Sheath Spot Research Articles

Stem and Sheath Diseases and Yield Response of Irrigated Rice Rotation Systems with Different Intensification Levels.

Rice cultivation in South America is undergoing several intensification processes for economic reasons that cause more rice to be planted continuously on the same soils. This intensification makes the long-term biological and economic sustainability of systems questionable. Among the most common problems that threaten sustainability in intensified systems is pest and disease pressure. In this context, the primary rice diseases were studied during a 6-year period in a long-term experiment of rice rotations established in 2012 in Uruguay. The experiment consisted of six rice rotations with other crops with different duration and intensification levels. The main disease found was stem rot and, to a lesser extent, aggregate sheath spot and sheath spot of rice. These diseases are of importance in intensified rice systems because they produce sclerotia that accumulate in soils. Disease occurrence was variable among years and rotations. Stem rot incidence increased rapidly in the experiment, reaching values close to 85% in the third year, while the severity varied each year. Sheath spot incidence and severity were low and varied with the year. Continuous rice (CR) reached the highest occurrence values for stem rot, but other systems with short pastures in rotation did not differ. Aggregate sheath spot and sheath spot were of importance only in CR and in some years. When levels of intensification were compared, systems with less time (25 to 60%) occupied with rice showed lower disease severity and higher yields. Comparing CR with rice rotation with pastures of different lengths (1 to 3 years), disease occurrence was higher in 2 of the 4 years analyzed. However, yields were consistently lower, denoting that other factors are influencing productivity. Stem and sheath diseases appear not to be a limiting factor in the medium term in intensified rice systems. Also, there are several alternatives of rice intensification incorporating crops and pastures in the rotation.

First report of Ceratorhiza oryzae-sativae, a fungus naturally causing aggregate sheath spot disease of rice in Manipur

Rice is cultivated and consumed worldwide being a cereal crop of chief importance. There are numerous soil-borne fungi causing diseases of huge economic losses to production of rice. A survey of soil-borne fungi of rice was conducted in different valley districts of Manipur, and symptoms appearing as lesions, water-soaked spots near the waterline which often coalesces, forming concentric lesion bands were spotted. Among the collected symptomatic samples, a fungus, Ceratorhiza oryzae - sativae was identified. An internal transcribed sequence of the isolated fungus was generated and an NCBI accession number, MH255604 acquired. This disease might become severe in near future in all rice-growing fields of the state, and identification of this pathogen shall be essential for developing the management strategies.

Fungi and Other Associates from Rice Fields of Bhor and Velhe Talukas of Pune District, Maharashtra State, India

Abstract: Plant pathogens are major cause of diseases in crops and plants. These plant pathogens devastated our crops and led us to devise methods and to search cultivars for resistant. Rice crops suffer from infection of many bacterial, viral and fungal diseases. The aim and objective of present study is to find out information about different plant pathogens caused by fungal infection. The survey of rice fields was carried out in Bhor and Velhe Talukas of Pune district. In case of rice crop, several diseases are reported such as Bacterial Blight, rice blast, Aggregate sheath spot, Black kernel Brown spot, Downy mildew, false smut, etc.

Interaction among sheath diseases complex of rice and ribosomal DNA analysis for the differentiation of Rhizoctonia solani, R. oryzae and R. oryzae-sativae

An experiment was conducted in the laboratory and field to study the interaction among three sheath diseases of rice namely sheath blight, sheath spot and aggregated sheath spot and their causal agents, Rhizoctonia solani, R. oryzae and R. oryzae-sativae. In addition to RFLP-PCR analysis and DNA sequencing of three sheath disease species, antagonism among the species was studied using dual culture technique. The results showed that antagonist, R. solani, inhibited radial growth against test fungi (R. oryzae and R. oryzae-sativae) from 37.08-46.49%. Disease incidence and severity of sheath blight was not reduced when R. solani was inoculated with either R. oryzae, R. oryzae-sativae, or a combination of the two. However, the incidence and severity of sheath spot and aggregated sheath spot were inhibited significantly when either R. oryzae or R. oryzae-sativae was inoculated with R. Solani. The highest percentage of relative lesion height and infected tiller was found at maturity followed by hard dough and flowering stage. Average yield losses ranged from 13.99-18.62 % when R. solani inoculated with both of R. oryzae-sativae and R. oryzae or any of them while R. solani alone caused 19.15% loss. DNA sequence analysis differentiated R. solani, R. oryzae and R. oryzae-sativae and phylogenetic tree revealed the close similarity of the species to the isolates of Philippines, Japan and Malaysia, respectively. In HhaI restriction enzyme, fragments 375 and 345 bp were specific for R. solani and 375, 235, and 120 bp was for R. oryzae-sativae which showed identical pattern across the isolates of two species.

Morphopathological and Molecular Morphometric Characterization of Waitea circinata var. prodigus Causing a Novel Sheath Spot Disease of Maize in India.

Maize brown sheath spot (MBSS), a new disease of maize, was discovered while surveying for maize leaf and sheath blight diseases in the Indian states of Assam, Jharkhand, Meghalaya, Manipur, and Odisha. Maize is the third most important cereal after rice and wheat in India. Unlike banded leaf and sheath blight disease caused by Rhizoctonia solani, MBSS symptoms on maize were discrete and limited to sheaths only. Symptoms of MBSS in the field were initially water-soaked necrotic lesions of 1 to 2 cm in diameter on the lowermost leaf sheaths, which then progressed to the upper sheaths. Lesions coalesced and covered approximately 2 to 5% of the sheath area. Infected dried lower leaves were shed, whereas infected upper leaves remained on the stem. The pathogen was isolated, characterized morphologically, pathologically, and molecularly, and identified as Waitea circinata var. prodigus, a basidiomycete known to cause basal leaf blight of seashore paspalum. The internal transcribed spacer (ITS) sequence 2 (ITS2) of rDNA from MBSS isolates formed a well supported clade with known W. circinata var. prodigus isolates. Molecular morphometric analysis of the ITS2 regions of the five known varieties of W. circinata detected distinguishing variations in GC content, compensatory base changes (CBCs), hemi- CBCs, indels, and altered base-pairing of helices. Variation in these characteristics may indicate that varieties are distinct biological species within W. circinata sensu lato. The geographical distribution and potential impacts of MBSS on the maize crop in India necessitate further investigations of pathogen identification and disease management.

Omnipresence of Partitiviruses in Rice Aggregate Sheath Spot Symptom-Associated Fungal Isolates from Paddies in Thailand.

Partitiviruses are one of the most prevalent double-stranded RNA viruses that have been identified mostly in filamentous fungi and plants. Partitiviruses generally infect host fungi asymptomatically but infrequently exert significant effect(s) on morphology and virulence, thus being considered a potential source of biological control agents against pathogenic fungi. In this study, we performed a screening for mycoviruses of a collection of Thai isolates of rice fungal pathogen Rhizoctonia oryzae-sativae, a causal agent of rice aggregated sheath spot disease. As a result, 36% of tested isolates carried potentially viral double-stranded RNAs with sizes ranging from 2 to 3 kbp. By conventional cDNA library construction and RNA-seq, we determined six new alphapartitiviruses that infected three isolates: tentatively named Rhizoctonia oryzae-sativae partitivirus 1 to 6 (RosPV1-6). Furthermore, RT-PCR detection of each virus revealed their omnipresent nature in different R. oryzae-sativae isolates. Although virus-curing of basidiomycetous fungi is generally difficult, our repeated attempts successfully obtained virus-free (for RosPV1, RosPV2, and uncharacterized partitiviruses), isogenic strain of R. oryzae-sativae TSS190442. The virus-cured strain showed slightly faster colony growth on the synthetic media and severe symptom development on the rice sheath compared to its virus-infected counterpart. Overall, this study shed light on the distribution of partitiviruses in R. oryzae-sativae in a paddy environment and exemplified a virus-curing protocol that may be applicable for other basidiomycetous fungi.

Phytotoxin of rice aggregate sheath spot pathogen Rhizoctonia oryzae-sativae and its biological activities.

Rice aggregate sheath spot disease occurs in many countries and causes serious yield losses. In China, the disease-causing fungus Rhizoctonia oryzae-sativae was reported in 1985, and since then, it has rarely been reported in major rice-growing areas after almost 30 years. Compared with Rhizoctonia solani, R.oryzae-sativae has a significantly different physiological morphology and growth status, although both fungi affect rice leaves in very similar ways. The optimum temperature for the suitable growth of R.oryzae-sativae is 31°C, which is consistent with previous reports. We extracted phytotoxins from R.oryzae-sativae and analyzed its biological activity via the detached leaf and radicle inhibition methods. Rhizoctonia solani and R.oryzae-sativae exhibit differences in terms of pathogenicity and toxin activity, which indicates that these fungi may produce different toxin components. Based on gas chromatography-mass spectrometry data, esters, phenols, and other components were present in the crude toxin extract of R.oryzae-sativae. Our research provides a new method for studying the phytotoxins of R.oryzae-sativae. However, further studies are needed to elucidate the pathogenic mechanisms responsible for aggregate sheath spot disease in rice.

Management of leaf and sheath spot (mini ring) on hybrid bermudagrass in Hawaii

AbstractLeaf and sheath spot (mini ring) is a turfgrass fungal disease that has been reported in some states in the USA. The causal pathogen is Waitea circinata var. zeae, also known as Rhizoctonia zeae. This disease typically causes thinned turf areas and/or rings of full, half, or quarter circles. Turfgrass under stress or irrigated with high salinity water is more prone to infestation. Seven fungicide programs were evaluated on ‘TifEagle’ hybrid bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt Davy] at a golf course on Maui, Hawaii. Four replications (plots) per treatment were arranged in a randomized complete block design. Disease severity and turfgrass quality were measured prior to treatment and after each application. Azoxystrobin showed the best suppression of disease and improvement of turfgrass quality, followed by chlorothalonil + acibenzolar‐S‐methyl. These two fungicide programs can be rotated to minimize possible resistance, and should also be used in conjunction with beneficial cultural practices to manage mini ring disease.

Nitrogen source impacts Rhizoctonia leaf and sheath spot severity in ultradwarf bermudagrass

AbstractThe disease Rhizoctonia leaf and sheath spot (RLSS), caused by Rhizoctonia zeae Voorhees, is problematic in ultradwarf bermudagrass (UDBG) [Cynodon dactylon (L.) Pers. × C. transvaalensis (Burtt‐Davy)] putting greens. Typically, RLSS symptoms are most severe in late summer and appear as frog‐eye patches 10 to 40 cm in diameter. In UDBG, RLSS is often called “mini‐ring” because of the small brown rings that are produced when symptoms are prominent. The effect of N source and N rate on RLSS severity was investigated in 2016 and 2017. Nitrogen sources, ammonium sulfate (AMS) [(NH4)2SO4] and urea (CH4N2O) were applied weekly during the growing season at rates of 4.9, 9.8, and 14.7 kg N ha−1. The study was conducted on ‘P18’ (MiniVerde) and ‘TifEagle’ UDBG cultivars maintained as research putting greens at the Clemson University Pee Dee Research and Education Center in Florence, SC. For both cultivars and in both years, urea decreased RLSS severity when compared to AMS. In November of 2016, RLSS severity increased with increasing rates of AMS; however, a rate response was not evident in 2017. In November of 2016 and 2017, RLSS severity was 25 and 30% when AMS was applied at 14.7 kg N ha−1 wk−1 verses 3 and 1% for UDBG receiving applications of urea at that same rate. These studies demonstrate that utilizing urea as an N source, rather than AMS, is an effective strategy to manage RLSS in UDBG.

Research on Integrated Algorithm Based on Convolutional Neural Network for Rice Disease Identification

Rice is the staple food in many Asian countries. The effective intelligent identification of rice diseases is helpful to improve the industrialized management of rice, reduce the input of manpower and improve the efficiency. In 2018, China’s first intelligent identification and service system for rice pests and diseases was released, but its average identification rate is only 80%. This paper takes 4 diseases of rice as the research object, adopts supervised learning, uses convolutional neural network to extract the characteristics of each disease, and integrates the extracted characteristics with AdaBoost algorithm to improve the accuracy. According to the research quantity and the characteristics of the research object, a 10-layer convolutional neural network and three convolution kernels are designed to miss the important information of the image as much as possible. According to the method in this paper, the correct recognition rate of the four diseases of Bacterial Blight, Flax Spot, Sheath Blight and Leaf Sheath Spot was 95.64% on average, which is higher than 5.56% and 14.69% respectively when the convolution neural network and AdaBoost algorithm are stable. Therefore, compared with the classical convolutional neural network and AdaBoost, the integrated algorithm of convolutional neural network proposed in this paper has a higher accuracy in rice disease research.

A broad diversity survey of Rhizoctonia species from the Brazilian Amazon reveals the prevalence of R. solani AG‐1 IA on signal grass and the new record of AG‐1 IF on cowpea and soybeans

AbstractLeaf blight, sheath blight, and web blight are major diseases caused by Rhizoctonia species on both Fabaceae and Poaceae plant hosts in the Brazilian Amazon agroecosystem. To determine the diversity of Rhizoctonia species associated with foliar diseases on fabaceous (cowpea and soybean) and poaceous (rice and signal grass [Urochloa brizantha]) hosts, a broad survey was conducted in Pará, Rondônia, Roraima, and Mato Grosso, in the Amazon, from 2012 to 2013. We extended our survey to Cerrado areas of Mato Grosso, and the lowlands of Paraíba Valley, in São Paulo, where these Rhizoctonia foliar diseases have not been reported so far. Our findings revealed that these diseases are caused by a diversity of Rhizoctonia solani AG‐1 complex. We detected that R. solani AG‐1 IA (sexual phase Thanatephorus cucumeris) was the predominant pathogen associated with signal grass leaf blight and collar rot diseases in the Amazon, especially in Rondônia and northern Mato Grosso. In addition, a subgroup of R. solani (AG‐1 IF), not previously reported in Brazil, was associated with leaf blight on cowpea and soybean, in Roraima. Another subgroup (AG‐1 ID) was also detected in Roraima. In Mato Grosso Cerrados we did not find any of the major Rhizoctonia foliar pathogens. Instead, R. oryzae (Waitea circinata) was the predominant species associated with a collar rot on U. brizantha. In the lowlands of São Paulo, R. oryzae‐sativae (Ceratobasidium oryzae‐sativae) was the predominant pathogen detected causing the rice sheath spot disease.

Morphological and Molecular characterization of rhizoctonia oryzae sativae in Bangladesh

Aggregate sheath spot disease of rice caused by Rhizoctonia oryzaesativae has emerged in higher incidence in North-Western region of Bangladesh. Thirty isolates of R. oryzae-sativae were studied by using morphological and molecular marker. Isolates were confirmed using specific primer where a single band of 1200bp was amplified. Two distinct groups relatively slow and faster were found in mycelal growth. Molecular characterization was done using four primers and DNA band ranged from 0.25 to 2.21 kb. A combined dendrogram was constructed which separated the isolates into three groups at 69.6% similarity level. All isolates placed in two major clusters except isolate RA-1 placed in cluster group III but were not grouped according to their geographic origins. Fast growing isolates have been placed in Group II while slow growing isolates in cluster group I. The similarity coefficient values of the dendrogram profile ranged from 0.36 to 0.98 with an average of 0.67. Diversity of different isolate showed that significant variation was present among the isolate and were not genetically identical.
 SAARC J. Agri., 16(2): 119-128 (2018)

First report of a novel alphapartitivirus in the basidiomycete Rhizoctonia oryzae-sativae.

Rhizoctonia oryzae-sativae is a soil-borne basidiomycete fungus that causes aggregate sheath spot disease on rice worldwide. Here, we report the complete genome sequence of a partitivirus designated as Rhizoctonia oryzae-sativae partitivirus 1 (RosPV1) infecting this fungus. The genome of RosPV1 consists of two double-stranded RNA(dsRNA) segments. The larger segment, designated as dsRNA-1 (1,961 bp), contains a single open reading frame (ORF) that encodes a putative polypeptide with a conserved RNA-dependent RNA polymerase (RdRp) domain. The smaller segment, dsRNA-2 (1,819bp), also has a single ORF, which is predicted to encode the capsid protein (CP). BLAST searches and phylogenetic analyses suggested that RosPV1 is a representative member of a new species within the genus Alphapartitivirus. This is the first report of an alphapartitivirus infecting the fungus R. oryzae-sativae.

Characterization of Rhizoctonia Species Complex Associated with Rice Sheath Disease in Karnataka

Sheath disease of rice is a complex biotic stress, caused by Rhizoctonia solani (sheath blight), R. oryzae (sheath spot) and R. oryzae-sativae (aggregate sheath spot). To determine the extent of distribution of these pathogen species, eighteen isolates of Rhizoctonia spp. were collected from different rice growing agroclimatic zones of Karnataka, India. Causal organisms were isolated from the diseased samples and identified based on morphological and molecular tools. Mycelial characters and sclerotial morphology were considered for species differentiation at culture level. Later, species identity was confirmed through polymerase chain reactions using previously standardized Rhizoctonia species-specific primers. Out of eighteen isolates, 14 isolates characterized as R. solani, two as R. oryzae and remaining two as R. oryzae-sativae. The results indicated that all three species of Rhizoctonia, viz. R. solani, R. oryzae and R. oryzae-sativae, are involved in causing rice sheath disease in Karnataka and R. solani was found predominantly distributed. Though sheath diseases caused by R. oryzae and R. oryzae-sativae are regarded as minor, their complementary role with R. solani in causing severe sheath disease epidemics has to be investigated. Accuracy in distinguishing these pathogens is essential to ensure the success of breeding programmes, which aim to develop rice varieties with resistance to sheath disease complex. As per the information available, this is the first report on the distribution of three species of Rhizoctonia associated with rice sheath disease in Karnataka.

Resistance to Multiple Temperate and Tropical Stem and Sheath Diseases of Rice

Stem rot and aggregated sheath spot are the two major stem and sheath diseases affecting rice (Oryza sativa L.) in temperate areas. A third fungal disease, sheath blight, is a major disease in tropical areas. Resistance to these diseases is a key objective in rice breeding programs but phenotyping is challenged by the confounding effects of phenological and morphological traits such as flowering time (FT) and plant height (PH). This study sought to identify quantitative trait loci (QTL) for resistance to these three diseases after removing the confounding effects of FT and PH. Two populations of advanced breeding germplasm, one with 316 tropical japonica and the other with 325 indica genotypes, were evaluated in field and greenhouse trials for resistance to the diseases. Phenotypic means for field and greenhouse disease resistance, adjusted by FT and PH, were analyzed for associations with 29,000 single nucleotide polymorphisms (SNPs) in tropical japonica and 50,000 SNPs in indica. A total of 29 QTL were found for resistance that were not associated with FT or PH. Multilocus models with selected resistance-associated SNPs were fitted for each disease to estimate their effects on the other diseases. A QTL on chromosome 9 accounted for more than 15% of the phenotypic variance for the three diseases. When resistance-associated SNPs at this locus from both the tropical japonica and indica populations were incorporated into the model, resistance was improved for all three diseases with little impact on FT and PH.

Morphological and Genetical Study on Rhizoctonia Sheath Disease Complex of Rice in Bangladesh

Morphological and molecular based experiments were conducted to identify the presence of Rhizoctonia solani, R. oryzae and R. oryzae-sativae which were the causal agents of sheath blight and sheath blight-like symptoms associated with sheath disease complex of rice. Disease survey was conducted at Bogra, Comilla, Gazipur and Rajshahi Districts in Bangladesh. The data collected revealed that sheath blight was the most important disease (19.8%) followed by aggregated sheath spot (2.4%), and sheath spot (2.3%). Three pathogens of the disease complex were identified morphologically by their spot size, general appearance of spot, number of spot, position in plant, sclerotium visibility and sclerotial size on infected plant. Those three species of Rhizoctonia of rice sheath disease complex were confirmed by diagnostic-PCR with their corresponding reference isolates by using specific primers GMRS-3, GMRO-3, GMROS-2 to Rhizoctonia solani, R. oryzae and R. oryzae-sativae, respectively. Molecular approaches might overcome the limitations of morphological characterization particularly when huge samples were needed to distinguish.

Characterization, Pathogenicity, and In Vitro Sensitivity of Rhizoctonia spp. Associated with Leaf and Sheath Spot of Bermudagrass Putting Greens in North Carolina and Alabama

Twenty isolates of a fungus resembling Rhizoctonia zeae were obtained from three golf courses with bermudagrass [Cynodon dactylon (L.) Pers.] greens that were exhibiting leaf and sheath spot symptoms in 2013 and 2014. Morphological and molecular identification showed that 17 isolates were identified as R. zeae and three were identified as Waitea circinata var. zeae. Pathogenicity of the 20 isolates was determined by inoculating hybrid bermudagrass (‘Champion’) plants in a growth chamber. After inoculation, plants were incubated in a growth chamber at 34/30°C (day/night) and sealed in a closed container to ensure 100% humidity. After 7 d, disease was visually estimated using a modified Horsfall‐Barrett scale (0–10). All isolates were pathogenic to the hybrid bermudagrass cultivar Champion and aggressiveness among isolates varied. Sensitivity of seven isolates collected in 2013 to 16 fungicides (flutolanil, pyraclostrobin, fluoxastrobin, azoxystrobin, propiconazole, triticonazole, triadimefon, penthiopyrad, fluxapyroxad, azoxystrobin + difenoconazole, tebuconazole, difenoconazole, iprodione, polyoxin‐D zinc salt, fluxapyroxad + pyraclostrobin, and chlorothalonil) was tested. The sensitivity of 13 isolates collected in 2014 to five fungicides (chlorothalonil, azoxystrobin, pyraclostrobin, difenoconazole, and flutolanil) was tested. In general, all the isolates were extremely sensitive to demtheylation and succinate dehydrogenase inhibitor fungicides (50% effective concentration [EC50] < 1 mg a.i. L−1) and all isolates showed insensitivity to quinone outside inhibitor fungicides (EC50 > 10 mg a.i. L−1). These results demonstrated that R. zeae is a pathogen of ultradwarf bermudagrass under the imposed environmental conditions. Rhizoctonia zeae is sensitive to many fungicides, but based on in vitro sensitivity data presented here, some may not perform well under field conditions.

Population Genetic Structure of Rhizoctonia oryzae-sativae from Rice in Latin America and Its Adaptive Potential to Emerge as a Pathogen on Urochloa Pastures

The fungus Rhizoctonia oryzae-sativae is an important pathogen that causes the aggregated sheath spot disease on rice. In this study, we investigated the genetic structure of rice-adapted populations of R. oryzae-sativae sampled from traditional rice-cropping areas from the Paraíba Valley, São Paulo, Brazil, and from Meta, in the Colombian Llanos, in South America. We used five microsatellite loci to measure population differentiation and infer the pathogen's reproductive system. Gene flow was detected among the three populations of R. oryzae-sativae from lowland rice in Brazil and Colombia. In contrast, a lack of gene flow was observed between the lowland and the upland rice populations of the pathogen. Evidence of sexual reproduction including low clonality, Hardy-Weinberg equilibrium within loci and gametic equilibrium between loci, indicated the predominance of a mixed reproductive system in all populations. In addition, we assessed the adaptive potential of the Brazilian populations of R. oryzae-sativae to emerge as a pathogen to Urochloa spp. (signalgrass) based on greenhouse aggressiveness assays. The Brazilian populations of R. oryzae-sativae were probably only incipiently adapted as a pathogen to Urochloa spp. Comparison between RST and QST showed the predominance of diversifying selection in the divergence between the two populations of R. oryzae-sativae from Brazil.

Effects of combined application of potassium phosphite and fungicide on stem and sheath disease control, yield, and quality of rice

Phosphite has been shown to suppress some diseases in different plant species but disease control in rice has not been investigated. In 2012/13 and 2013/14 potassium phosphite was sprayed at 1775 g ha−1 and 3550 g ha−1 in field plots of rice in Uruguay to determine if phosphite alone or in combination with label rates of a mixed strobilurin and triazole fungicide could be used to effectively control stem rot and aggregate sheath spot in irrigated rice. Six treatments consisting of different combinations of potassium phosphite alone or with a fungicide and an unsprayed control were assayed in one application at late-boot to early-heading. Phosphite alone in single and double rate slightly reduced severity and incidence of stem rot and produced a small yield increase over the unsprayed control. Fungicide at a 50% label rate with phosphite reduced stem rot severity and incidence to a similar level as the fungicide alone applied at the label rate. Disease severity was reduced by approximately 25% and incidence by 17–20% when compared with untreated control. Yield increase was 5% for both treatments over the unsprayed control. Fungicide combined with phosphite at single and double rates reduced stem rot severity by 40–45% and incidence by 34–38% when compared with untreated control, with yield increased by 10% over the unsprayed control and 5% over plots treated with a fungicide. These results indicate that a single application of potassium phosphite combined with fungicide can be used efficiently to manage of rice stem diseases.

Comparison of Phenotyping Methods for Resistance to Stem Rot and Aggregated Sheath Spot in Rice

Stem and sheath diseases caused by Sclerotium oryzae Cattaneo (SCL) and Rhizoctonia oryzae‐sativae Sawada Mordue (ROS) can severely reduce rice (Oryza sativa L.) yield and grain quality. Genetic resistance is the best strategy to control them. Phenotypic selection for resistance is hampered due to a heterogeneous distribution of the inoculum in the soil that generates high environmental variability and decreases genetic gain. To have higher selection accuracy it is necessary to develop phenotyping methods with high repeatability and discriminative power. Comparison of greenhouse methods have been reported for Rhizoctonia solani Kühn, a more invasive pathogen than SCL and ROS, and for SCL, but no such comparisons are reported for ROS. Our study compares five inoculation methods for SCL and ROS to identify the more discriminant and repeatable method and to apply it for high‐throughput phenotyping of hundreds of rice lines. A method that uses an agar disc with growing mycelium attached to the base of stems was found to have the best balance between discrimination among genotypes and variability among replicates of the same genotype for both pathogens. This method was used in five greenhouse experiments for phenotyping resistance to SCL and ROS in a population of 641 rice advanced breeding lines. Heritabilities of resistance ranged from 0.36 to 0.71 in these experiments. These findings have a direct application in screening for resistance of rice to SCL and ROS, and in high‐throughput phenotyping for mapping loci associated to disease resistance.