Heterokaryon fonnation between different fungal individuals is an important component of many fungal life cycles and may serve as the first step in the parasexual cycle and the transmission of hypovirulent factors such as dsRNAs. Heterokaryosis also is a means by which nonnally haploid fungi may enjoy the benefits of functional diploidy, such as complementation or heterosis. In plant pathogenic fungi, the entity that emerges following heterokaryosis may differ from its constituents in aggressiveness or host range; some of these aspects have been reviewed in previous volumes in this series (9, 28, 45, 62, 72, 112, 114, 117, 143, 154, 158, 160). In most heterothallic fungi, the fonnation of a heterokaryon between two genetically different haploid strains is an essential part of the life cycle. Such heterokaryons may be quite stable and persist vegetatively for an indefinite period of time or may last only long enough for the component haploid nuclei to fuse and then immediately undergo meiosis. In many fungi, sexual and vegetative heterokaryons are quite distinct from one another. Strains capable of fonning a successful sexual heterokaryon may be unable to fonn a successful vegetative heterokaryon and vice versa. Strains that are capable of fonning these types of heterokaryons are referred to as "sexually" or "vegetatively" compatible, respectively. Strains that are vege­ tatively compatible with one another are frequently described as members of the same vegetative compatibility group, or VCG. Sexual compatibility is usually governed by one or more mating-type loci that may have two or more alleles (58, 62). Vegetative compatibility may be governed by the mating-type loci in some fungi, e.g. many basidiomycetes, but there also are examples in which a separate set of genes controls the fonnation and stability of these vegetative heterokaryons.

Fungal vegetative compatibility

Rhizoctonia solani Kuhn and other Rhizoctonia spp. are widespread in the world and in both cultivated and noncultivated soils. Rhizoctonia spp. are readily isolated from diseased plants and soils, with isolates differing in pathogenicity and morphology, as well as cultural and physiological charac­ teristics. Based on these differences, many species and intraspecific groups (ISGs) have been described; however, identification of species and ISGs is sometimes very difficult, owing to the absence of the authentic cultures, which hinders mycological and pathological studies. Fortunately, this situa­ tion has been put in order through the efforts of many researchers through characterization of the genus Rhizoctonia, the species R. solani, and other Rhizoctonia spp. The introduction of the concept of anastomosis groups (AGs) or intraspecific groups, as this paper introduces, has also aided species and subspecies identification. Ecologically, Rhizoctonia spp. are diversified; they may be pathogens on many hosts, saprophytes in soils, or mycorrhizae of orchidiaceous plants. This paper outlines the present status of our knowledge of ecology and pathology of Rhizoctonia solani, especially of anastomosis groups or in­ traspecific groups.

Ecology and pathogenicity of anastomosis and intraspecific groups of

Terrestrial orchid conservation in the age of extinction

Most plant-pathogenic fungi gain access into their host by penetration of unwounded tissue Some pathogens such as rusts invade the host via stomata (Hoch and Staples, 1987 and Chapter 2), whereas others penetrate the intact leaf surface without the requirement of natural openings (Aist, 1976; Emmett, 1975; Kunoh, 1984) The latter type of direct penetration encounters the plant cuticle, a noncellular hydrophobic structure covering the layer of epidermal cells The cuticle thus serves as the first surface barrier that the pathogen has to breach There is little evidence for the mere physical strength of the plant cuticle as a major factor in plant defense against pathogens In some cases, the thickness of plant cuticles has been correlated with an increased passive resistance against fungal attack This correlation, however, appears to be an exception rather than the rule (Martin, 1964) Furthermore, the cuticle has not been considered to play a major role in the active defense mechanisms of disease-resistant cultivars There is good evidence that the cuticle is penetrated by the attacking pathogen before the sequential steps of disease development are halted by the active defense reactions of the challenged plant Recent examples for this lack of cuticle involvement in cultivar resistance are the host—pathogen interactions of Venturia ivaequalis—apple (Valsangiacomo and Gessler, 1988) or Phytophthora infestans—potato (Gees and Hohl, 1987) The breaching of the cuticle can also be accomplished in many interactions of pathogens with nonhost plants (Heath, 1987)

The Plant Cuticle

Fusarium oxysporum is well represented among the rhizosphere microflora. While all strains exist saprophytically, some are well-known for inducing wilt or root rots on plants whereas others are considered as nonpathogenic. Several methods based on phenotypic and genetic traits have been developed to characterize F. oxysporum strains. Results showed the great diversity affecting the soil-borne populations of F. oxysporum. In suppressive soils, interactions between pathogenic and nonpathogenic strains result in the control of the disease. Therefore nonpathogenic strains are developed as biocontrol agents. The nonpathogenic F. oxysporum strains show several modes of action contributing to their biocontrol capacity. They are able to compete for nutrients in the soil, affecting the rate of chlamydospore germination of the pathogen. They can also compete for infection sites on the root, and can trigger plant defence reactions, inducing systemic resistance. These mechanisms are more or less important depending on the strain. The nonpathogenic F. oxysporum are easy to mass produce and formulate, but application conditions for biocontrol efficacy under field conditions have still to be determined.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2003.00700.x

Fusarium oxysporum and its biocontrol

Identification of Rhizoctonia Species.

Identification of Rhizoctonia species , Identification of Rhizoctonia species , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Identification of Rhizoctonia Species

Ecology and Pathogenicity of Anastomosis and Intraspecific Groups of Rhizoctonia Solani Kuhn

The genus Rhizoctonia was described by DeCandolle in 1815. He designated R. crocorum (Pers.) DC. as the type species, while R. solani,the most important species of Rhizoctonia, was described by Kuhn in 1858. The history of R. solani is almost as long as the history of plant pathology. It was published around the time when the potato late blight broke out in Europe, which caused the noteworthy potato famine in Ireland. Since these initial reports, the pathology, taxonomy, ecology, and disease control of Rhizoctonia have been extensively studied.

Akira Ogoshi

Papers

Identification of Rhizoctonia Species.

Identification of Rhizoctonia Species

Ecology and Pathogenicity of Anastomosis and Intraspecific Groups of Rhizoctonia Solani Kuhn

Ecology and pathogenicity of anastomosis and intraspecific groups of

Introduction — The Genus Rhizoctonia