In this article we will discuss about:- 1. Subject-Matter of the Species Rhizoctonia Solani 2. Morphology of Rhizoctonia Solani 3. Host Resistance 4. Fungicides 5. Bio-Control.
Contents:
- Subject-Matter of the Species Rhizoctonia Solani
- Morphology of Rhizoctonia Solani
- Host Resistance of Rhizoctonia Solani
- Fungicides of Rhizoctonia Solani
- Bio-Control of Rhizoctonia Solani
1. Subject-Matter of the Species Rhizoctonia Solani:
German scientist Kuhn on diseased potato tubers reported Rhizoctonia solani in 1958. Since then, this fungus has gained the reputation of being widespread, destructive and versatile plant pathogen. It is capable of attacking a wide range of host plants, causing seed decay, damping-off, stem cankers, root rots, aerial blights and fruit decay.
It is the combination of its competitive saprophytic ability and high pathogenic potential that makes Rhizoctonia solani a persistent and destructive plant pathogen.
An insufficient description was made by Kuhn was insufficient and it was often difficult to determine whether or not a particular isolate was Rhizoctonia solani until, Duggar (1915) reported that Rhizoctonia solani was characterized as:
(a) The diameter of vegetative hyphae 8-12 µm,
(b) Constriction at point of branching, and
(c) Right angle branching of matured hyphae.
The systematic position of the sclerotial and basidial stages are as follows:
Sclerotial Stage:
Kingdom: Fungi
Phylum: Ascomycota
Class: Deuteromycetes
Genus: Rhizoctonia
Species: Solani Kuhn
Basidial Stage:
Kingdom: Fingi
Phylum: Basidiomycota
Class: Basidiomycetes
Order: Ceratobasidiales
Family: Ceratobasidiaceae
Genus: Thanatephorus
Species: cucumeris (Frank) Donk
It was Rolfs (1903) who first connected Rhizoctonia solani with a basidiomycetous fungus. Donk (1956) assigned its teleomorphic stage to Thanatephorus cucumeris (Frank) Donk. At present, this name is considered valid for the perfect state of Rhizoctonia solani.
However, Tu and Kimbrough (1978) differentiated Corticium sasakii, the sheath blight pathogen, from Thanatephorus cucumeris on the basis of larger basidia and spores, frequent occurrence of adventitious septa in protosterigmata, more intense in iodide reaction, presence of neutral lipids and primitive rind type of sclerotium. They accordingly proposed a new Thanatephorus sasakii (Shirai).
Due to widespread and destructive nature on wide range of host plants and great variability among isolates, Rhizoctonia solani has been divided into different intraspecific groups (Iga). As far as the maize leaf and sheath blight pathogen in India is concerned, most of the taxonomic characters have been documented by Ahuja and Payak (1988) and Payak (1988).
The characteristic of the pathogen are; the appearance of fungal colonies on potato dextrose agar medium as effused white when young, later buff to brown, hyphae 3-11mm (average 7.4 mm) in diameter, hyaline at first, later brown, septate, branched at right angle, constriction and septum near origin of branch; hyphal cells multinucleate, nuclei 3 to 16 in number but mostly 4 to 8 (mean 6.3 to 6.6; mode 6) per cell; sclerotia of primary rind type, variable in size and shape but mostly discoid and irregularly curled, growth optimum at 25-30°C, no growth below 15°C and above 35°C, and growth rate about 45 mm/ 24 hr.
2. Morphology of Rhizoctonia Solani:
Dasgupta (1992) described the mycelium in culture initially as silvery becoming yellow and brown with maturity, 1-2 mm broad and infrequently septate. Three types of mycelium have been observed.
(а) Runner Mycelium:
Straight, creeping tropic, non-infectious hyphae or sometimes thick and flattened.
(b) Lobate Mycelium:
branching out from the former as short, swollen, much branched, single or multiple lobate aspersoria and penetration peg, which later becomes intracellular all through the lesions. The lobate state associated with the infection process may be counter part of the sporogenous moniloid state.
(c) Moniloid Mycelium:
Forming the sclerotia. Ou (1985) reported that mycelium aggregates and forms sclerotia which are spherical, dark brown to black, 4-5 mm in diameter.
The sclerotia of Rhizoctonia solani are brown to black composed of clusters of melanin encrusted, thick walled cells, rich in nutrients, formed by repeated branching from short, thick, lateral hyphae, when produced on plant parts it is difficult to separate the sclerotia from their surrounding embedded sclerotia. Sclerotia are 4-5 mm in diameter, spherical but flattened when pressed between leaf sheath and culm.
Five phases of sclerotial morphogenesis have been recognized in Rhizoctonia solani:
(а) Repeated hyphal branching resulting short thick and lateral moniloid hyphae
(b) Hyphal aggregation as clusters of thick walled cells and network formation.
(c) Formation of sclerotial initial.
(d) Formation of whitish immature sclerotia.
(e) Maturation and pigmentation by mycelium encrustation.
Sclerotia are non-buoyant when immature but become buoyant after approximately 30 days when exterior cells become void of cellular content. Freshly formed sclerotia are dense and sink in water but not when they become mature. Floating and sinking sclerotia may have differential roles in pathogenicity due to variation in membrane permeability.
Gangopadhyay and Chakrabarty (1982) reported the size of basidia as 11-15 x 8-9 µm, sterigmata as 7-10 µm x 2-3 µm and basidiospores as 9-12 µm x 5-7 µm. Dasgupta (1992) reported that basidiospores are 2-4 terminal in imperfect cymose or racemose clusters formed by branching of short celled ascending hyphae.
The basidial stage was observed on upland rice appearing as white powdery layers on the healthy leaves or the areas adjacent to lesion under extremely moist conditions. Basidiospores of T. cucumeris, produced in hymenia on host plants, can initiate infection but considered to be unimportant in the epidemiology of rice sheath blight.
Ogoshi (1987) first time reported anastomosis groupings to differentiate strain of Rhizoctonia solani. Four main groups, each non-inter breeding population, were recognized by Talbot (1970).
However, Naiki and Kanoh (1978) classified T. cucumeris into 5 anastomosis groups according to their virulence. Yokoyama and Ogoshi (1987) conducted an experiment to show that when isolates of Rhizoctonia solani are poured 2-3cm apart on a medium usually 2 per cent water agar in a petridish, their mycelia grow and overlap, which can be observed under a light microscope at low magnification.
If hyphal fusion occurs these isolates belong to the same anastomosis group and often, attraction of hyphae and of fused cells are observed. If fusion, attraction and hyphal death do not occur, the isolates belong to different anastomosis grouping (AGs.) Wang and Hsich (1993) observed anastomosis behaviour of 12 isolates of Rhizoctonia spp. obtained from diseased stem of turf grass in central and southern parts of Taiwan.
Ten isolates were binucleate and fell into six anastomosis groups AG-B, AG-C, AG-F, AG-G, AG-L and AG-Q. The other two isolates were multinucleate and belonged to AG-4 and WAG-O.
The major anastomosis groups were; rice sheath blight, Rhizoctonia solani AG-IIA; maize sheath blight, AG-IIA and AG-4; sheath blight of wheat and barley binucleate Rhizoctonia, damping-off of solanaceous vegetable seedlings, AG-4 ground nut leaves, root and shoots, AG-4, Sheath blight of soybean, AG-1 IB; potatoes, AG-3; crucifers, AG-2-1 in winters and AG-4 during other season, Vigna, Phaseolus, cucurbitacceous and solanaceous, usually AG-4.
Ogoshi (1987) divided Rhizoctonia solani into 13 ISGs that differ in anastomosis behaviour, cultural appearance and pathogenicity. They are AG-IIA, AG-IIB, AG-IIC, AG-2-1, AG-2- 2 IIIb, AG-2-2 IV, AG-3, AG-4, AG-5, AG-6, AG-7, AG-8 and AG-BI.
The AG system is now considered as the most useful grouping system for Rhizoctonia solani. However, the behaviour of this fungus cannot be completely understood solely in terms of AGs. For example, isolates of AG-I cause not only sheath blight of rice but also web blight or leaf blight on many hosts.
The information regarding the affect of environmental factor on development of banded leaf and sheath blight disease of maize is very scanty. Thakur (1973) reported that the epiphytotic of the disease that broke out in low-lying altitudes of Himachal Pradesh was highly favoured by warm temp accompanied by high humidity.
Ahuja and Payak (1981) studied the relationship of temperature and RH with the development of disease under laboratory condition. They showed that temp between 25-35°C was most suitable for disease development whereas at 35CC only traces of mycelial growth were noticed without any symptom development and at 15 and 20°C no growth was observed.
RH of 100.0 per cent was shown most suitable for disease development whereas at 70.0 per cent RH, the disease was negligible or absent. The cardinal temperature for growth of the pathogen is 15, 25- 30 and 35°C.
Disease Management:
The main objective is to prevent economic crop losses caused by diseases and increase the value of the crop. Therefore, this disease could be managed effectively through Prevention and therapy.
3. Host Resistance of Rhizoctonia Solani:
Host resistance could be the most simple, practical, effective and economical preventive approach for management of this disease. Therefore, several attempts have been made successfully to develop resistant Varieties which are reviewed hereunder.
In an screening experiment in Orissa (India), Kar (1998) found inbred lines CM 177 and CM 211 as resistant to banded leaf and sheath blight Rhizoctonia solani f. sp. sasakii.
The inbreds, CM 122, CM 500 and CM 600 were moderately resistant, and CM 123 was moderately susceptible to Rhizoctonia solani f. sp. sasakii. Singh (1998) reported that the genotypes Navjot, Ganga 11, Prabhat and x 1266 showed resistance against the disease.
Balla (2000) reported that inbred lines CA-003134, CA-00396 and CA-00310 showed higher degree of tolerance to BLSB in other countries as well. Sharma (2002) reported management of BLSB through host plant resistance. They proposed that inbred lines CA-14510, CA-14524, Suwan-1 showed high degree of resistance.
Sharma (2003) conducted an experiment to evaluate 128 maize genotypes for resistance to banded leaf and sheath blight disease, 28 genotypes were resistant and 50 moderately resistant to the disease.
The resistant genotypes included 5 inbred lines, 3 extra early maturing, 6 early maturing, 7 medium maturing, and 7 full season maturing. Sharma (2003) screened a total of 44 elite inbred lines under artificial epiphytotic condition. Out of 44 inbred lines, 24 showed moderately resistant reaction and none of the lines were found resistant to the disease.
4. Fungicides of Rhizoctonia Solani:
Fungicides are used to create a chemical barrier between pathogen and host plant and a protectant or as therapeutic application to destroy the pathogen in or on the host pant instantly. This method is expensive but provides instant protection from devastating pathogens. Therefore, attempts have been made in past to manage this disease.
Puzari (1998) tested the antibiotic validamycin and the fungicides carbendazin (bavistin), captan (captaf), contaf, rhizolex, mancozeb, thiophanate-methyl and kavach against banded leaf and sheath blight of maize and reported that validemycin at the rate 0.1% as foliar spray gave the best disease management.
Sharma and Rai (1999) found rhizolex [tolelofoxmethyl] at the rate 10 g/10 litters and thiophanate at the rate 7 g/ 10 litters effective in controlling this disease on maize varieties A-112 and A-123.
Kumar and Jha (1999) tested the fungicides bavistin (carbendazim) at the rate 0.15%, bengard at the rate 0.1%, topsin M [thiophanate-methyl] at the rate 0.1%, kitazin (ipropenfos) at the rate 0.15%, captaf (captan) at the rate 0.2%, brassicol (quintozene) at the rate 0.2%, indohil M-45 (mancozeb) and thiophanate-methyl at the rate 0.2% on inoculated maize.
All the fungicides except blitox-50 were not only effective in reducing the disease severity but were also effective in increasing the grain yield. However, spraying of bavistin at the rate 0.1% resulted in the minimum disease severity and the maximum grain yield.
Meena (2003a) evaluated six fungicides in-vitro and in-vivo against banded leaf and sheath blight of maize caused by Rhizoctonia solani. Carbendazim and kitazin completely inhibited the mycelial growth of the test pathogen.
5. Bio-Control of Rhizoctonia Solani:
Among various fungal antagonists, Trichoderma spp have gained wide attention due to their ability to control many fungal pathogens on a variety of crop plants under greenhouse condition and field conditions as well as to their growth promotion effects on host plants.
The mechanisms of biocontrol agents for the control of plant diseases fail into three categories, viz., by the production of secondary metabolites suppressing disease causing pathogens, competition for the available nutrients and/ or induced systemic resistance in the host.
Rosales and Mew (1982) reported that there was significant reduction in lesion development when spore suspension of fungal bioagent (Trichoderma sp.) was sprayed on the pathogen (Rhizoctonia solani) inoculated rice plants.
Isolates of p. fluorescens used as biocontrol agent suppressed soil borne diseases caused by fungal pathogens. Sivakumar (2000) isolated Pseudomonas fluorescens PF-1 from the maize crop rhizosphere, which exhibited inhibitory action against Rhizoctonia solani causing banded leaf and sheath blight of maize in Himachal Pradesh, India.
Sharma and Saxena (2001) tested T. harzianum, T. viride Gliocladium virens and 4 strains of P. fluorescens against Rhizoctonia solani in-vitro. The fungal biocontrol agents started inhibiting the growth of Rhizoctonia solani after 7 days and covered the entire petridish within 10 days of inoculation.
Manisha (2002) recovered fluorescent Pseudomonads from rhizoplane (PEn -1) and rhizosphere of pea (PRS-1) and wheat (WRS-24) and evaluated in-vitro against Rhizoctonia solani. Field performance of biocontrol agents was assessed in maize against sheath blight caused by Rhizoctonia solani in Uttar Pradesh, India.
Meena (2003b) conducted several experiments to determine the efficacy of biocontrol agents, namely T. harzianum, T. viride, G. virens, P. fluorescens and Bacillus subtilis, against banded leaf and sheath blight disease of maize. The volatile activity of T. harzianum was effective in suppressing both the growth and sclerotia formation.
And, Sharma and Saxena (2002) assessed the efficacy of maize seed treatment with thiram, bioagents (Trichoderma harzianum, T. viride and T. virens), foliar spray with propiconazole and carbendazim, cultural practices such as stripping of the lower 3-4 leaves and leaf sheath in an integrated approach which could effectively manage banded leaf and sheath blight in maize.