M. C. Filippi

Genetic and phenotypic diversity of Magnaporthe oryzae from leaves and panicles of rice in commercial fields in the State of Goiás, Brazil

Genetic and phenotypic structure of Magnaporthe oryzae populations of two upland rice cultivars was determined. Monoconidial isolates were obtained from rice blast affected fields, four from cv. BRS Bonanca and four from cv. Primavera, in Goias State (2001-2003). The pathotypes IB-41 and IB-9 were predominant in both leaf and panicle isolates of BRS Bonanca and IF-1 in Primavera. A great majority of pathotypes were common to both leaf and panicle subpopulations of Bonanca (42.8%) and Primavera (66.6%). The ANOVA of virulence data showed high variability within population of each cultivar. There was no significant difference in virulence pattern of isolates from leaves and panicles, independent of collection site and cultivar. The molecular characterization of isolates was done employing the rep-PCR analysis with two primer sequences from Pot2. The genetic analysis of 538 isolates showed a high genotypic diversity in both leaf and panicle pathogen populations with 103 haplotypes in Bonanca and 49 in Primavera. The migration of pathotypes from leaves to panicles in each field was 70.8% and 36.6% for Primavera and BRS Bonanca, respectively. The diversity of M. oryzae population was influenced by cultivar of origin. A great amount of population diversity was encountered within the same field.

Increased enzymatic activity in rice leaf blast suppression by crude extract of Epicoccum sp

Epicoccum sp. showed in vitro antagonism to the rice pathogens Magnaporthe oryzae, Rhizoctonia solani, Sarocladium oryzae, Monographella albescens and Cochliobolus miyabeanus in dual culture. The colony growth of the rice blast fungus, M. oryzae, was reduced by 42.50%. The lethal doses (LD50) determined based on probit-log dosage response curves at 3 and 6 hours after germination were 393.0 and 326.6 ppm, respectively. The crude extract (600 ppm) reduced appressorial formation by 95.68%. A greenhouse experiment comparing the relative efficiency of conidial suspension and crude extract of Epicoccum sp. in the suppression of leaf blast showed no statistical difference between both application methods. However, the crude extract of Epicoccum sp. (4000 ppm) 48 hours before the application of M. oryzae induced resistance and suppressed leaf blast by 97.6%. Scanning electron microscopy of rice leaves inoculated with crude extract of Epicoccum sp. and challenged with M. oryzae showed appressorial deformation on penetration phase. Peroxidase and β-1,3-glucanase activities increased in plants sprayed with crude extract of Epicoccum sp., 24 hours after the application of the challenger. PAL as well as chitinase activities increased 72 hours after challenge inoculation. Epicoccum sp. was shown to be a potential antagonist and inducer of resistance against M. oryzae.

Alterations in Gas Exchange and Oxidative Metabolism in Rice Leaves Infected by Pyricularia oryzae are Attenuated by Silicon.

Rice blast, caused by Pyricularia oryzae, is the most important disease in rice worldwide. This study investigated the effects of silicon (Si) on the photosynthetic gas exchange parameters (net CO2 assimilation rate [A], stomatal conductance to water vapor [gs], internal-to-ambient CO2 concentration ratio [Ci/Ca], and transpiration rate [E]); chlorophyll fluorescence a (Chla) parameters (maximum photochemical efficiency of photosystem II [Fv/Fm], photochemical [qP] and nonphotochemical [NPQ] quenching coefficients, and electron transport rate [ETR]); concentrations of pigments, malondialdehyde (MDA), and hydrogen peroxide (H2O2); and activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and lypoxigenase (LOX) in rice leaves. Rice plants were grown in a nutrient solution containing 0 or 2 mM Si (-Si or +Si, respectively) with and without P. oryzae inoculation. Blast severity decreased with higher foliar Si concentration. The values of A, gs and E were generally higher for the +Si plants in comparison with the -Si plants upon P. oryzae infection. The Fv/Fm, qp, NPQ, and ETR were greater for the +Si plants relative to the -Si plants at 108 and 132 h after inoculation (hai). The values for qp and ETR were significantly higher for the -Si plants in comparison with the +Si plants at 36 hai, and the NPQ was significantly higher for the -Si plants in comparison with the +Si plants at 0 and 36 hai. The concentrations of Chla, Chlb, Chla+b, and carotenoids were significantly greater in the +Si plants relative to the -Si plants. For the -Si plants, the MDA and H2O2 concentrations were significantly higher than those in the +Si plants. The LOX activity was significantly higher in the +Si plants than in the -Si plants. The SOD and GR activities were significantly higher for the -Si plants than in the +Si plants. The CAT and APX activities were significantly higher in the +Si plants than in the -Si plants. The supply of Si contributed to a decrease in blast severity, improved the gas exchange performance, and caused less dysfunction at the photochemical level.

Induction of leaf blast resistance in rice by an avirulent isolate of Magnaporthe oryzae

The induction of disease resistance can be activated in plants with biotic and abiotic agents such as avirulent isolates of pathogens. The objective of this investigation was to determine the concentration of inducer isolate and the time required between the inoculation with inducer (avirulent isolate) and virulent isolate of Magnaporthe oryzae, for the induction of resistance in rice leaves. Eighteen-day-old plants of rice cultivars Metica-1 and Cica-8 were sprayed in a greenhouse with an avirulent isolate utilized as inducer, at the concentrations of 0, 105, 3x105 and 6x105 conidia mL-1; sprayed 24, 48, and 72 hours prior to inoculation with virulent isolate. The induction of resistance was manifested in the reduction of leaf area affected and lesion type. The degree of induced resistance was greater in cultivar Metica-1 than in cultivar Cica-8 in relation to respective controls. The induction of resistance was superior in Cica-8 in treatments where inducers were applied 48 hours prior to inoculation with the virulent isolate, at concentrations of 6x105 and 3x105 conidia mL-1. On the other hand, the induction in Metica-1 was significantly higher in relation to control, at all concentrations and all periods of application of the inducer, and did not show differences between treatments.

RAPD analysis of blast resistant somaclones from upland rice cultivar IAC 47 for genetic divergence

Seventeen somaclones of upland rice cultivar IAC 47 showing different plant types, and either resistance or susceptibility to leaf blast, were utilized for random amplified polymorphic DNA (RAPD) analysis. Somaclones exhibited differences in reaction to isolates of Pyricularia grisea. Two somaclones (SC02 and SC04) were resistant to all three field isolates of somaclones, while the cultivar IAC 47 was susceptible. The inheritance study of two distinct plant types, one with erect bright green leaves and the other with droopy yellow green leaves, showed that a single possibly different, dominant gene governs each plant type. Of 32 random decamer primers utilized, OPA02 and OPD02 detected polymorphisms between somaclones showing erect bright green leaves and droopy yellow green leaves. Reliable grouping exhibiting 80% similarity was achieved with 17 primers. Leaf blast resistance to race IC-2 of P. grisea was associated with the plant type of erect bright green leaves.

Silicon potentiates the activities of defense enzymes in the leaf sheaths of rice plants infected by Rhizoctonia solani

This study aimed to assess the ability of silicon (Si) to potentiate defense enzyme activities in rice leaf sheaths and thus reduce sheath blight, caused by Rhizoctonia solani, development. Rice plants of BR-Irga 409 and Labelle cultivars were grown in a hydroponic solution containing 0 (-Si) or 2 mM (+Si) Si and inoculated with R. solani. Silicon concentration in the leaf sheaths was significantly higher in the +Si plants than the -Si plants by 727% for BR-Irga 409 and 714% for Labelle. The area under relative lesion expansion progress curve was significantly lower for +Si plants than -Si plants, by 34.2% for BR-Irga 409 and 30.59% for Labelle. Increases in the activities of phenylalanine ammonia-lyases, peroxidases, polyphenoloxidases and chitinases in the leaf sheaths of plants supplied with Si, especially of those from BR-Irga 409, led to reduced progress of sheath blight lesions.

Biology of Gaeumannomyces graminis var. graminis isolates from rice and grasses and epidemiological aspects of crown sheath rot of rice

A collection of Gaeumannomyces graminis var. graminis isolates obtained from symptomatic rice and grass plants in central and northern Brazil were studied in regard to pathogenicity on rice and cultural characteristics. For all isolates, only one type of lobed hyphopodia was observed both in the field and artificially inoculated plants. Perithecia were formed on artificial media and inoculated leaf sheaths. Hyphopodia were formed from ascosporic germ tubes. The hyphae under moist stress conditions produced dark brown chlamydospores that were initially hyaline. Large variation in the number of chlamydospores produced and duration of perithecia formation was observed among the isolates. Pathogenicity assays showed that 60-day old rice plants were more susceptible than 35-day old plants. The isolates from rice and grasses varied significantly in regard to disease severity on both rice seedlings and adult rice plants. In general, the isolates from rice were more aggressive than the isolates from grasses. Spontaneous infection of rice plants by ascosporic inoculum from perithecia on rice stubbles was observed in the greenhouse, suggesting their role as a source of primary inoculum in the field, which deserves further investigation.

Trichoderma reduces scald and protects the photosynthetic apparatus in rice plants

ABSTRACT Scald reduces the photosynthetic area, causing yield losses in rice. Changes in gas exchange parameters caused by the pathogen begin before the onset of symptoms. Chemical methods are most commonly applied to control this disease; further research into biological control methods is required. Since Trichoderma asperellum induces plant pathogen defences, increases growth, and improves photosynthetic capability, this study investigated the efficacy of T. asperellum (Ufra T06, UfraT09, Ufra T12, and Ufra T52 (Ta)) in reducing the scald lesion size and the area under the disease progress curve and in minimising the negative effects of scald on gas exchange, chlorophyll a fluorescence, chlorophyll content, and oxidative stress enzyme activity. The experiment was a completely randomised design with five replications and two treatments. Scald was reduced by 62% in plants treated with T. asperellum compared with that in control. There was a 62% increase in the net CO2 assimilation rate (A) and a drop of 78% in the transpiration rate (E) in plants treated with T. asperellum. The maximum fluorescence (Fm) was 128% higher, and ascorbate peroxidase activity was also higher in plants treated with T. asperellum than in the control. This shows that the use of T. asperellum may be effective in improving the sustainability of the integrated management of rice diseases.