Binucleate Rhizoctonia Isolates Research Articles

Isolation, identification, and pathogenicity of Rhizoctonia spp. recovered from soil samples of the greenhouse tomato growing area of the West Mediterranean region, Türkiye

AbstractThis study's objective was to identify the anastomosis groups and pathogenicity of Rhizoctonia species from soil samples in tomato production areas of Antalya, Burdur, and Isparta provinces in the West Mediterranean Region of Türkiye during the 2022 season. Soil samples were gathered from tomato plant rhizospheres in 223 different greenhouses, and isolations were done from the soil using the baiting technique. According to the results of morphological characterization and sequence analysis of the internal transcribed spacers of the ribosomal DNA, 47 Rhizoctonia isolates were defined as binucleate and 21 isolates as multinucleate. The collection of binucleate Rhizoctonia (Ceratobasidium sp.) isolates included representatives of Anastomosis Group‐F (61.76%), AG‐A (1.47%), and an unidentified binucleate Rhizoctonia (Ceratobasidium sp.) (5.88%). Multinucleate R. solani included AG 4 HG‐I (26.47%) and AG 4 HG‐II (4.41%). Pathogenicity tests showed statistically significant differences in virulence between binucleate Rhizoctonia (Ceratobasidium sp.) isolates and multinucleate Rhizoctonia isolates. All R. solani AG 4 HG‐II isolates and most of the AG 4 HG‐I isolates were pathogenic, causing severe necrosis symptoms and damping off in tomato seedlings, but binucleate AG‐A and AG‐F isolates were nonpathogenic. This study is the first detailed report about Rhizoctonia species isolated from the soil of tomato plants grown in greenhouse production areas in Türkiye.

Characterization and report of binucleate Rhizoctonia AG‐Fb affecting soybean in Brazil

AbstractRhizoctonia root rot is a disease that can affect soybean leading to plant death usually in bare patches. Frequently, Rhizoctonia solani has been reported as the main causal agent of the disease. However, due to the great genetic diversity of the genus Rhizoctonia, there are other species that could also attack soybean plants. Thus, the objective of this study was to determine the pathogenicity and characterize an isolate of binucleate Rhizoctonia obtained from a soybean plant with symptoms of root rot cultivated in soil from southern Brazil. The isolate was identified based on the morphology and phylogenetic analysis of the internal transcribed spacer region (ITS) of the rDNA. Pathogenicity tests were then performed on seeds and roots of soybean and the optimum temperature for mycelial growth was determined. The isolate was identified as binucleate Rhizoctonia AG‐Fb and it was pathogenic to soybean seeds and roots. In the pathogenicity assay in soybean seed, the isolate reduced seed germination by 20% and caused seed rot index of 69.3. In the pathogenicity assay in soybean root, the inoculation of binucleate Rhizoctonia AG‐Fb caused 67.3 root rot severity index, and reduced the emergence speed index by 19%, seedling height by 31%, root dry mass by 25% and root volume by 43% in soybean seedlings. The optimum temperature for the mycelial growth of the isolate was 26°C. To the best of our knowledge, this is the first report of binucleate Rhizoctonia AG‐Fb associated with soybean root rot in Brazil.

Anastomosis Grouping and Phylogenetic analysis of Rhizoctonia Isolates on Wheat in Türkiye

The aim of this study is to determine the species and to evaluate the genetic diversity of the pathogenic and nonpathogenic Rhizoctonia spp. and anastomosis groups (AG) from wheat plants and rhizosphere soils in Turkey. Rhizoctonia species were isolated from plants and rhizosphere soils in wheat fields in 5 provinces in the Central Anatolian Region of Türkiye. As a result of the isolations, a total of 88 multinucleate (MN) and binucleate (BN) Rhizoctonia isolates were obtained. Identifications of the isolates were determined by rDNA-ITS sequence analyzes. The identified isolates belonged to MN Waitea circinata var. zeae, W. circinata var. oryzae, W. circinata var. circinata, MN Rhizoctonia solani AG 2-1, 2-2, 3, 4 HGII, 4 HGIII, 5, 8, 11 and BN AG A, DI, E, G, H, I, I-like, K. The most isolated group was W. circinata var. circinata. In pathogenicity studies, the most virulent group was determined as R. solani AG 4. Among the binucleate isolates, groups other than R. cerealis AG DI were not found to be pathogenic. Neighbor-joining phylogenetic trees of isolates were constructed from rDNA-ITS sequences. As a result of this study, the regional distribution of MN and BN Rhizoctonia AG isolates in important wheat production areas in the Central Anatolia Region, Turkey was determined. In addition, this study is the first comprehensive study in which the genetic diversity of Rhizoctonia AGs isolates obtained from wheat and rhizosphere soils in the region was evaluated with a molecular approach.

Anastomosis Groups of Rhizoctonia solani and Binucleate Rhizoctonia Associated with Potatoes in Idaho.

A survey of the relative incidence of anastomosis groups (AGs) of Rhizoctonia spp. associated with potato disease was conducted in Idaho, the leading potato producing state in the U.S.A. In total, 169 isolates of Rhizoctonia solani and seven binucleate Rhizoctonia (BNR) isolates were recovered from diseased potato plants. The AG of each isolate was determined through real-time PCR assays for AG 3-PT and phylogenetic analysis of the internal transcribed spacer region of ribosomal DNA. AG 3-PT was the predominant AG, accounting for 85% of isolates recovered, followed by AG 2-1 (5.7%) and AG 4 HG-II (4.5%). Two different subsets of AG 2-1 isolates were recovered (subset 2 and 3). Three isolates each of AG A and AG K were recovered, as well as one isolate each of AG 5 and AG W. An experiment carried out under greenhouse conditions with representative isolates of the different AGs recovered from Idaho potatoes showed differences in aggressiveness between AGs to potato stems, with AG 3-PT being the most aggressive followed by an isolate of AG 2-1 (subset 3). The three BNR isolates representative of AG A, AG K, and AG W appeared to be less aggressive to potato stems than the R. solani isolates except for the AG 2-1 (subset 2) isolate. This is the first comprehensive study of the relative incidences of Rhizoctonia species associated with Idaho potatoes and the first study to report the presence of BNR AG W outside of China.

Effect of Rhizoctonia spp. on Polyphenol Oxidase Enzyme Activity in Alfalfa Seedling

Soil borne diseases cause significant losses on quantity and quality of many crop species annually. Rhizoctonia is a widespread and ecologically diverse soilborne fungus causing different types of diseases in many plant species including alfalfa (Medicago sativa). Rhizoctonia species have been traditionally identified based on the cell nuclear condition. Polyphenol oxidases (PPOs) are ubiquitous copper-containing enzymes that are widely occurring enzymes among plants. PPOs are involved in the oxidation of polyphenols into quinones (antimicrobial compounds) and lignification of plant cells that contribute to the formation of defense barriers against pathogens. The study was conducted with the aim to determine the effect of indigenous isolates of a multinucleate (Rhizoctonia solani AG-4) and nineteen isolates of binucleate Rhizoctonia on PPO activity in alfalfa (cv. Gea) seedling under in vitro conditions. The activity of PPO enzyme was determined in inoculated and uninoculated control alfalfa plants after ten days from inoculation. There was a significant increase in the activity of PPO after treatment of alfalfa seedling with isolates of Rhizoctonia. Among Rhizoctonia isolates, highest induction of PPO activity was recorded with pathogenic R. solani AG-4. In present study, increased amounts of PPO were also observed in plants that were challenged with Rhizoctonia spp. PPO has a role in catalyzing phenolic oxidation in limiting disease development. PPO may therefore be involved in induction of defense resistance against plant diseases.

A Binucleate Rhizoctonia anastomosis group (AG-W) is the causal agent of sugar beet seedling damping-off disease in China

Two binucleate Rhizoctonia (BNR) isolates (HLJ-21 and HLJ-57) were recovered from diseased sugar beet seedlings in Heilongjiang Province, Northeastern China in May, 2015. The two isolates were identified as members of the anastomosis group (AG)-W based on morphological characteristics, hyphal anastomosis, polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP), and phylogenetic analysis of the internal transcribed spacer of the ribosomal DNA (rDNA ITS). The optimum temperature for hyphal growth was 25 °C and the mycelia retained a whitish appearance on potato dextrose agar as cultures aged. No sclerotia, however, were observed during the period of culture. Both of the isolates exhibited anastomosis with AG-W, but failed with other reference strains of BNRs. Restriction fragments resulting from the eight enzymes indicated that HLJ-21 and HLJ-57 isolates had the same restriction pattern as AG-W isolates. Phylogenetic analysis also indicated that the rDNA ITS region of the HLJ-21 and HLJ-57 isolates clustered with other AG-W isolates in the same clade. The AG-W isolates from sugar beet and potato were the causal agent of damping-off disease on sugar beet seedlings as well as root disease on wheat and corn seedlings. The AG-W isolates from sugar beet could also induce stem canker disease on potato plants. Notably, the isolates from sugar beet seedlings had a higher virulence on all of the tested crops than those isolates obtained from potato. This appears to be the first report of a AG-W BNR causing sugar beet seedling damping-off disease in China, and perhaps globally.

Phenotypic and Molecular Analyses of Rhizoctonia spp. Associated with Rice and Other Hosts.

Forty-two Rhizoctonia isolates were collected from rice, mung bean, and grasses from Laguna, Philippines. Sixteen isolates were binucleate Rhizoctonia (BNR), while 26 were multinucleate Rhizoctonia (MNR). BNR isolates produced white to brown, small sclerotia (<1.0 mm) except for mung bean isolates. Twenty MNR isolates produced big (>1.0 mm), light to dark brown sclerotia, three produced salmon-colored masses in the medium, and three did not produce sclerotia. Twenty-three MNR isolates were identified as R. solani AG1-IA using specific primers. Deduced Internal Transcribed Spacer (ITS) sequences of BNR isolates D1FL, NVL, and ScNL shared 100, 97, and 100% identity with R. oryzae-sativae, respectively, while MNR isolates BMgL, IbMgL, and MaSL that produced salmon-colored masses shared 100, 90, and 100% identity with R. oryzae, respectively. Preliminary analysis of the DNA fingerprint patterns generated by repetitive-element PCR (rep-PCR) clustered the 42 isolates into three: R. solani, R. oryzae-sativae, and R. oryzae, together with Ceratobasidium sp. R. solani isolates were pathogenic on rice (TN1), barnyard grass, mungbean (Pagasa 3), and tomato (Athena), while R. oryzae and R. oryzae-sativae isolates were only pathogenic on rice, Echinochloa crus-galli, and tomato. R. solani and R. oryzae were found to be more virulent than R. oryzae-sativae.

Induction of Systemic Resistance in Cucumber by Hypovirulent Binucleate Rhizoctonia against Anthracnose Caused by Colletotrichum orbiculare.

Treatment with hypovirulent binucleate Rhizoctonia (HBNR) isolates induced systemic resistance against anthracnose infected by Colletotrichum orbiculare in cucumber, as there were no direct interaction between HBNR and C. orbiculare. This is because of the different distances between HBNR and C. orbiculare, where the root was treated with HBNR isolate and C. orbiculare was challenged and inoculated in leaves or first true leaves were treated with HBNR isolate and C. orbiculare was challenged and inoculated in second true leaves. The use of barley grain inocula and culture filtrates of HBNR significantly reduced the lesion diameter compared to the control (p = 0.05). The total lesion diameter reduction by applying barley grain inoculum of HBNR L2, W1, W7, and Rhv7 was 28%, 44%, 39%, and 40%, respectively. Similar results was also observed in treatment using culture filtrate, and the reduction of total lesion diameter by culture filtrate of HBNR L2, W1, W7, and Rhv7 was 45%, 46%, 42%, and 48%, respectively. When cucumber root was treated with culture filtrates of HBNR, the lignin was enhanced at the pathogen penetration, which is spread along the epidermis tissue of cucumber hypocotyls. Peroxidase activity in hypocotyls in the treated cucumber plant with culture filtrates of HBNR significantly increased before and after inoculation of pathogens as compared to the control. Significant enhancement was also observed in the fast-moving anodic peroxidase isozymes in the treated plants with culture filtrates of HBNR. The results showed the elicitor(s) contained in culture filtrates in HBNR. The lignin deposition as well as the peroxidase activity is an important step to prevent systemically immunised plants from pathogen infection.

Characterization and pathogenicity of Rhizoctonia spp. isolated from kiwifruit in the Middle and Eastern Black Sea region of Turkey

AbstractIn this study, 58 Rhizoctonia isolates were obtained from plants exhibiting symptoms of root and stem rot in kiwifruit orchards in six different provinces in the Black Sea region of Turkey. The isolates were analysed for their cultural characteristics, anastomosis groups (AGs) and subgroups according to nucleotide sequence variations in the internal transcribed spacer (ITS) regions including the 5.8S gene of nuclear ribosomal DNA (rDNA) and pathogenicity. Based on ITS1‐5.8S‐ITS2 rDNA sequencing, 52 binucleate (BN) Rhizoctonia isolates were identified as belonging to AG‐O (17), AG‐L (1), AG‐G (7), AG‐I (1), AG‐A (9), AG‐Fa (9), AG‐Fb (1), AG‐R (3) and AG‐P (4). Of the remaining 6 isolates, three belonged to multinucleate (MN) R. solani AG 4 HG‐I, one belonged to AG 4 HG‐II, one belonged to AG 5 and one belonged to AG 1‐IB. The sequences of these isolates were aligned and compared with other Rhizoctonia sequences retrieved from GenBank (NCBI), and phylogenetic analysis was used to determine evolutionary relationships among them. All isolates used in the temperature experiments grew between 15–35°C, and their growth was completely inhibited at 5 and 40°C, with a few exceptions. In the pathogenicity tests carried out using 58.6% of the isolates on kiwifruit seedlings, it was determined that there were statistically significant virulence differences among BN and MN Rhizoctonia isolates (p &lt; 0.05). The virulence of BN Rhizoctonia ranged from 0.2 to 4.0, whereas that of MN varied from 2.4 to 4.0. Unlike some isolates of AG‐R, AG 4 HG‐II and AG 1‐IB, which had no significant difference between them in virulence, AG‐P and AG 4 HG‐I isolates significantly reduced plant height, shoot and root dry weights and root length when compared to control plants. To our knowledge, it is the first detailed report of AGs of BN and MN Rhizoctonia, the causal agent of root and stem rot on kiwifruit in Turkey and in the world.

Rhizoctonia fragariae causes black root rot on strawberry seedlings in Turkey

Samples of strawberry transplants were randomly collected from 47 growers in Aydin province of Turkey. A total of 10 Rhizoctonia isolates were identified by morphological characterization, analysis of rDNA-ITS sequences. Pathogenicity tests were carried on strawberry seedlings and detached stolon. Ten pathogenic binucleate Rhizoctonia isolates were defined as AG-A, AG-G and AG-K on the basis of ITS region sequence analysis. To our knowledge, this is the first report of the presence of Rhizoctonia fragariae in strawberry transplants in Turkey.

Comparison of the virulence and cognate virulence factors of multinucleate, binucleate and uninucleate Rhizoctonia isolates, causing sheath blight on maize plants

Maize sheath blight is a destructive disease of maize world-wide. The causal pathogens of the disease include multinucleate, binucleate and uninucleate Rhizoctonia isolates according to the latest reports. In this study, the virulence and virulence factors of one uninucleate, two binucleate and nine multinucleate Rhizoctonia isolates were compared. The results showed that there was significant difference in the virulence of the tested Rhizoctonia isolates on maize, with multinucleate Rhizoctonia isolates the most virulent, binucleate Rhizoctonia isolates moderate and uninucleate Rhizoctonia isolate the lowest. The cell wall-degrading enzymes (CWDEs) and phytotoxins of the tested isolates were extracted and their enzymatic or biological activities were detected. The correlation coefficients between the enzymatic activity of polymethylgalacturonase (PMG), polygalacturonase (PG) and endo-β-1,4-glucanase (Cx) and pathogenicity of the isolates were 0.8210, 0.7314 and 0.8497, respectively. The correlation coefficients between the inhibition rate of corn seed germination and root growth caused by the phytotoxins and pathogenicity of the isolates were 0.9257 and 0.9690, respectively. These results indicated that CWDEs and phytotoxins likely act as virulence factors during infection. In conclusion, the lowest virulence of the uninucleate Rhizoctonia isolate JN on maize is possibly due to the low activities of CADEs and phytotoxins.

Anastomosis Groups and Pathogenicity of Rhizoctonia solani and Binucleate Rhizoctonia from Potato in South Africa.

A survey of anastomosis groups (AG) of Rhizoctonia spp. associated with potato diseases was conducted in South Africa. In total, 112 Rhizoctonia solani and 19 binucleate Rhizoctonia (BNR) isolates were recovered from diseased potato plants, characterized for AG and pathogenicity. The AG identity of the isolates was confirmed using phylogenetic analysis of the internal transcribed spacer region of ribosomal DNA. R. solani isolates recovered belonged to AG 3-PT, AG 2-2IIIB, AG 4HG-I, AG 4HG-III, and AG 5, while BNR isolates belonged to AG A and AG R, with frequencies of 74, 6.1, 2.3, 2.3, 0.8, 12.2, and 2.3%, respectively. R. solani AG 3-PT was the most predominant AG and occurred in all the potato-growing regions sampled, whereas the other AG occurred in distinct locations. Different AG grouped into distinct clades, with high maximum parsimony and maximum-likelihood bootstrap support for both R. solani and BNR. An experiment under greenhouse conditions with representative isolates from different AG showed differences in aggressiveness between and within AG. Isolates of AG 2-2IIIB, AG 4HG-III, and AG R were the most aggressive in causing stem canker while AG 3-PT, AG 5, and AG R caused black scurf. This is the first comprehensive survey of R. solani and BNR on potato in South Africa using a molecular-based approach. This is the first report of R. solani AG 2-2IIIB and AG 4 HG-I causing stem and stolon canker and BNR AG A and AG R causing stem canker and black scurf on potato in South Africa.

Controlling Rhizoctonia Damping-off of Chinese Mustard by Using Endomycorrhizal Rhizoctonia spp. Isolated from Orchid Mycorrhizae

Inoculation of hypovirulent Rhizoctonia spp. has been recognized as an effective strategy for protecting plants against damping-off caused by pathogenic Rhizoctonia spp. In this study, endomycorrhizal Rhizoctonia spp. isolated from fungal pelotons in orchid plants were used for controlling Rhizoctonia damping-off of Chinese mustard. According to phylogenetic analysis and anastomosis group (AG) determination, the virulence of three isolates of multinucleate Rhizoctonia solani in AG-6; eight isolates of binucleate Rhizoctonia in AG-A, AG-B, AG-G, AG-P, and AG-R; and two isolates of binucleate R. repens were evaluated using test plants. All isolates, except that in AG-R, caused low disease severity in 10-day-old radish (0.10 to 0.61), cucumber (0.28 to 0.54), and Chinese mustard (0.18 to 0.65). By contrast, pathogenic isolates in AG-4 killed almost all test plants with symptoms of collapsed hypocotyl and wilted leaves (0.88 to 0.96). Of the 13 endomycorrhizal Rhizoctonia isolates assessed, AG-P isolates Cno10-3 and CalS1-2 provided 91 and 100% protection, respectively, against R. solani AG-4 in 26-day-old Chinese mustard. This study revealed that endomycorrhizal Rhizoctonia spp. in orchid have the potential to biologically control damping-off of Chinese mustard.

Characterization of a New Anastomosis Group (AG-W) of Binucleate Rhizoctonia, Causal Agent for Potato Stem Canker.

Two binucleate Rhizoctonia (BNR) isolates were recovered from potato cankered stems in Heilongjiang Province, northeastern China. Their cultural appearance on potato dextrose agar remained whitish as the cultures aged. White monilioid cells formed in the fluffy aerial hyphae, whereas no sclerotia appeared during the incubation. The two isolates could anastomose with each other, but they failed to anastomose with reference strains of BNR from AG-A to AG-Q, and AG-U. Analyses of restriction fragment length polymorphism (RFLP) of internal transcribed spacer of ribosomal DNA (rDNA-ITS) regions confirmed that these two isolates differed from the reference strains. The phylogenetic tree based on the sequences of rDNA-ITS regions showed that they were located in a distinct clade from other BNR AGs. These collective results suggested that the isolates recovered from potato in this study belonged to a new BNR AG designated as AG-W. Pathogenicity tests under glasshouse conditions revealed that both isolates were able to cause brown, dry, and slightly sunken lesions on potato subterranean stems. To our knowledge, this is the first report of the AG-W causing potato disease in China as well as worldwide.

Molecular identification and pathogenicity of Rhizoctonia spp. from tobacco growing areas in northwestern Argentina

In Argentina, more than 60 % of the tobacco crops are grown in the northwestern part of the country and where Rhizoctonia solani leads to a reduction in crop yield and quality. In this study, 35 isolates of Rhizoctonia were obtained from 32 tobacco fields in northwestern Argentina and characterized by both morphological and molecular approaches. Based on the variability in the ITS region, isolates were identified as R. solani (80 %), Waitea circinata var. zeae (Rhizoctonia zeae) (8 %) and binucleate Rhizoctonia (8 %). Most isolates of R. solani belonged to the anastomosis groups (AGs) AG 4 HG-I (44 %), AG 2-1 (41 %) and AG 4 HG-III (13 %). Isolates of binucleate Rhizoctonia belonged to AG-F and AG-P of Ceratobasidium sp. Morphological variability was higher within isolates of AG 2-1 and AG 4 HG-III than within those of AG 4 HG-I. Aggressiveness of the isolates towards tobacco seedlings was assessed in the greenhouse. Isolates of AG 2-1 were the most aggressive on leaves, causing target spot, whereas isolates of AG 4 HG-I were the most aggressive on stems and roots, causing damping-off.

Potato stem canker caused by binucleate Rhizoctonia AG-G in China

A binucleate Rhizoctonia (BNR) isolate was recovered from potato subterraneous stems showing canker symptoms, which was collected from Chongqing municipality in southwestern China. This isolate was identified as anastomosis group (AG)-G based on cultural appearance, hyphal anastomosis and sequence analysis of the internal transcribed spacers of ribosomal DNA. Pathogenicity tests under glasshouse conditions revealed that the AG-G isolate induced brown, dry and slightly sunken lesions on potato stems. Necrotic areas covered less than 10 % of the surface of underground stems. This is the first report of BNR AG-G causing stem canker on potato plants in China.

A nonpathogenic species of binucleate Rhizoctonia inhibits the formation of infection structures caused by Rhizoctonia solani on cucumber

Binucleate Rhizoctonia (BNR) W7 was evaluated for the efficacy to control damping-off disease in cucumber seedlings. Results revealed that Lobate appresoria and dome-shaped infection cushion were produced by virulent Rhizoctonia solani C9 AG4 on the surface of the cucumber seedling hypocotyls, causing damping-off, 12 h after inoculation. A significant positive correlation was observed between disease severity rating and number of infection cushions (r = 0.94) and appresoria (r = 0.97) on the hypocotyls of seedlings inoculated with virulent R. solani C9 alone. Inoculation of a nonpathogenic species of BNR prior to virulent R. solani significantly reduced the number of appresoria on hypocotyls surface and inhibited the formation of infection cushions that accompanied by highly significant (P ≤ 0.01) reduction in disease severity in comparison with seedling inoculated with R. solani C9 alone. Formation of mucilaginous material upon recognition of the nonpathogenic isolate of BNR followed by hyphal lysis of BNR W7 6 h after inoculation was responsible for reduction in appresorium formation and complete inhibition of infection cushion formation. However, the number of infection structures produced on cucumber was not significantly different for hypocotyls of seedlings inoculated with R. solani C9 with and without the nonpathogenic BNR W7.

Investigating binucleate Rhizoctonia induced defence responses in kidney bean against Rhizoctonia solani

Four isolates of binucleate Rhizoctonia spp. (BNR), including AG-Bb, AG-Ba, AG-G and AG-A, were evaluated for pathogenicity on kidney beans. AG-Bb was the least virulent on kidney bean, and it was used as biocontrol agent against damping-off caused by Rhizoctonia solani AG-4 HG-II. The inoculation of the bean seedlings with BNR (AG-Bb) reduced disease symptoms in the following treatments: seedlings inoculated with BNR and R. solani simultaneously and also in seedlings inoculated with BNR and R. solani with a one-week interval. An investigation into different time points showed that BNR-induced resistance gradually decreased and disappeared after two weeks. Enzyme assays showed that the peroxidase (POX) and catalase (CAT) activities of bean seedlings increased after R. solani inoculation. The greatest activity of POX and CAT was discovered in seedlings inoculated with BNR and R. solani simultaneously. Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed a correlation between the expression of bean POX and CAT genes and the activity of the corresponding enzymes. The inoculation of leaf discs with BNR revealed the induction of resistance to R. solani. The leaf discs pretreated with sodium azide (NaN3) as POX and CAT inhibitors had less resistance to R. solani compared to the control. These results showed the importance of the former enzymes to induce resistance in bean plants. BNR inoculation leads to H2O2 accumulation in leaf discs which is the main reactive oxygen species (ROS). H2O2 can act as a defence signal and also lead to an increase in the solidity of cell walls. BNR stimulates bean seedlings to produce antioxidant enzymes and ROS which can increase bean resistance to R. solani.

Anastomosis groups and pathogenicity of binucleate Rhizoctonia isolates associated with stem canker of potato in China

Potato stem canker caused by Rhizoctonia solani commonly occurs in potato-growing regions around the world, but little is known about whether binucleate Rhizoctonia (BNR) can incite this disease on potato plants in China. In the present study, a total of 69 BNR isolates were recovered from cankered subterraneous stems of potato collected from 17 provinces, autonomous regions, and municipalities of China. Based on the morphological characteristics and sequence analysis of the internal transcribed spacers of the ribosomal DNA, 48 isolates were classified as anastomosis group (AG)-A with a ratio of 69.56 %, 15 isolates (21.74 %) were AG-K, four isolates (5.80 %) were AG-F, one isolate (1.45 %) was AG-I and one isolate (1.45 %) was AG-U. Pathogenicity tests under glasshouse conditions revealed that all BNR isolates, except for the AG-I isolate, could induce symptoms of stem canker on potato plants with disease incidence ranging from 5.26 to 55.56 % and disease index ranging from 1.32 to 13.89, respectively. To our knowledge, this is the first detailed report about BNR AGs causing stem canker on potato plants in China.

English

The interaction of binucleate Rhizoctonia (BNR) anastomosis group (AG)-A isolate W7, Rhizoctonia solani AG-4 and cucumber seedlings were investigated to elucidate the mechanism of biocontrol of Rhizoctonia solani by BNR. Hypocotyls of Cucumis sativus L. cv. Jibai were inoculated with a virulent isolate of R. solani AG-4 isolate C4 and examined with light microscopy and scanning and transmission electron microscopy. The hyphae of the virulent isolate colonized the outer surface of the hypocotyl and penetrated the epidermal and cortical cells to the pith of hypocotyls. The hyphae of a non-pathogenic species of BNR isolate W7 did not penetrate the cortical cells but instead colonized the outer epidermal cells 12 h after inoculation. Accumulation of mucilage occurred on the surface after dense colonization of the hypocotyls surface by the BNR. Observation of the hypocotyls surface pre-inoculated with BNR and challenged inoculated with R. solani showed constricted and deformed hyphae of R. solani , which were prevented to penetrate the epidermal cells. Pre-inoculating of BNR isolate caused profused formation of the mucilage which lysed BNR hyphae and restricted the growth of R. solani . The mucilage was identified as pectic substances. Analysis of pectin contents from dried hypocotyls tissues showed highly methylated pectin with low uronic acid contents in the seedlings inoculated with R. solani and unprotected by BNR while BNR- treated seedlings showed less methylated pectin and high amount of uronic acid. Furthermore, the BNR-inoculated seedlings showed significant ( P = 0.05) increase in calcium, indicating high amount of wall-bound cations in cell walls. Pectic substances accumulation and increased calcium in the cucumber tissues greatly contributed in the protection of cucumber seedlings against Rhizoctonia damping off by a non-pathogenic species of Rhizoctonia. Keywords: Rhizoctonia solani , binucleate Rhizoctonia , cucumber, biological control African Journal of Biotechnology , Vol. 13(5), pp. 640-650, 29 January, 2014