Oryzae Populations Research Articles

Effect of azoxystrobin and kresoxim‐methyl on rice blast and rice grain yield in China

AbstractSensitivity to azoxystrobin and kresoxim‐methyl of 80 single‐spore isolates of Magnaporthe oryzae was determined. The EC50 values for azoxystrobin and kresoxim‐methyl in inhibiting mycelial growth of the 80 M. oryzae isolates were 0.006–0.056 and 0.024–0.287 µg mL−1, respectively. The EC50 values for azoxystrobin and kresoxim‐methyl in inhibiting conidial germination of the M. oryzae populations were 0.004–0.051 and 0.012–0.105 µg mL−1, respectively. There was significant difference in sensitivity to azoxystrobin or kresoxim‐methyl between the tested isolates representing differential sensitivity to carbendazim (MBC) and kitazin P (IBP); however, there was no correlation between this difference in sensitivity to azoxystrobin or kresoxim‐methyl and sensitivity to MBC or IBP, indicating that there was no cross‐resistance between azoxystrobin or kresoxim‐methyl and MBC or IBP. In the protective and curative experiments, kresoxim‐methyl exhibited higher protective and curative activity than azoxystrobin when applied at 150 and 250 µg mL−1 accordingly, while azoxystrobin exhibited stronger inhibitory activity against M. oryzae isolates than that of kresoxim‐methyl in the in vitro test. The results of field experiments also suggested that both azoxystrobin and kresoxim‐methyl at 187.5 g.a.i. ha−1 gave over 73% control efficacy in both sites, exhibiting excellent activity against rice blast. Taken together, azoxystrobin and kresoxim‐methyl could be a good substitute for MBC or IBP for controlling rice blast in China, but should be carefully used as they were both at‐risk.

Mating Types Analysis of Magnaporthe oryzae Populations by Molecular Methods

Rice blast is a devastating disease which is caused by the heterothallic fungus Magnaporthe oryzae. Compatible sexual recombination which occurs between two M. oryzae strains of different mating types, can enhance genetic variability. Assessment of mating type alleles is used as a marker to measure population diversity. Forty six isolates of M. oryzae were collected from infected rice leaves from various ecosystems of coastal Odisha, India, and the mating type analysis using molecular markers was carried out. MAT1-1 mating type was dominating in all the ecosystems and MAT1-2 was found to be present in uplands as well as in irrigated fields. Both mating types could be found in the same field in irrigated ecosystem. The disease spread was very fast vertically as well as horizontally in those fields resulting in blast lesions looking as ‘green islands (gi) produced in senescence leaves’, and MAT1-2 was found to be associated with all gi lesions. Consequently, the management of the disease in those plots was very difficult. Interestingly, ribosomal RNA IGS region could not be amplified in MAT1-2 isolates but consistent amplification was obtained in MAT1-1 mating type isolates.

Sensitivity of Nonexposed and Exposed Populations of Magnaporthe oryzae from Rice to Tricyclazole and Azoxystrobin.

Magnaporthe oryzae is the major pathogen of cultivated rice worldwide, which can cause substantial losses to rice production. Rice blast management is based predominantly on the application of fungicides; however, only a little is known about responses of pathogen populations to the most widely used fungicides. In this work, the baseline sensitivity of the Italian M. oryzae population to tricyclazole and azoxystrobin in terms of mycelium growth was determined, and the possible adaptation of the pathogen population after several years of repeated exposure to fungicide treatments was evaluated. All the analyzed strains demonstrated higher sensitivity and variability to azoxystrobin (concentration of fungicide causing 50% growth inhibition [ED50] = 0.063 mg liter-1) than to tricyclazole (99.289 mg liter-1). After comparing two additional populations collected from fields repeatedly treated with fungicides to the baseline, no decrease in sensitivity toward these fungicides was observed and no resistant strains were detected. The shift of the pathogen sensitivity toward these fungicides has not occurred, although we observed slightly increased variance associated with ED50 of azoxystrobin. Therefore, both azoxystrobin and tricyclazole can be used to manage rice blast in Italy but it will be important to continue monitoring M. oryzae population to early detect possible azoxystrobin resistance.

DNA typing and virulence determination of Xanthomonas oryzae pv. oryzae population for the management of bacterial leaf blight of rice in Udham Singh Nagar, India

Bacterial leaf blight caused (BLB) by Xanthomonas oryzae pv. oryzae (Xoo) is the most serious biotic constraint of rice in Udham Singh Nagar (U.S. Nagar), India. We have experienced severe BLB outbreaks, causing total crop failure during 2007–2010. The severity of losses incurred due to BLB in U.S. Nagar necessitates the development of an ecology-conscious, effective and durable management strategy. Our interests here were focused to obtain information on the haplotypic and pathogenic variability of Xoo to develop effective breeding strategy for BLB resistance. Our results indicate that the currently existing population of Xoo in U.S. Nagar is highly virulent and genetically diverse. Twenty one haplotypes were detected among a collection of 193 strains by each of the PCR technique used i.e. rep-PCR and IS-PCR. The high value of total haplotypic diversity (HT =0.79) reflected the genetic heterogeneity of Xoo population infecting popular varieties. Strains responsible for severe BLB outbreaks were grouped into eight distinct lineages with significant bootstrap values (p > 75). Virulence assay revealed the presence of 11 pathotypes. The interaction of lines with strains was found significant (p < 0.01) confirming the pathogenic specialization of Xoo in U.S. Nagar. Most of the single major resistance genes used in rice breeding program at International Rice research Institute (IRRI) was defeated by Xoo strains, at the same time; effectiveness of Xa21 to all the pathotypes is encouraging. The potential of Xa21 alone or in combination with xa13 and xa5 could be exploited for pyramiding into well-adapted rice cultivars for the effective management of the pathogen in this region. This is the first report on the population structure of Xoo in U.S. Nagar, India.

Genetic structure of various Magnaporthe oryzae populations in Iran and Uruguay

Rice blast, caused by Magnaporthe oryzae is one of the most destructive rice diseases worldwide. The genetic variability of 55 M. oryzae isolates from Iran and 32 isolates from Uruguay was analyzed using amplified fragment length polymorphism (AFLP). Cluster analysis using different grouping methods and similarity coefficients, based on the AFLP data from 679 monomorphic and polymorphic bands generated with eight primer combinations, was performed. The resulted grouping of the isolates revealed 4 separate AFLP lineages among a total of 87 isolate, designated as A1, A2, B, and C. Within each AFLP lineage, three or more clones were detected with a genetic similarity of 100 %. Overall genetic similarity was greater than 78 % between Iranian and Uruguayan populations. Evidence of gene flow between the Iranian and Uruguayan populations obtained from indica rice cultivars was observed as one Iranian isolate grouped lineage 1A together with Urugyayan isolates. Analysis of molecular variance (AMOVA) revealed that rice varietal type, sampling year and geographic region were the dominant factors determining genetic structure of M. oryzae populations; but rice cultivar had not significant effect.

South-East Asia is the center of origin, diversity and dispersion of the rice blast fungus, Magnaporthe oryzae.

Inferring invasion routes and identifying reservoirs of diversity of plant pathogens are essential in proposing new strategies for their control. Magnaporthe oryzae, the fungus responsible for rice blast disease, has invaded all rice growing areas. Virulent genotypes regularly (re)emerge, causing rapid resistance breakdowns. However, the world-wide genetic subdivision of M. oryzae populations on rice and its past history of invasion have never been elucidated.In order to investigate the centers of diversity, origin and migration of M. oryzae on rice, we analyzed the genetic diversity of 55 populations from 15 countries.Three genetic clusters were identified world-wide. Asia was the center of diversity and the origin of most migrations to other continents. In Asia, two centers of diversity were revealed in the Himalayan foothills: South China–Laos–North Thailand, and western Nepal. Sexual reproduction persisted only in the South China–Laos–North Thailand region, which was identified as the putative center of origin of all M. oryzae populations on rice.Our results suggest a scenario of early evolution of M. oryzae on rice that matches the past history of rice domestication. This study confirms that crop domestication may have considerable influence on the pestification process of natural enemies.

Comparative Analysis of the Korean Population of Magnaporthe oryzae by Multilocus Microsatellite Typing.

Rice blast fungus, Magnaporthe oryzae, inflicts serious damage to global rice production. Due to high variability of this fungal pathogen, resistance of newly-released rice cultivars is easily broken down. To understand the population structure of M. oryzae, we analyzed the genetic diversity of the Korean population using multilocus microsatellite typing. Eleven microsatellite markers were applied to the population of 190 rice isolates which had been collected in Korea for two decades since the 1980’s. Average values of gene diversity and allele frequency were 0.412 and 6.5, respectively. Comparative analysis of the digitized allele information revealed that the Korean population exhibited a similar level of allele diversity to the integrated diversity of the world populations, suggesting a particularly high diversity of the Korean population. Therefore, these microsatellite markers and the comprehensive collection of field isolates will be useful genetic resources to identify the genetic diversity of M. oryzae population.

Molecular diversity in rice blast resistance gene Pi-ta makes it highly effective against dynamic population of Magnaporthe oryzae

Rice blast is one of the important diseases of rice which can be effectively managed by the deployment of resistance genes. Pi-ta is one of the major blast resistant genes effective against pathogen populations in different parts of India. We analysed allelic variants of Pi-ta from 48 rice lines selected after phenotyping of 529 rice landraces across three eco-geographical blast hot spot regions. Besides, Pi-ta orthologue sequences of 220 rice accessions belonging to wild and cultivated species of rice were also included in the study for a better evo-devo perspective of the diversity present in the gene and the selection pressures acting on this locus. We obtained high nucleotide variations (SNPs and insertion-deletions) in the intronic region. We also identified 64 haplotypes based on nucleotide polymorphism in these alleles. Pi-ta orthologues of Indian landraces were scattered in eight major haplotypes indicating its heterogenous nature. We identified a total of 47 different Pi-ta protein variants on the basis of deduced amino acid residues amongst the orthologues. Five unique and novel Pi-ta variants were identified for the first time in rice landraces exhibiting different reaction types against the Magnaporthe oryzae population. A high value of Pi(non/syn) was observed only in the leucine-rich domain of the alleles cloned from Indian landraces, indicating strong selective forces acting on this region. The detailed molecular analysis of the Pi-ta orthologues provides insights to a high degree of inter- and intraspecific relationships amongst the Oryza species. We identified rice landraces possessing the effective alleles of this resistance gene which can be used in future blast resistance breeding programmes.

Detection and characterization of bismerthiazol-resistance of Xanthomonas oryzae pv. oryzae

The resistance to bismerthiazol in the Xanthomonas oryzae pv. oryzae population in China has not been clearly determined. Three bismerthiazol-resistant mutants (1-1-1, 2-1-1, 4-1-1) of X. oryzae pv. oryzae were induced on rice plants and the characteristics were compared with their parental strain ZJ173. After 10 subcultures on fresh bismerthiazol-free nutrient agar plates and after being inoculation onto and re-isolation from untreated rice plants two times, the mutants remained pathogenicity on rice plants treated with 300 μg/ml bismerthiazol, indicating that the in vivo-induced bismerthiazol resistance was stable in vivo. The in vitro characteristics of the bismerthiazol-resistant mutants and a wild-type bismerthiazol-sensitive strain (ZJ173) were also compared. The three mutants grew faster than ZJ173 during the logarithmic growth phase but at the same rate during the decline phase. Bismerthiazol had protective and curative activity against strain ZJ173 but not against the three mutants. To evaluate the current degree of bismerthiazol resistance in field populations of X. oryzae pv. oryzae in China, 781 isolates of X. oryzae pv. oryzae were obtained from 1419 leaf samples collected from Yunnan, Sichuan, Jiangsu, Anhui, Hubei, Guangdong, Hainan, and Hunan Provinces in China from 2007 to 2009. The in vivo sensitivity of 505 of the 781 isolates to bismerthiazol was determined by applying them to rice plants treated with 300 μg/ml bismerthiazol and evaluating blast severity. The tested strains were considered resistant, if they showed <70% inhibition relative to strain ZJ173. Of the 505 isolates, 62 were resistant to bismerthiazol. The resistance frequencies among field population ranged from 0 to 21.1%, differed among the provinces, and was 11.2, 20.0 and 10.4% in 2007, 2008 and 2009, respectively. Resistance determined in vivo was not related to resistance determined in vitro. The in vivo-induced bismerthiazol resistance identified in this study will serve as a base line for further studying bismerthiazol resistance in fields, and the current degree of bismerthiazol resistance in field populations of X. oryzae pv. oryzae in China determined would be useful for the rice growers for control of rice bacterial blight.

Preliminary Assessment of Resistance Among U.S. Wheat Cultivars to the Triticum Pathotype of Magnaporthe oryzae.

Magnaporthe oryzae is the causal agent of blast disease on several graminaceous plants. The M. oryzae population causing wheat blast has not been officially reported outside South America. Wheat production in the United States is at risk to this pathogen if it is introduced and established. Proactive testing of U.S. wheat cultivars for their reaction to blast and identification of resistance resources is crucial due to the national and global importance of the U.S. wheat industry. In this preliminary study, the phenotypic reaction of 85 U.S. wheat cultivars to M. oryzae (Triticum pathotype) was determined. Although there was a significant correlation in the reaction to blast at the seedling and adult plant stages, only 57% of the head reaction was explained by the seedling reaction. Because of the importance of disease development at the head stage in the field, assessment of all 85 cultivars occurred at the head stage. Among cultivars tested, a continuum in severity to head blast was observed; cultivars Everest and Karl 92 were highly susceptible with more than 90% disease severity, while cultivars Postrock, JackPot, Overley, Jagalene, Jagger, and Santa Fe showed less than 3% infection. No evidence of the presence of physiological races among isolates T-7, T-12, T-22, and T-25 was found.

Pathogenic and Genetic Diversity of Magneporthe oryzae Populations from Sri Lanka

The present study was undertaken to determine the pathogenic and the genetic diversity of the isolates of M. oryzae collected from the wet, intermediate and dry zones of Sri Lanka with a view to develop rice varieties conferring durable resistance to rice blast. No significant morphological or growth variations existed amongst the isolates studied. The genetic diversity of isolates determined by carrying out Pot2 transposable element based on repetitive-PCR revealed that the majority of isolates (92%) clustered into a single group with 45.4% similarity. The existence of nine pathotypes was identified by observing the reaction type of isolates on 16 different rice lines. Pathotype 1 which was distributed in all zones affected only one differential line. Pathotype 2 which was able to infect six lines was restricted only to the dry zone of Sri Lanka. Of the 16 rice lines, seven lines, K3 (Pik-h), C101A51 (Piz5), K1 (Pita), C105TTP2L9 (Pita), K59 (Pit), Shin (Pish) and WHD-1S-75-1-127 (Pi9) had highly effective blast resistance. None of the isolates of the fungus showed any virulence against the seven lines. These seven blast resistant lines can be used in the breeding programmes in Sri Lanka for development of lines conferring durable resistance to rice blast.

Sex at the origin: an Asian population of the rice blast fungus Magnaporthe oryzae reproduces sexually

Sexual reproduction may be cryptic or facultative in fungi and therefore difficult to detect. Magnaporthe oryzae, which causes blast, the most damaging fungal disease of rice, is thought to originate from southeast Asia. It reproduces asexually in all rice-growing regions. Sexual reproduction has been suspected in limited areas of southeast Asia, but has never been demonstrated in contemporary populations. We characterized several M.oryzae populations worldwide both biologically and genetically, to identify candidate populations for sexual reproduction. The sexual cycle of M.oryzae requires two strains of opposite mating types, at least one of which is female-fertile, to come into contact. In one Chinese population, the two mating types were found to be present at similar frequencies and almost all strains were female-fertile. Compatible strains from this population completed the sexual cycle in vitro and produced viable progenies. Genotypic richness and linkage disequilibrium data also supported the existence of sexual reproduction in this population. We resampled this population the following year, and the data obtained confirmed the presence of all the biological and genetic characteristics of sexual reproduction. In particular, a considerable genetic reshuffling of alleles was observed between the 2 years. Computer simulations confirmed that the observed genetic characteristics were unlikely to have arisen in the absence of recombination. We therefore concluded that a contemporary population of M.oryzae, pathogenic on rice, reproduces sexually in natura in southeast Asia. Our findings provide evidence for the loss of sexual reproduction by a fungal plant pathogen outside its centre of origin.

Magnaporthe oryzae Populations Adapted to Finger Millet and Rice Exhibit Distinctive Patterns of Genetic Diversity, Sexuality and Host Interaction

In this study, host-specific forms of the blast pathogen Magnaporthe oryzae in sub-Saharan Africa (SSA) were characterised from distinct cropping locations using a combination of molecular and biological assays. Finger millet blast populations in East Africa revealed a continuous genetic variation pattern and lack of clonal lineages, with a wide range of haplotypes. M. oryzae populations lacked the grasshopper (grh) element (96%) and appeared distinct to those in Asia. An overall near equal distribution (47-53%) of the mating types MAT1-1 and MAT1-2, high fertility status (84-89%) and the dominance of hermaphrodites (64%) suggest a strong sexual reproductive potential. Differences in pathogen aggressiveness and lack of cultivar incompatibility suggest the importance of quantitative resistance. Rice blast populations in West Africa showed a typical lineage-based structure. Among the nine lineages identified, three comprised ~90% of the isolates. Skewed distribution of the mating types MAT1-1 (29%) and MAT1-2 (71%) was accompanied by low fertility. Clear differences in cultivar compatibility within and between lineages suggest R gene-mediated interactions. Distinctive patterns of genetic diversity, sexual reproductive potential and pathogenicity suggest adaptive divergence of host-specific forms of M. oryzae populations linked to crop domestication and agricultural intensification.

Effects of spinosad and abamectin on different populations of rice weevil Sitophilus oryzae (L.) in treated wheat grain

In laboratory conditions (25?1.C and 60?5% r.h.) effects of natural insecticides spinosad and abamectin on five S. oryzae populations (laboratory, Sid, Gornji Milanovac, Zabari and Novi Pazar) were investigated. Both insecticides for all tested populations were applied to untreated wheat grain at following rates 0.25, 0.5, 1.0 and 2.0 mg AI/kg, subsequently 25 adults were added in each plastic vessels (V=200 cm3) containing 50 g of treated wheat, in four replicates, for each population tested. Mortality of weevils was determined after 2-, 7- and 14-days, and the effect on progeny production was determined 8-weeks from parental exposure. Efficacy of spinosad and abamectin after 2-days of weevil exposure for all tested populations and all application rates was &lt;15%. After 7-days of exposure, the efficacy was ?95% for weevils from Zabari, in wheat treated with 2 mg/kg of spinosad and abamectin, and for weevils from Gornji Milanovac, only in wheat treated with 2 mg/kg of spinosad. After 14-days of exposure the efficacy ?95% was found for laboratory weevils and weevils from Zabari and Gornji Milanovac, in wheat treated with 1 and 2 mg/kg of spinosad, and for S. oryzae from Novi Pazar and Sid, in wheat treated with 2 mg/kg of spinosad. At the same time for all tested populations abamectin at rates of 0.5, 1.0 and 2.0 mg/kg was 94-100% efficien t. No insecticide achieved total (100%) progeny reduction in tested populations of S. oryzae, while high progeny reduction (?95%) was found only in weevils which were in contact with wheat treated with 1 and 2 mg/kg of abamectin. The results showed that for highly efficient control of different populations of S. oryzae in wheat grain, ?2 mg/kg of abamectin, and, particularly, spinosad should be applied.

Pathogenic Races and Virulence Gene Structure of Magnaporthe oryzae Population and Breeding Strategy for Blast Resistance in Heilongjiang Province

Rice blast, caused by Magnaporthe oryzae, became more severe in Heilongjiang Province, China during the past few years. The objectives of this study were to understand the race and virulence gene compositions of local M. oryzae populations, and purposefully select blast resistance genes for efficient development and deployment of resistant rice cultivars. A total of 173 isolates, collected from different rice-cropping districts of the province in 2006, were tested for their pathogenicity against 9 Japanese and 7 Chinese differential varieties (DVs) together with 31 rice monogenic lines (MLs) with different blast resistance genes and 12 local leading cultivars. Out of these 173 isolates, 55 Japanese pathotypes were identified using the Japanese DVs, and the predominant races were 017, 077, 037, 377, and 047, which accounted for 38.15% of all isolates tested. The comparison of differential ability between Japanese and Chinese DVs testified that the former one was much more suitable for M. oryzae pathotyping in Heilongjiang Province. Among the 12 leading cultivars tested, only Longjing 14 and Longdun 104 still kept good resistance to blast disease. The resistance gene Pi9 showed the broadest resistance spectrum (on average 94.80%) to all the blast isolates tested, and was of the highest utilization value in rice blast resistance breeding. The resistance genes Pi-z5(CA), Pi-z5(R), Pi-ta2(R), Pi-ta2(P), Pi-12(t), and Pi20(t) also showed high utilization values due to their resistance spectra of around 70%. The most effective breeding strategy for blast resistance should be as follows: 1) to use rationally Longjing 14, Longdun 104, and Pi9 as resistance donors, and pyramid one to several broad-spectrum resistance genes into elite leading cultivars by means of marker-assisted selection; 2) to strengthen monitoring of predominant virulent races and their temporal and spatial variation; and 3) to explore new resistance resources extensively and transfer the new broad-spectrum resistance genes into leading cultivars purposefully.

Evidence for Rapid Changes in the Population Genetic Structure of Magnaporthe oryzae in Southern Spain

AbstractPrevious studies of European populations of the blast fungus (Magnaporthe oryzae) have shown the existence of five clonal lineages. Three of them were common to different countries, whereas one was specific to Hungary and another was specific to Spain. But these studies were carried out on a limited number of individuals and pointed out a need for more extensive studies in each European rice‐growing area. In addition, temporal evolution of M. oryzae populations is also poorly documented. In this study, we focused on Guadalquivir delta region in southern Spain. A total of 186 M. oryzae isolates were collected from various farmer and experimental fields, on diverse cultivars, over the period 1999–2003, and characterized for their genotypic and pathotypic diversity. Five lineages were identified, one of which was detected for the first time in Europe (E6). The E6 lineage, which was collected over the period 2000–2003, became dominant in 2000. Pathotyping confirmed previous results of a narrow diversity for virulence spectrum in European lineages. Five dominant pathotypes were identified, each one corresponding to a single pathotype. Drastic changes in lineage relative frequencies were observed from one year to another. The origin of such changes is discussed.

PELLETS PRODUZIDOS COM ESTRUTURAS DE PLANTAS E ÓLEO ESSENCIAL DE FOLHAS DE Ageratum conyzoides L. AFETANDO O DESENVOLVIMENTO DE Sitophilus oryzae

The objectives of this study were to evaluate the effect of pellets, made from either vegetative and reproductive plant structures powder or essential oil obtained from leaves of billy goat weed, on the development of Sitophilus oryzae (L., 1763) (Coleoptera: Curculionidae), as well as to determine the period in which they could be stored without losing their insecticidal action. Pellets made from powders of different billy goat weed structures (root, leaf, inflorescence, and seeds) were stored for 2, 60, 120, and 180 days, and those made from essential oil were stored for 2 and 60 days. The billy goat weed presents insecticidal activity on S. oryzae, depending on the plant structure and concentration in the pellet. Pellets made from leaves and seeds, at the concentration of 0.5%, provided a better S. oryzae control, affecting more the emergence of insects than their development cycle. Pellets made from any plant structure, except for root, at 5.0%, can be stored for up to 180 days without losing their insecticidal action, while, at 0.5%, the storage period should be less than 60 days. Pellets made from essential oil of billy goat weed leaves can not be stored under uncontrolled conditions and reduce the S. oryzae population, but do not eliminate it, at concentrations of up to 0.5 µg L-1. KEY-WORDS: Insecta; biopesticide plant; weevil; billy goat weed.

A benefit of high temperature: increased effectiveness of a rice bacterial blight disease resistance gene

*Continuous planting of crops containing single disease resistance (R) genes imposes a strong selection for virulence in pathogen populations, often rendering the R gene ineffective. Increasing environmental temperatures may complicate R-gene-mediated disease control because high temperatures often promote disease development and reduce R gene effectiveness. Here, performance of one rice bacterial blight disease R gene was assessed in field and growth chamber studies to determine the influence of temperature on R gene effectiveness and durability. *Disease severity and virulence of Xanthomonas oryzae pv. oryzae (Xoo) populations were monitored in field plots planted to rice with and without the bacterial blight R gene Xa7 over 11 yr. The performance of Xa7 was determined in high- and low-temperature regimes in growth chambers. *Rice with Xa7 exhibited less disease than lines without Xa7 over 11 yr, even though virulence of Xoo field populations increased. Xa7 restricted disease more effectively at high than at low temperatures. Other R genes were less effective at high temperatures. *We propose that Xa7 restricts disease and Xoo population size more efficiently in high temperature cropping seasons compared with cool seasons creating fluctuating selection, thereby positively impacting durability of Xa7.

Flight activity of Sitophilus oryzae (L) and Sitophilus zeamais Motsch (Coleoptera: Curculionidae) and its relationship with susceptibility to insecticides

Insect-pest mobility can influence insect susceptibility to the insecticides used to control them. The objective of this work was to evaluate the flight activity of Sitophilus oryzae (L) and Sitophilus zeamais Motsch populations reared on corn and wheat grains, and its relationship with insecticide susceptibility. Unsexed adult insects with ages between 10 and 20 days were placed on a plastic tray. A 40-watt incandescent light bulb was positioned at a 50 cm height relative to the tray. Flight activity was evaluated during 30 min. In the LC50 estimation bioassays, the insects were exposed to dry residues of the organophosphorus insecticide fenitrothion and of the pyrethroid insecticide esfenvalerate on the internal surface of glass vials. Mortality was evaluated 24 h after installation of the experiments. Sitophilus zeamais showed greater flight activity when compared with S.oryzae; likewise, insects reared on corn had greater flight activity than those reared on wheat. Sitophilus oryzae was less susceptible to the insecticides studied than S. zeamais. Adults reared on wheat were more susceptible to the insecticides than those reared on corn. This study demonstrates that the lower flight activity of S. oryzae is related to its greater tolerance to insecticides.

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.