Seedling and root rots - Rhizoctonia solani

Introduction

Rhizoctonia spp. cause a range of disease, some truely soilborne, others attacking aerial parts. These fungi are a diverse group of organisms that are still being studied to determine their relationships. Most seem to be the imperfect stage of Basidiomycetes. These fungi attack a broad range of hosts and are commonly associated with other crop and weed plants. They are found in almost all soils and in most environmental conditions.

Disease Symptoms

Rhizoctonia can cause damping off and seedling blight. The fungus penetrates the epidermis to colonise the soft tissue of the cotyledons, hypocotyl and roots of the seedling. Lesions are sunken, red-brown and may be covered in cream-brown mycelium. As the fungus colonised the cells, sclerotia may develop that can remain in the crop debris until the next susceptible crop is planted.
Sheath blight of rice is a key disease, which begins as lesions on the leaf sheath from the seedling through to the grain stages. Grey-green water-soaked spots 1 cm long and circular to oval develop, then enlarge to be bleached with an irregular purple-brown border. Hyphae may spread over the plant until it turns white or grey and dies. Sclerotia, white, then brown, develop on the leaf sheaths, and infected plants may lodge, poorly fill with grain or poor yield.
In maize, Rhizoctonia produces crown and adventitious root rot, where the roots of germinating seeds and young plants brown and decay with sunken brown lesions on the roots and crown. Symptoms are visible on the cob as larged grey spots with a purple-brown border similar to those of sheath blight of rice. Lodging may occur and nematodes are known to contribute to disease.
In bottom rot of lettuce, lesions appear on the lower leaves that are in contact with the soil. These red-brown spots on the midribs rapidly expand under favourable conditions to quickly rot the midribs and leaf blades. The lesions may also provide an entry point for secondary soft rot bacteria.
In crucifers, the growth of older seedlings can be stunted and wire stem can develop, with the tissue of the outer stem becoming brown and shrunken. Stem and root rot can occur in the later stages of growth, along with a firm dark head rot.
Belly rot of cucumber and other cucurbits is a rot that appears on the underside of fruit, with water-soaked tan lesions. The sunken lesions are irregular and the cratered area has a crusty appearance.
Infection of potatoes appears as seedling death, yield reduction and then cankers develop on developing stems. The stems may be girdled to lead to leaf roll, chlorosis, stem cortical necrosis and death of the plant. Reddish brown lesions can also be seen on the stolons. Brown or black sclerotia develop on the tuber surface that will not wash off.
Infected tomato plants can develop dark brown sunken lesions on the stem near soil level.
Root rot and web blight, along with damping off diseases occur on soybean, peanut, bean, lentil, chickpea, mung bean, cowpea and lima bean. Web blight appears as leaf lesions similar to those on the stems and roots of other infected crops.

The Pathogen

Rhizoctonia species are usually sterile fungi that rarely form perfect states. There are a large number of types of Rhizoctonia and many are poorly characterised. Most have a basidiomycete perfect state that can often be encouraged to be produced under specialised conditions in culture. The most characteristic feature of these fungi is hyphae that branch at right angles with a septum after the branch. They can have variable numbers of nuclei in each cell and some pathogen strains are multinucleate and have very robust hyphae.

Distribution within Vietnam

In all parts of the country.

Host Range

It is difficult to accurately discuss the host range of Rhizoctonia because there are so many types that are poorly understood; in many cases, the host range for each type has not been determined.

Knowledge of the host specificity of strains in the field may be important in determining rotation planting. Rhizotonia has been reported on the following crops in Vietnam:
Rice, maize, mungbean, cotton, cassava, onion, soybean, grasses and weeds, crucifers, cucurbits (especially cucumber), plus damping off and seedling death of peanut, sesame, citrus and many other crops.

Epidemiology

The sources of inoculum are hyphae, sclerotia and potentially basidiospores. Hyphae are often visible growing on and within the soil and plant debris. Sclerotia may also be visible on the plant surface. Weeds and other crops in the rotation may also function as the source of inoculum. The role of seeds in the disease cycle is also important, where potato seed tubers and true seeds may carry sufficient inoculum to severely contaminate soil previously free of inoculum. Seed may harbour the pathogen externally or internally so that treatments must kill the pathogen within the seed.

With respect to environmental factors conducive to disease:

R. solani is aided by the energy-yielding compounds in plant exudates, but the exudates are not necessary for penetration.
Much of the research into the optimal temperature for fungal growth and disease development of this pathogen has been strain specific. It is more important to remember that disease is more likely to occur when the host is most vulnerable. The optimal temperature for disease development in potatoes, peas and beans is 18°C, with little disease developing above 21-24°C. The optimum temperature for fungal growth is however 26°C. The optimum temperature for plant growth must also be considered, in that cool temperatures do not promote fast emergence of seedlings. The fungus may take advantage of the wider infection time, which would be less likely above 24°C despite the optimum growth of the fungus.
The growth of Rhizoctonia is reported to be restricted by poor aeration but promoted by high soil moisture.
As with the temperature, Rhizoctonia attacks the host in pH ranges outside those optimal for plant growth, but within the range for fungal growth.
High nitrogen levels and high density also promote disease development.

Survival mechanisms can be in either active or inactive states. The inactive state is in the form of sclerotia or thick walled hyphae, which are dormant, resting structures produced by the pathogen for overseasoning. High temperatures or high moisture conditions decrease the longevity of these survival structures. Generally, viability is maintained for 6 months in the field for thick walled propagules within plant debris particles, while those in the soil matrix are only short-lived. Rhizoctonia is a poor competitor, with other microbes reducing the survival potential of the pathogen significantly in unsterilised soil.

Active survival occurs through the colonisation of living roots by the hyphae in both host and non-host plants, with survival better in the former. The growth of the fungus as a saprophyte will also provide a means for survival and perhaps a way to increase the inoculum levels in the soil.

Control

Validamycin is the most widely used fungicide treatment for this pathogen, and is especially important in rice, corn and cabbage.

For damping off and canker diseases, the period of infection is usually brief, while for root rots and storage rots, the host tissue is open for infection for an extended period of time. The best control measures for the damping off and cankers are those that decrease the inoculum levels or the survival of the inoculum, including the principles of minimising the spread of the pathogen into uninfected areas since the environmental conditions are less important. For rot diseases, the environment dictates the period of infection, so that pathogen activity and host susceptibility will determine the disease severity.

Practices such as these are useful:

Avoid transmission from propagating material such as seeds, potato tubers and transplanted seedlings from unpasteurised or contaminated soil.
Select the time of seeding to suit the optimum germination and emergence conditions.
Plant seed as shallow as soil moisture levels allow.
Rotate with grain crops for 1-4 years to reduce inoculum levels.
Introduce organic matter or antagonistic organisms such as Trichoderma as a form of biological control.
Select resistant or more tolerant varieties if they are available.

Rhizoctonia symptoms on maize