Rhizoctonia solani in Potato Crops:

Introduction to Disease, Classification, Symptoms and Pathogen Cycle

1. Introduction

From Biocontrol Technologies we are back with a new blog to talk about one of the fungal pathogens that most impact agriculture: Rhizoctonia spp. We will focus especially on potato, a crop of high global relevance that suffers significantly the effects of Rhizoctonia solani species.

In this entry we will discuss their taxonomic classification, the diseases they cause in potato crops and the characteristic symptoms that facilitate their diagnosis and their biological cycle, which gives us valuable information for control strategies.

2. Taxonomic Classification

The fungus Rhizoctonia solani, (Kühn 1858) belonging to the genus Rhizoctonia, is a pathogen of the family Ceratobasidiaceae, order Ceratobasidiales, and of the kingdom Fungi, phylum Basidiomycota, class Agaricomycotina.

The sexual reproduction (teleomorphic form) of this fungus, known as Thanatephorus cucumeris, occurs with the fusion of its hyphae that form basidia. The fusion of these hyphae is quite specific and forms groups of anastomosis (GA) that can be differentiated from each other. These groups are specific to the crops it infects, for example, potato is mainly affected by GA 3 (Chaudhary, S. et al., 2024), rarely other GA produce disease in potato.

In the absence of host plants, the pathogen survives in the soil, in the form of latent sclerotia, also as a saprophyte feeding on organic remains of the soil and as a parasite on the remains of its own roots. Its permanence in the soil will depend on the levels of disease (initial concentration) and will be subject to optimal climatic conditions for this pathogen.

Kingdom:

Filo:

Class:

Order:

Family:

Gender:

Fungi

Basidiomycota

Agaricomycotina

Ceratobasidiales

Ceratobasidiaceae

Rhizoctonia

Potato affected by Rhizoctonia Solani.

Did you know that...?

…R. solani affects many crop families: fabaceae, solanaceae, poaceae, asteraceae, etc.

Unlike other fungi, Rhizoctonia solani (asexual or anamorphic form) does not produce easily identifiable spores, which complicates its taxonomic classification, which is why it is called “sterile mycelium”.

3. Disease and Symptoms of Rhizoctonia solani on Potatoes

Rhizoctonia solani is a versatile pathogen capable of infecting different parts of the plant at various stages of its development, causing different symptoms depending on the affected organ: stem and stolon canker and black scurf.

Rhizoctonia Canker

Stem Canker

This symptom causes the most damage to potatoes. Stem canker appears in the subway parts of the plant, with brown to reddish-brown lesions that sink into the tissue and have a dry, rough texture. These lesions can surround the stem and disrupt the flow of water and nutrients, weakening the plant and leading to irregular, poor and stunted growth. Affected plants often appear stunted and may produce few or no tubers. When infections are severe, canker can kill young shoots or stolons, causing significant yield losses.

Stolon and Root Canker.

Image .https://www.infojardin.com/

Stolon and Root Canker.

Infection can also occur in stolons, reducing the number of tubers per plant, while when roots are affected, nutrient uptake from leaves to tubers is affected, reducing the size and number of tubers produced. When stolons are severely affected, small, misshapen tubers are formed, often green and shallow, which are not marketable. The formation of these aerial tubers indicates a disruption of the flow of carbohydrates to the subway tubers, which negatively affects production.

Black Scurf

Image. EPPO Int.

Black Scurf

This is the most visible and recognizable symptom of R. solani infection. In the final stage of its cycle, the fungus produces dark, hard masses called sclerotia, resistance structures that adhere to the tuber surface as small irregular black spots. These structures do not directly penetrate or damage tuber tissue, but their presence diminishes its commercial quality and perpetuates the disease either by detaching and remaining in the soil and/or if these infected tubers are used as seeds. Therefore, although it does not directly affect the internal integrity of the tuber, its persistence contributes to the dissemination of the fungus and to the reduction of quality.

Other symptoms of this disease: The presence of lesions in the subway parts of the plant can lead to deformities in the foliage, such as dwarfing, leaf curling and rosette appearance in affected shoots, which can be confused with symptoms of viruses such as potato leaf roll virus.

Das, S. (2013). Rhizoctonia solani on potato in New Zealand: pathogen characterisation and identification of double-stranded RNA viruses that may affect their virulence.

Disease symptoms on potato caused by Rhizoctonia solani under glasshouse and shadehouse conditions.

(A) Cankerous lesions on below ground stems;

(B) cankerous lesions on root system;

(C) stolon lesion and tip burning;

(D) severe black scurf on progeny tubers produced by plants inoculated with AG-3PT under shadehouse conditions;

(E) negligible black scurf on progeny tubers produced by plants inoculated with an AG-2-1 tester isolate;

(F) malformed tubers by AG-2Nt isolates from New Zealand;

(G) hymenia on potato stems under shadehouse conditions;

(H) hymenia on potato stems under glasshouse conditions;

(I) aerial tuber formation by AG-2Nt isolates from New Zealand under shadehouse conditions;

(J) hymenium formation on an aerial tuber under shadehouse conditions;

(K) basidiospores identified in hymenia by microscopic observation.

The term hymenia (hymenium) refers to the set of reproductive structures of certain fungi, in particular the fertile layer where the sexual spores are formed. In basidiomycetes (the group to which Rhizoctonia solani belongs), the hymenium is the part of the fungus that produces and releases basidiospores, the sexual reproductive units.

However, it is important to note that Rhizoctonia solani rarely produces sexual structures under natural conditions, as its predominant reproduction is asexual by mycelium and sclerotia (resistance structures). The vegetative and asexual phases of the fungus are responsible for symptoms in potato crops.

4. Biological Cycle of Rhizoctonia solani

The life cycle of the pathogen is characterized by its ability to persist in soil and crop debris, which facilitates its reappearance year after year. Its cycle in potato can be described as follows:

Chaudhary, S., Lal, M., Sagar, S. et al. (2024) Black scurf of potato: Insights into biology, diagnosis, detection, host-pathogen interaction, and management strategies. Tropical Plant Pathology, 49, 169–192 . https://doi.org/10.1007/s40858-023-00622-4

Onset of Infection:

Infection in potato starts from contaminated soil or infected seed tubers. R. solani can survive in the soil as mycelium or in the form of sclerotia (resistance structures that may be dormant for years). When infected potatoes are planted or in infected soil, the fungus moves from the inoculum to the growing shoot, infecting the subway stem.

2. Optimal Conditions for Development:

The fungus grows actively in conditions of low soil temperature (below 10°C), high humidity and soils with neutral to slightly acid pH. These conditions hinder the rapid emergence of shoots, favoring the fungus in its infection and development in non-chlorophyllous and more susceptible plant tissue.

3. Spread of Infection:

Roots and stolons can become infected at any time during the crop cycle, but the most serious damage occurs in the early stages, when the fungus penetrates the tender tissue of emerging shoots, forming cankers that can arrest the growth of stolons and stems. This directly affects tuber production and quality, reducing yield.

4. Sclerotia formation at the end of the cycle:

At the end of the season, especially after plant wilting, R. solani produces sclerotia on young tubers, which serve as a reservoir inoculum for future infections. Although not all tubers formed on infected plants develop sclerotia, sclerotia mainly form under the right conditions and may continue to form during storage if the environment is conducive.

5. Survival for the Next Season:

At the end of the season, especially after plant wilting, R. solani produces sclerotia on young tubers, which serve as a reservoir inoculum for future infections. Although not all tubers formed on infected plants develop sclerotia, sclerotia mainly form under the right conditions and may continue to form during storage if the environment is conducive.

6. Mechanisms of Infection and Resistance:

Although the fungus can easily penetrate young, susceptible tissue in subway shoots, plant resistance increases as the plant emerges and develops chlorophyll. For this reason, damage tends to be reduced in later stages of growth as the tissues strengthen. However, sclerotia remaining on tubers and in the soil remain an active source of infection.

5. References

1. Root rot, Rhizoctonia solani, Servicio Nacional de Sanidad, Inocuidad y Calidad Agroalimentaria, Mexico: https://www.gob.mx/cms/uploads/attachment/file/600968/Pudrici_n_de_la_ra_z.pdf

2.Rhizoctonia solani in potatoes and its control, University of Waeningen: https://edepot.wur.nl/305202

3. Rhizoctonia Canker and Black Scurf of Potato, Cornell University: https://www.vegetables.cornell.edu/pest-management/disease-factsheets/rhizoctonia-canker-and-black-scurf-of-potato/

4. Rhizoctonia stem canker and black scurf of potato, University of Idaho: https://objects.lib.uidaho.edu/uiext/uiext32100.pdf

5. Bulletin #2273, Potato Facts: Rhizoctonia Diseases on Potatoes, University of maine: https://extension.umaine.edu/publications/2273e/

6. Ajayi‐Oyetunde, O. O., & Bradley, C. A. (2018). Rhizoctonia solani: taxonomy, population biology and management of rhizoctonia seedling disease of soybean. Plant pathology, 67(1), 3-17.

7. Chaudhary, S., Lal, M., Sagar, S., Sharma, S., & Kumar, M. (2024). Black scurf of potato: Insights into biology, diagnosis, detection, host-pathogen interaction, and management strategies. Tropical Plant Pathology, 49(2), 169-192

Next Entry:

Control Strategies

In our next blog, we will explore prevention and control strategies for Rhizoctonia solani in potato(Solanum tuberosum), addressing biological control alternatives with the biological fungicide RootDei Biocontrol®, the new solution from BiocontrolTechnologies®, S.L. that allows an effective management of the disease without resorting to chemical fungicides.