M. C. Rush

Effect of Silicon and Host Resistance on Sheath Blight Development in Rice

Rice cultivars high in partial resistance (Jasmine, LSBR-5), moderately susceptible (Drew and Kaybonnet), and susceptible (Lemont and Labelle) to sheath blight were grown in a silicon-deficient Histosol with and without calcium silicate slag. The treatment with silicon increased the concentration of this element in plant tissue by 80%over all experiments. Fertilization with silicon significantly reduced the severity of sheath blight, and the total area under the vertical lesion extension progress curve on moderately susceptible and susceptible cultivars compared to those cultivars high in partial resistance without silicon. The percentage of infected tillers was significantly reduced by 82, 42, 28, 41, 26, and 17%respectively for Jasmine, LSBR-5, Drew, Kaybonnet, Lemont, and Labelle, when silicon was applied, over all experiments. Dry matter accumulation was significantly greater with added silicon. In the absence of disease, silicon enhanced dry matter accumulation by 15%over the control, whereas silicon more than doubled the mean dry matter accumulation in infected plants. The application of silicon to complement host resistance to sheath blight appears to be an effective strategy for disease management in rice, especially when the soil is low or limiting in plant-available silicon.

Burkholderia glumae : next major pathogen of rice?

UNLABELLED Burkholderia glumae causes bacterial panicle blight of rice, which is an increasingly important disease problem in global rice production. Toxoflavin and lipase are known to be major virulence factors of this pathogen, and their production is dependent on the TofI/TofR quorum-sensing system, which is mediated by N-octanoyl homoserine lactone. Flagellar biogenesis and a type III secretion system are also required for full virulence of B. glumae. Bacterial panicle blight is thought to be caused by seed-borne B. glumae; however, its disease cycle is not fully understood. In spite of its economic importance, neither effective control measures for bacterial panicle blight nor rice varieties showing complete resistance to the disease are currently available. A better understanding of the molecular mechanisms underlying B. glumae virulence and of the rice defence mechanisms against the pathogen would lead to the development of better methods of disease control for bacterial panicle blight. TAXONOMY Bacteria; Proteobacteria; Betaproteobacteria; Burkholderiales; Burkholderiaceae; Burkholderia. MICROBIOLOGICAL PROPERTIES Gram-negative, capsulated, motile, lophotrichous flagella, pectolytic. DISEASE SYMPTOMS Aborted seed, empty grains as a result of failure of grain filling, brown spots on panicles, seedling rot. DISEASE CONTROL Seed sterilization, planting partially resistant lines (no completely resistant line is available). KNOWN VIRULENCE FACTORS: Toxoflavin, lipase, type III effectors.

Tagging major quantitative trait loci for sheath blight resistance in a rice variety, Jasmine 85

This study was conducted with a clonal F2 population of rice from a cross between Jasmine 85, a resistant variety, and Lemont, a susceptible cultivar. The rice plants belonging to each F2 clone were divided into two plots, which were put in two replicates, respectively. Clonal parents were tested as controls. The plants were inoculated by short toothpicks incubated with RH-9, a virulent isolate of the pathogenic fungus,Rhizoctonia solani, which causes rice sheath blight. The extreme resistant and susceptible clonal lines were selected for construction of resistant and susceptible DNA pools, respectively. A total of 94 polymorphic markers evenly distributed on 12 rice chromosomes were used for bulked segregant analysis, three positive ones were found polymorphic between the two DNA pools, and three major QTLs for sheath blight resistance, Rh-2, Rh-3 and Rh-7, were identified. The three major QTLs were located on chromosomes 2, 3 and 7, and could explain 14.4%, 26.1% and 22.2% of the phenotypic variation.

Foliar Fungicides for Control of Rice Diseases in the United States

The most common and important foliar diseases of rice in the southern United States are sheath blight, blast, stem rot, brown leaf spot, narrow brown leaf spot and leaf smut. Fungicides have been evaluated for more than 20 years for efficacy against these diseases at the Rice Research Station in Crowley, Louisiana, USA in small plot experiments. Five fungicides are presently registered for commercial use. The commercial fungicides benomyl, propiconazole, iprodione, copper plus sulfur, and thiabendazole have received registrations since 1976. Benomyl, propiconazole, and Iprodione have similar activity against sheath blight but thiabendazole and copper plus sulfur are not recommended because of poor performance. Benomyl also has activity against blast and narrow brown leaf spot. Propiconazole gives excellent control of narrow brown leaf spot and leaf smut. Iprodione suppresses the brown leaf spot disease. Several new fungicides which are narrow spectrum and specific for sheath blight control will soon be registered.

Diversities in Virulence, Antifungal Activity, Pigmentation and DNA Fingerprint among Strains of Burkholderia glumae

Burkholderia glumae is the primary causal agent of bacterial panicle blight of rice. In this study, 11 naturally avirulent and nine virulent strains of B. glumae native to the southern United States were characterized in terms of virulence in rice and onion, toxofalvin production, antifungal activity, pigmentation and genomic structure. Virulence of B. glumae strains on rice panicles was highly correlated to virulence on onion bulb scales, suggesting that onion bulb can be a convenient alternative host system to efficiently determine the virulence of B. glumae strains. Production of toxoflavin, the phytotoxin that functions as a major virulence factor, was closely associated with the virulence phenotypes of B. glumae strains in rice. Some strains of B. glumae showed various levels of antifungal activity against Rhizoctonia solani, the causal agent of sheath blight, and pigmentation phenotypes on casamino acid-peptone-glucose (CPG) agar plates regardless of their virulence traits. Purple and yellow-green pigments were partially purified from a pigmenting strain of B. glumae, 411gr-6, and the purple pigment fraction showed a strong antifungal activity against Collectotrichum orbiculare. Genetic variations were detected among the B. glumae strains from DNA fingerprinting analyses by repetitive element sequence-based PCR (rep-PCR) for BOX-A1R-based repetitive extragenic palindromic (BOX) or enterobacterial repetitive intergenic consensus (ERIC) sequences of bacteria; and close genetic relatedness among virulent but pigment-deficient strains were revealed by clustering analyses of DNA fingerprints from BOX-and ERIC-PCR.

Comparative genomic analysis of two Burkholderia glumae strains from different geographic origins reveals a high degree of plasticity in genome structure associated with genomic islands

Burkholderia glumae is the major causal agent of bacterial panicle blight of rice, a growing disease problem in global rice production. To better understand its genome-scale characteristics, the genome of the highly virulent B. glumae strain 336gr-1 isolated from Louisiana, USA was sequenced using the Illumina Genome Analyser II system. De novo assembled 336gr-1 contigs were aligned and compared with the previously sequenced genome of B. glumae strain BGR1, which was isolated from an infected rice plant in South Korea. Comparative analysis of the whole genomes of B. glumae 336gr-1 and B. glumae BGR1 revealed numerous unique genomic regions present only in one of the two strains. These unique regions contained accessory genes including mobile elements and phage-related genes, and some of the unique regions in B. glumae BGR1 corresponded to predicted genomic islands. In contrast, little variation was observed in known and potential virulence genes between the two genomes. The considerable amount of plasticity largely based on accessory genes and genome islands observed from the comparison of the genomes of these two strains of B. glumae may explain the versatility of this bacterial species in various environmental conditions and geographic locations.

Evidence for the soilborne nature of the rice sheath rot and panicle blight pathogen, Burkholderia gladioli 1

Strains of Burkholderia gladioli were isolated from rice (Oryza sativa) field soil, and its association with rice sheath rot and panicle blight symptoms was demonstrated, both under greenhouse and field conditions for the first time. The B. gladioli strains were identified using Biolog, cellular fatty acid composition, polymerase chain reaction, and sequencing of both 16S rDNA and 16–23S rDNA internal transcribed spacer (ITS) regions. Inoculation of rice with B. gladioli strains isolated from soil caused vertical, linear gray lesions surrounded by dark brown margins on flag-leaf sheaths in greenhouse experiments and significant panicle blighting under field conditions.

Phylloplane Yeasts as Potential Biocontrol Agents for Rice Sheath Blight Disease

Rice is an important cereal crop in the world. More than 50% of the world’s population utilises rice for their main daily intake of carbohydrate. It is an important cash crop in several states in the United States. There are many constraints to rice production in the world with diseases being among the most significant due to the severity of losses and the lack of cost efficient control measures. Yield losses due to diseases range from 10% or more in the United States (Groth et al., 1991; Lee and Rush, 1983) to 20–25% in other countries (Ou, 1985; Rao, 1995; Shahjahan et al., 1986 ). Among the major fungal diseases of rice, sheath blight caused by Rhizoctonia. solani, is the most destructive necrotrophic disease (Lee and Rush, 1983; Ou, 1985; Rao, 1995; Rush and Lee 1992). The disease is endemic in nature and the pathogen overwinters as sclerotia in soil and in debris from the previous crop. The sclerotia and debris float to the surface of flood water and initiate infections on the lower leaf sheaths starting with the maximum tillering stage of growth. Under the conditions of high temperature and humidity prevailing during the panicle initiation to heading stages, sheath blight develops very rapidly. When the plants are heading, lesions may reach the flag leaf in as few as 10–15 days (Groth et al., 1991; Rush and Lee, 1992; Shahjahan and Mew, 1989). Most modern rice cultivars are susceptible to this disease. Commercial cultivars are susceptible or have low levels of partial resistance (Rush et al., 1995). Farmers presently use fungicides to control this disease in the United States (Groth et al., 1993) and in other countries (Ou, 1985), but this technology increases the cost of production as well as leaving potential pollutants in the environment. Fungicide use also affects the nontarget phylloplane microorganisms that naturally restrain pathogen development (Andrews and Kinnerly, 1978; Fokkema and de Nooij, 1981; Rush et al., 1991; Van Eeckhout et al., 1991). Because of its sustainable nature, scientists all over the world are now concentrating on developing biological methods of disease control.

Differential sensitivity of callus derived from immature panicles of rice cultivars to the non-specific toxin of Pyricularia grisea

An in vitro screening procedure was adapted to study the sensitivity of callus to the toxin picolinic acid of Pyricularia grisea in four rice cultivars. The lethal dose LD50 was determined on the basis of probit-log dosage response curve. The values of LD50 were 10, 51, 129 and 151 ppm for Tetep, Newbonnet, Labelle and M 201, respectively. The callus culture of cultivar Tetep, with a known broad spectrum of resistance, exhibited a high toxin sensitivity whereas the highly susceptible cultivar M 201 showed low sensitivity indicating the absence of relation between host plant specific resistance to blast and resistance of the callus to toxin.

Transgene Transfer to United States Commercial Rice Cultivars via Conventional Breeding Techniques

Abstract Breeding programs can benefit from transfer of a foreign gene from one transgenic plant to commercial cultivars through continuous backcrossing, especially to cultivars in which it is difficult to transfer a foreign gene directly through biotechnology. In this study, two homogeneous transgenic plants, T-28 and T-64 (from Taipei 309), and one homogeneous transgenic plant, N-84 (from Nipponbare), were used as donors of Bar gene. Commercial cultivars, Cypress and Laffite, were used as the recurrent parents. Three to five backcrosses were made using the transgenic plants as donor parents and the commercial cultivars as recurrent parent. The results from selected progeny rows, and two-years of yield tests with selected lines, indicated that the target Bar gene could be transferred to lines from homozygous transformants in 2–3 years of backcrossing, giving lines similar to the recurrent parents in phenotype and yield potential.