedlings are particularly vulnerable to infection in their early stages, especially when planted in cool, wet soils that slow their emergence and growth, giving the fungus more time to attack.
Managing Rhizoctonia Diseases
Effective management of Rhizoctonia solani requires a combination of strategies, as complete elimination of the pathogen from the soil is generally not possible. Cultural practices form the first line of defense, aiming to create an environment less favorable for the fungus. Improving soil drainage, particularly in heavy or compacted soils, helps reduce excessive moisture that promotes fungal growth. Sterilization of tools, growing media, and planting trays is also important to prevent introducing or spreading the pathogen.
Crop rotation with non-host plants can help reduce inoculum levels in the soil, although the broad host range of Rhizoctonia solani makes this more challenging compared to other pathogens. Proper spacing of plants allows for better air circulation, which reduces humidity around the plant canopy, thereby inhibiting fungal development. Avoiding overwatering, especially of young seedlings, helps prevent saturated soil conditions that favor the pathogen. Using disease-free seeds or seedlings is a straightforward way to reduce initial inoculum in the planting area. Maintaining optimal soil pH and nutrient balance supports healthy plant growth, which can increase a plant’s natural resilience to disease. Prompt removal and destruction of infected plant debris after harvest can significantly reduce the amount of inoculum available for future infections.
Biological control methods involve introducing beneficial microorganisms that can suppress Rhizoctonia solani. Species of Trichoderma, for example, are widely used as biological control agents. These fungi can directly antagonize Rhizoctonia through mycoparasitism, antibiosis, and competition for nutrients and space. Trichoderma species also have the ability to induce systemic resistance in plants, enhancing their natural defenses against the pathogen.
Chemical control using fungicides can be employed for severe outbreaks, but it should be approached with caution, especially for home gardeners. Fungicides are available as seed treatments to protect young seedlings or as soil applications. Systemic fungicides, which are absorbed by the plant, offer better control than non-systemic options.
Repeated use of the same fungicides can lead to the development of resistance in the pathogen, making integrated strategies important. Consulting local agricultural extension services for guidance on appropriate product selection and application rates is highly recommended.
Some plant varieties exhibit a degree of resistance or tolerance to Rhizoctonia solani, though complete immunity is rare. While these resistant varieties may offer lower yields compared to standard susceptible varieties, their use can be a valuable component of an integrated disease management plan, especially in areas with a history of severe Rhizoctonia problems. Combining resistant varieties with cultural practices and, when necessary, biological or chemical controls, offers the most comprehensive approach to managing Rhizoctonia diseases.