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Phytopathol. Mediterr. (2010) 49, 287−300
Corresponding author: L. Kredics
Fax: +36 62 544823
E-mail: kredics@bio.u-szeged.hu
Introduction
Rhizoctonia solani Kühn (teleomorph: Thanatephorus cucumeris [A.B. Frank) Donk.]) is a widespread soil-borne pathogen that causes economically important diseases in many crops (Adams, 1988). Rice sheath blight caused by R. solani is one of the most serious diseases of rice worldwide, causing considerable yield losses (Sudhakar et al., 1998). The widespread adoption of new, susceptible, high-yielding cultivars with large numbers of tillers, and the changes in cultural practices associated with these cultivars, favor the development of sheath blight and contribute greatly to the rapid increase in the incidence and severity of this disease in rice-producing areas throughout the world (Groth et al., 1991; Rush and Lee, 1992). Furthermore, environmental conditions such as low light, cloudy days, high temperature and high relative humidity also favor the disease (Ou, 1985). The pathogen overwinters as soil-borne sclerotia and mycelium in plant debris; these constitute the primary inoculum. Control of the pathogen is difficult because of its ecological behavior, its extremely
Key words: biocontrol, Oryza sativa, Rhizoctonia solani, Trichoderma harzianum, Trichoderma virens.
Summary. Sheath blight caused by Rhizoctonia solani is one of the most serious rice diseases worldwide. The disease is currently managed only by the excessive application of chemical fungicides which are toxic and not environmentally friendly. Therefore, greater emphasis should be given to biological control as being both safe and effective. Trichoderma species are ubiquitous fungi in the soil and have an antagonistic activity against several soil-borne plant pathogens including R. solani. The present study was undertaken to evaluate the potential of indigenous Trichoderma strains from Mazandaran province, Northern Iran (a Mediterranean region on the southern coast of the Caspian Sea) against R. solani AG1-IA in vitro, and against sheath blight disease in the glasshouse, in order to find biocontrol isolates for application in the field. More than 200 Trichoderma strains were isolated from the soil, plant debris and the phyllosphere in rice fields. Strains were first screened for their antagonism to R. solani by in vitro antagonism tests including dual culture, antibiosis, the effect of Trichoderma strains on the production and viability of R. solani sclerotia, and hyperparasitism on microscopic slides. According to the in vitro experiments, several strains belonging to T. harzianum, T. virens and T. atroviride showed excellent biocontrol. These potential antagonist strains were further evaluated for their effectiveness in controlling sheath blight under glasshouse conditions. Among the 55 selected strains, seven significantly controlled the disease. T. harzianum AS12-2 was the most effective strain in controlling rice sheath blight, better even than propiconazole, the most commonly used fungicide in Iran.
Shahram NAEIMI1, Sayyed Mahmood OKHO VVAT1, Mohammad JAVAN-NIKKHAH 1, Csaba VÁGVÖLGYI2,
Vahid KHO SRA VI3 and Lás zló KREDICS2
1 Department of Plant Protection, Faculty of Agriculture, University of Tehran, Karaj 3158711167, Iran
2 Department of Microbiology, Faculty of Science and Informatics, University of Szeged,
Közép fasor 52. H-6726 Szeged, Hungary
3 Deputy of Iranian Rice Research Institute in Mazandaran, P.O. Box 145, Amol, Iran
Biological control of Rhizoctonia solani AG1-1A,
the causal agent of rice sheath blight with Trichoderma strains
Phytopathologia Mediterranea
S. Naeimi et al.
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broad host range and the high survival rate of sclerotia under various environmental conditions (Groth et al., 2006). So far, no rice variety completely resistant to this fungus has been found, although extensive evaluation of rice germplasm has been conducted (Oard et al., 2004). In the absence of a desired level of host resistance, the disease is currently managed by excessive application of chemical fungicides, which have drastic effects on the soil biota, pollute the atmosphere, and are environmentally harmful. Some potentially effective fungicides are highly phytotoxic to rice and, if the disease is not severe, these fungicides may reduce yield (Groth et al., 1990). It is difficult to achieve control through host resistance or fungicides, therefore, biological control may be effective in minimizing the incidence of sheath blight (Das and Hazarika, 2000).
The anamorphic fungal genus Trichoderma (Hypocreales, Ascomycota) contains cosmopolitan soil-inhabiting fungi that are a major component of the mycoflora in soils of various ecosystems (Harman et al., 2004). The genus Trichoderma is especially known for its antagonistic activity against several plant pathogens, including R. solani (Papavizas, 1985; Chet, 1987; Harman and Björkman, 1998; Harman, 2006), and some strains are already commercialized as biocontrol agents (BCAs). These are also potential agents in suppressing rice sheath blight, they are highly competitive on rice plant residue and thus exhaust the nutrient supply for the pathogen and greatly reduce its survival (Mew and Rosales, 1984, 1985; Mostafa Kamal and Shahjahan, 1995).
Sheath blight is the most serious disease of high yielding rice cultivars in Mazandaran, the largest rice-growing province in Iran, on the southern coast of the Caspian Sea, which has a Mediterranean climate. The objective of this study was to evaluate the potential of indigenous Trichoderma isolates recovered from paddy rice fields in controlling R. solani, the rice sheath blight pathogen, in vitro and in vivo.
Materials and methods
Fungal isolates
Rhizoctonia solani RBL1, isolated from naturally infected rice plants with typical symptoms of sheath blight in a paddy field of Mazandaran province, Iran, was used in all experiments. R. solani strain RBL1 was obtained from the culture collection of the Iranian Rice Research Institute. The fungus was purified with the hyphal tip method and maintained on potato dextrose agar (PDA, Merck, Germany). To prove pathogenicity, inoculations were done in a glasshouse on Oryza sativa cv. Neda by placing a 5-mm mycelial plug of R. solani between the junction of the basal leaf sheath and the stem above the water line at the maximum tillering stage. R. solani was re-isolated from characteristic lesions of sheath blight. To confirm the anastomosis group (AG) and subgroup, a nuclear rDNA region, containing the ITS1 and 2 as well as the 5.8S gene (accession No. HM211085) was subjected to a BLAST search (Altschul et al., 1997) to find out the most similar sequences in the NCBI GenBank.
For the isolation of Trichoderma strains, soil samples were collected from rice fields located all over Mazandaran province (Figure 1), on the southern coast of the Caspian Sea. Soil was taken with an auger from a depth of 15 cm. Samples were air dried for 3–5 days at room temperature. Trichoderma isolates were obtained by the dilution plate method (Dhingra and Sinclair, 1995) on McFadden & Sutton’s RB-S-F Trichoderma selective medium (Davet and Rouxel, 2000). Sieved soil samples (10 g) were shaken in 90 mL sterile water for 10 minutes. For the isolation of Trichoderma from the rice phyllosphere, the leaves and stems were cut into small pieces (1 cm2), transferred to 500 mL Erlenmeyer flask with 100 mL sterile distilled water and placed on a shaker for one hour. A dilution series up to 10-6 was made from the samples. Aliquots (1 mL) were spread on Petri plates containing a selective medium and were then incubated at 25°C in the dark. Trichoderma isolates were also purified directly from fungal masses on rice debris. Putative Trichoderma colonies (from soil and foliage samples) were purified on PDA plates by the single spore method and deposited in the Microbiological Collection of the University of Szeged (SzMC).
DNA extraction and PCR conditions
For DNA preparation, a mycelium plug of each strain was placed on a cellophane disk sterilized by autoclaving in water and placed on the surface
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Biocontrol of rice sheath blight with Trichoderma
of yeast extract agar (5 g yeast extract, 5 g dextrose and 20 g agar L-1) for 2–3 days at room temperature. The fresh mycelium was scraped off and ground with a mortar and pestle in liquid nitrogen. Total DNA was extracted using the GenElute Plant Genomic DNA Miniprep Kit (Sigma-Aldrich, St. Louis, USA) according to manufacturer’s instructions.
A nuclear rDNA region containing the internal transcribed spacer (ITS) regions 1 and 2 and the 5.8S rRNA gene was amplified using the primers ITS1 and ITS4 (White et al., 1990). PCR amplifications were performed as described previously (Hermosa et al., 2000). Amplicons were purified with the GenElute PCR Clean-up Kit (Sigma-Aldrich) and sequenced at Macrogen Inc., Seoul, Korea. ITS sequences representing different ITS genotypes were submitted to the NCBI GenBank database (Table 1).
Species identification
A combination of morphological and molecular analysis was used for the identification of Trichoderma isolates. For morphological identification, strains were grown on 2% malt extract agar and on PDA under ambient laboratory conditions of light and temperature (about 21°C). Microscopic observations and measurements were made from preparations mounted in lactic acid. Individual isolates were identified at species level using standard mycological key (Gams and Bissett, 1998), species descriptions (Bissett, 1992; Samuels et al., 1999; Kraus et al., 2004), and TrichOKEY 2.0, an online method (http://www.isth.info/tools/molkey/index.php) for the quick and reliable molecular identification of Hypocrea and Trichoderma at the genus, clade, and species levels based on ITS1 and 2 sequences (Druzhinina et al., 2005). TrichOKEY identifies most species unequivocally, although some species have identical or very simi
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