- Evolutionary ecology research
- Horticultural research
- Plant diversity research
- Plant pathology research
- Australian fungi
- Fungal leaf spot on eucalypts
- Fusarium oxysporum
- Fusarium wilt
- Fusarium workshops
- Leaf spot fungi systematics
- Phytophthora Dieback
- Plant Pathology and Mycology 2011-2012
- Plant Pathology and Mycology 2010-2011
- Plant Pathology and Mycology 2009-2010
- Plant Pathology and Mycology 2008-2009
- Soilborne plant diseases in Vietnam
- General information on soilborne diseases
- Soilborne plant diseases
- Host ranges of soilborne diseases
- Laboratory techniques
- About the Ausaid CD-ROM Project
- References and further information
- Herbarium & resources
- Scientific publications
Seedling and root rots - Rhizoctonia solani
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.
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.
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:
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:
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.
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: