Molecular Mapping of the rps1.a Recessive Gene for Resistance to Stripe Rust in BBA 2890 Barley

ABSTRACT Stripe rust, caused by Puccinia striiformis f. sp. hordei, is one of the most important diseases of barley in the south-central and western United States. Growing resistant cultivars is the best approach for controlling the disease. The barley genotype BBA 2890 has all-stage resistance against all races of P. striiformis f. sp. hordei (PSH) identified thus far in the United States. The resistance in BBA 2890 is controlled by a single recessive gene, rps1.a. The objectives of this study were to identify resistance gene analog polymorphism (RGAP) markers for the all-stage resistance gene rps1.a, to map the gene on a barley chromosome using chromosome-specific simple sequence repeat (SSR) markers, and to determine the presence or absence of the flanking RGAP markers for the gene in 24 barley genotypes. Seedlings of the parents and 200 F(8) recombinant inbred lines (RILs) were tested for resistance to pathogen races PSH-14, PSH-48, and PSH-54 in the greenhouse in 2005. Genomic DNA was extracted from the parents and 150 F(8) RILs. The RGAP technique was used to identify molecular markers for the rps1.a gene. Twelve primer pairs generating repeatable polymorphic bands were selected for genotyping the 150 F(8) RILs. A genetic linkage group was constructed for the resistance gene with 13 RGAP markers and four chromosome-specific SSR markers. The four SSR markers mapped the gene on the long arm of barley chromosome 3H. The closest RGAP marker for the resistant allele was within a genetic distance of 2.1 centimorgans (cM). The closest marker for the susceptible allele was 6.8 cM away from the locus. The two closest RGAP markers for the resistant allele detected polymorphisms in 67 and 71% of the 24 barley genotypes when used individually, and detected polymorphism in 88% of the genotypes when used in combination. This information should be useful in incorporating the resistance gene into barley cultivars and in pyramiding the gene with other resistance genes for superior stripe rust resistance.     

Status of black grass (Alopecurus myosuroides) resistance to acetyl-coenzyme A carboxylase inhibitors in France

We assessed the contributions of target site‐ and non‐target site‐based resistance to herbicides inhibiting acetyl‐coenzyme A carboxylase (ACC) in Alopecurus myosuroides (black grass). A total of 243 A. myosuroides populations collected across France were analysed using herbicide sensitivity bioassay (24 300 seedlings analysed) and ACC genotyping (13 188 seedlings analysed). Seedlings resistant to at least one ACC‐inhibiting herbicide were detected in 99.2% of the populations. Mutant, resistant ACC allele(s) were detected in 56.8% of the populations. Among the five resistant ACC alleles known in A. myosuroides, alleles containing an isoleucine‐to‐leucine substitution at codon 1781 were predominant (59.5% of the plants containing resistant ACC alleles). Comparison of the results from herbicide sensitivity bioassays with genotyping indicated that more than 75% of the plants resistant to ACC‐inhibiting herbicides in France would be resistant via increased herbicide metabolism. Analysis of herbicide application records suggested that in 15.9% of the populations studied, metabolism‐based resistance to ACC‐inhibiting herbicides was mostly selected for by herbicides with other modes of action. Our study revealed the importance of non‐target site‐based resistance in A. myosuroides. Using herbicides with alternative modes of action to control populations resistant to ACC‐inhibiting herbicides, the recommended management approach, may thus be jeopardised by the widespread occurrence of metabolism‐based resistance mechanisms conferring broad‐spectrum cross‐resistance.     

Detection of cyclodiene insecticide resistance-associated mutations by single-stranded conformational polymorphism analysis

Cyclodiene insecticide resistance is associated with replacements of a single amino acid within the putative lining of a δ-aminobutyric acid (GABA)-gated chloride ion channel gene Resistance to dieldrin (Rdl). Only two resistance-associated amino acid replacements have been identified; alanine to serine in Drosophila melanogaster, D. simulans, Aedes aegypti, and Tribolium castaneum and alanine to glycine as a second allele in D. simulans. Here we report that single stranded conformational polymorphism (SSCP) analysis of genomic DNA, amplified by the polymerase chain reaction (PCR) for exon 7 of the Rdl gene, can be used to genotype strains or individuals of all of these insects. This technique also appears simultaneously to distinguish between D. melanogaster and D. simulans, sibling species only reliably identifiable by examination of male genitalia. The relative advantages of this genotyping technique against other PCR-mediated techniques in monitoring for insecticide resistance are discussed.     

Increased frequency of the T929I and L932F mutations associated with knockdown resistance in permethrin-resistant populations of the human head louse, Pediculus capitis, from California, Florida, and Texas

The resistance levels of different human head louse populations from the USA to 1% permethrin were evaluated using permethrin-impregnated, filter paper disk-contact bioassay. Populations from southern California, south Florida and south central Texas showed 1.5-, 3.1-, and 1.5- to 5.1-fold resistance compared to insecticide-susceptible head louse populations from Panama or Ecuador. Permethrin-resistant or permethrin-susceptible homozygous or heterozygous genotypes were determined from sequences of PCR-amplified genomic DNA fragments of the voltage-sensitive sodium channel α-subunit gene by the presence of a T or C, or both, respectively, at nucleotide positions 36 and 44 in the sequence. The presence of a T at both these positions resulted in the amino acid substitutions, T929I and L932F, respectively. Of the 424 louse samples examined that had the T929I mutation, all also possessed the L932F mutation, indicating that the two mutations were tightly linked. The southern California population was phenotypically determined by bioassay to be comprised of 45% resistant individuals and had a resistant allele frequency of 0.53 by DNA sequence analysis. The south Florida population was phenotypically determined to consist of 87% resistant individuals and had a resistant allele frequency of 0.97. The four Texas populations varied in the level of resistance and in resistant allele frequency. The Mathis population was phenotypically determined to consist of 15% resistant individuals and had a resistant allele frequency of 0.33. However, the populations from San Antonio, Mansfield, and Corpus Christi were likewise phenotyped to have 91%, 94%, and 100%, respectively, resistant individuals and a 0.98, 1.00 and 1.00, respectively, resistant allele frequency. The log survival time versus logit mortality regression lines of susceptible-homozygotes, resistant-homozygotes, and heterozygotes determined that the resistance trait was complete recessive. Thus, the presence of homozygotes of the T929I and L932F mutations in the voltage-sensitive sodium channel correlated well with increased survival time following exposure to permethrin and indicates that a knockdown-type nerve insensitivity mechanism is functioning as the major mechanism causing permethrin resistance in USA head louse populations. Our results substantiate that permethrin resistance in human head louse population in the USA is widespread but variable. Permethrin resistance is highly correlated with the presence of the T929I and L932F point mutations, which are suitable for detection by a variety of DNA-based diagnostic techniques [Pest Manag. Sci. 57 (2001) 968]. Large-scale monitoring of permethrin resistance is possible utilizing these techniques and would provide critical information necessary for the development of an effective resistance management program for pediculosis.     

Evaluation of different methods for the characterization of carrot resistance to the alternaria leaf blight pathogen (Alternaria dauci) revealed two qualitatively different resistances

Alternaria leaf blight (ALB), caused by Alternaria dauci, is one of the most damaging foliar diseases of carrot worldwide. The aim of this study was to compare different methods for evaluating levels of carrot resistance to ALB. Three techniques were investigated by comparison with a visual disease assessment control: in vivo conidial germination, a bioassay based on a drop‐inoculation method, and in planta quantification of fungal biomass by quantitative PCR (Q‐PCR). Three carrot cultivars showing different degrees of resistance to A. dauci were used, i.e. a susceptible cultivar (Presto) and two partially resistant genotypes (Texto and Bolero), challenged with an aggressive or a very aggressive isolate of A. dauci. Both partially resistant genotypes produced a higher mean number of germ tubes per conidium (up to 3·42±0·35) than the susceptible one (1·26±0·18). The drop‐inoculation results allowed one of the partially resistant genotypes (Bolero, log₁₀(S+1) = 1·34±0·13) to be distinguished from the susceptible one (1·90±0·13). By contrast, fungal growth measured by Q‐PCR clearly differentiated the two partially resistant genotypes with log₁₀(I) values of 2·77±0·13 compared to the susceptible cultivar (3·65±0·13) at 15 days post‐inoculation. This result was strongly correlated (r² = 0·91) with the disease severity index scored at the same date. Data obtained with the different assessment methods strongly suggest that the Texto and Bolero genotypes have different genetic resistance sources.     

High-throughput approach to detection of knockdown resistance (kdr) mutation in mosquitoes, Culex quinquefasciatus, based on real-time PCR using single-labelled hybridisation probe/melting curve analysis

BACKGROUND: Knockdown resistance (kdr) mutation (L1014F) is a well‐defined mechanism of resistance to pyrethroids and DDT in many insect species. Sensitive detection of the mutations associated with resistance is a prerequisite for resistance management strategies. The authors have developed a new real‐time molecular diagnostic assay based on SimpleProbe^®/melting curve analysis for large‐scale kdr genotyping in the wild population of Culex quinquefasciatus Say, the principal vector of bancroftian filariasis. Melting curve analysis is based on the thermal stability difference between matched and mismatched DNA duplexes. The application of SimpleProbe^® chemistry in insects described here is novel in entomology research. RESULTS: The mosquitoes homozygous for knockdown‐resistant and knockdown‐susceptible allele showed melting peaks at 60.45 °C ( ± 0.25) and 64.09 °C ( ± 0.24) respectively. The heterozygous mosquitoes yielded both peaks at approximately 60.5 °C ( ± 0.2) and 64.20 °C ( ± 0.23). Among the 92 samples genotyped, 16 were found to be homozygous resistant, 44 homozygous susceptible and 32 heterozygous. Comparative assessments were made of all the reported methods for kdr genotyping. CONCLUSION: The present method is cheaper, faster, more reliable and versatile than other alternatives proposed in detecting correct kdr genotypes in mosquitoes. This is the first report using a single‐labelled hybridisation probe to detect point mutations in insect populations. Copyright © 2010 Society of Chemical Industry     

High‐throughput detection of highly benzimidazole‐resistant allele E198A with mismatch primers in allele‐specific real‐time polymerase chain reaction

BACKGROUND: A point mutation often confers resistance of organisms against medical drugs and agricultural pesticides. Allele‐specific nucleotide polymerase chain reaction (ASPCR) and allele‐specific quantitative real‐time PCR using SYBR Green (ASQPCR) are widely and effectively applied to detect and monitor this type of resistance. However, the former is unsuitable for high‐throughput detection, and the latter often reduces the accuracy of detection. RESULTS: In order to decrease background amplification, a rapid and high‐throughput genotyping method with mismatch primers was developed (ASQPCR‐MP) and applied specifically to survey the frequency of the highly benzimidazole‐resistant MBC^HR mutation (E198A) in the β‐tubulin gene of Sclerotinia sclerotiorum (Lib.) de Bary populations. Genomic DNA from 223 sclerotia was analysed. Similar genotype results were also obtained using ASPCR with mismatch primers and a mycelial growth inhibition assay. It was found that ASQPCR‐MP clearly differentiated MBC^HR and benzimidazole‐sensitive MBC^S phenotypes. Moreover, ASQPCR‐MP took less than 6 h to complete. CONCLUSION: ASQPCR‐MP appears suitable for large epidemiological studies involving resistant genotypes and requiring high‐throughout formats. Copyright © 2009 Society of Chemical Industry     

Reduced agent-area treatments (RAAT) for management of rangeland grasshoppers: Efficacy and economics under operational conditions

The Reduced Agent-Area Treatment (RAAT) approach involves applying low rates of insecticides in intermittent swaths to control grasshopper infestations in an effort to achieve a more economically and environmentally sound pest management strategy compared to traditional blanket applications at high rates. Operationalscale (>250 ha) insecticide tests on Wyoming rangeland grasshopper infestations (18-56 grasshoppers/m2) at two sites in 1997 revealed that carbaryl yielded 94% control as a blanket treatment (560 g/ha) and 81% control as a RAAT application (280 g/ha applied to 50% of the infested land). Malathion yielded 90% control as a blanket treatment (683 g/ha) and 91% control as an RAAT application (342 g/ha applied to 80% of the infested land), and fipronil (4 g/ha) yielded 98% control as a blanket treatment and 92% control when applied to 33% of the infested land. One year after treatment, the RAAT and blanket-treated plots had 2-4% (Pollet Ranch) of the densities in untreated plots. Economic analysis showed that the greatest benefit:cost ratio was obtained with the fipronil-RAAT method, followed by carbaryl-RAAT, malathion-RAAT, malathion-blanket, fipronil-blanket, carbaryl-blanket. The worst-case outcome of fipronil and carbaryl with a RAAT approach had a greater economic return than the best-case outcome of no treatment. Following blanket treatments, non-target arthropod abundance was highest in the fipronil plots, followed by carbaryl and malathion; arthropod biomass was higher in the carbaryl plots than fipronil and malathion. In RAAT plots, arthropods were generally less impacted than in the blanket plots, with the least reduction caused by fipronil and carbaryl. Thus, the RAAT tactic appears to be economically and environmentally superior to blanket treatments under large-scale, operational conditions.     

Genotyping Common Bean for the Potyvirus Resistance Alleles I and bc-1(2) with a Multiplex Real-Time Polymerase Chain Reaction Assay

ABSTRACT A multiplex real-time polymerase chain reaction (PCR) assay was developed to simultaneously genotype plants for the I and bc-1(2) alleles, which condition resistance in beans to Bean common mosaic virus and Bean common mosaic necrosis virus. A segregating F(2) population was derived from the cross between pinto bean breeding line P94207-189A (bc-1 bc-1 I I) and Olathe (bc-12 bc-1(2) i i). Real-time PCR assays were developed that were specific for each allele, and a multiplex PCR reaction could unambiguously assign F(2) plants to one of nine genotypes. Remnant F(1) plants were used as a comparative reference sample. PCR results among this sample fit a normal distribution for both real-time PCR assays, and 99% probability distributions were determined for heterozygotes. F(2) plants were genotyped based on results relative to the probability distributions for heterozygotes. F(2) plants also were genotyped for the I and bc-1(2) alleles by performing F(3) family progeny tests for virus resistance. Agreement between the two methods was 100% (198/198) for the bc-1(2) allele, and 92.4% (183/198) for the I allele. Erroneous genotyping was due to recombination between the amplicon and the I allele. Realtime PCR assays provide a robust method for genotyping seedlings and, in some cases, may eliminate the need for progeny testing.     

Estimation of knockdown resistance in diamondback moth using real-time PASA

We have developed a simple and accurate real-time PASA (PCR amplification of specific allele) (rtPASA) protocol for the estimation of pyrethroid resistance allele frequency in pooled DNA samples using the T929I-mutated allele of sodium channel α-subunit gene from the diamondback moth, Plutella xylostella, as a model. Conditions for the rtPASA for the detection of the T929I mutation were optimized by adjusting annealing temperature, template, and primer concentrations. Using standard DNA mixtures of susceptible and resistant alleles in various ratios, a plot of allele frequency versus cycle threshold value (C_t value) was generated for the prediction of allele frequency. The semi log plot was linear within the 1-80% range of resistance allele frequencies with a high correlation coefficient (r²=0.997) when highly stringent conditions (67 °C annealing temperature and 2 ng template DNA) were used. For the detection of lower frequencies of the resistant allele (0.004-1%), another hyperbolic semi log plot was constructed (r²=0.983) using the C_t values obtained from the rtPASA with less stringent conditions (66 °C annealing temperature and 5 ng template DNA). The rtPASA was able to detect the resistant allele at frequency as low as 0.02%. The performance of the rtPASA was evaluated by comparing with the data generated from 50 individual genotypings. We demonstrated that the actual resistance allele frequency of a field population of P. xylostella precisely matched the predicted frequency deduced by our protocol. The rtPASA format is applicable for the detection of additional mutations associated with resistance in other insect pest species and will allow rapid and efficient monitoring of resistance in field populations in a high throughput format.     

Polymerase chain reaction-based monitoring techniques for the detection of insecticide resistance-associated point mutations and their potential applications

The design of three PCR-based monitoring techniques for the genotyping of cyclodiene-resistant insects are described: (1) PCR followed by a diagnostic restriction enzyme digest or PCR/REN, (2) PCR amplification of specific alleles or PASA and (3) single-stranded conformational polymorphism analysis of PCR-amplified DNA or SSCP. The relative disadvantages and potential applications of each of these techniques are discussed and compared to the use of insecticide bioassays. It is concluded that, although such techniques will probably never replace bioassays for routine monitoring, they can more readily address several fundamental questions relating to the evolution and spread of specific resistance alleles in insect populations.     

杀菌剂抗性分子检测技术的研究进展

靶标病菌基因突变是许多内吸性杀菌剂出现抗性的根本原因,检测与抗药性相关的靶标病菌基因突变对阐明抗药性发生的分子生物学机制及进行早期诊断具有重要意义。目前已成功用于检测靶标病菌抗药性菌株的分子检测技术有6种:等位基因特异性扩增、限制性片段长度多态性、等位基因特异性寡核苷酸杂交、单链构象多态性、实时定量PCR、变性高效液相色谱分析。这些技术能够快速、灵敏地检测田间早期出现的抗药性或抗药性种群的发展动态,在病害的可持续管理系统中科学使用杀菌剂方面发挥着重要作用。     

Diversity and fitness of <Emphasis Type="Italic">Plasmopara viticola</Emphasis> isolates resistant to QoI fungicides

The effectiveness of Quinone outside Inhibitor (QoI) fungicides against grape downy mildew in European vineyards has significantly decreased in the last decade. One nucleotide polymorphism, G143A in the cytochrome b gene of Plasmopara viticola, is involved in resistance to QoIs. Previous genetic examination on the mitochondrial genomes showed four major haplotypes (IR, IS, IIR, IIS) coexisting in European vineyards. A resistant allele (G143A) was present in IR and IIR haplotypes. The purpose of the present study was to estimate the diversity of the different mitochondrial haplotypes and their distribution in QoI-resistant populations before evaluating the potential cost of the resistant mutation G143A in P. viticola population. From 2000 to 2004, the frequencies of resistant isolates ranged from 0% to 23.25% with an average of 4.64 % among the populations examined. To evaluate the fitness of sensitive and resistant isolates, a comparison of different biological parameters including latent period, spore production and infection frequency was performed, enabling a fitness index (F_I) to be determined. Resistant isolates exhibited greater infection frequency than sensitive isolates, whereas no significant difference was found in sporulation ability and latent period between sensitive and resistant isolates. To further investigate competitiveness among isolates, an assay including two resistant isolates in different proportion with a sensitive isolate was conducted on eight asexual growing cycles in the absence of a QoI fungicide. The competitiveness of resistant isolates varied according to their fitness parameters, suggesting that there is no noticeable cost of QoI resistance in controlled conditions in Plasmopara viticola.     

Recent situation of the Colorado beetle (Leptinotarsa decemlineata) in Poland

During the last 20 years there have been two outbreaks (1972 and 1976) of the Colorado beetle and one decline (1980). The new outbreak in 1983 was unusually massive as the population density of the pest was 2–3 times higher than in previous years. Almost 100% of potato crops and more than 50% of plants were attacked by the pest. This was due to low mortality of hibernating beetles during winter of 1982/83 and very favourable weather conditions in 1983 which resulted in an early emergence of hibernating beetles in the spring, intensive migration and high female fecundity. In 1983 insecticide treatments were applied to an area of 3.49 million ha as compared to 1.8 million ha treated in an average year. In 1984 there was a serious decline of the Colorado beetle population.     

Spatiotemporal patterns of oxidative burst and micronecrosis in resistance of wheat to brown rust infection

The accumulation of H₂O₂ (oxidative burst) and the progress of pathogen development were studied in compatible and incompatible wheat‐brown rust interactions. The accumulation of H₂O₂ was detected in 98·7% of guard cells with appressoria 8 h post inoculation (hpi). The reaction in both susceptible and resistant plants declined 2–3 days post inoculation (dpi). The second phase of the oxidative burst was observed in the mesophyll and/or epidermis. In susceptible plants it began 4–5 dpi and was detected only in the epidermis. In resistant plants the response was observed in the mesophyll. In moderately resistant plants it was induced 1–3 dpi, and the percentage of infection units reached 80–90% 8 dpi. This corresponded with severe necrotic symptoms. In highly resistant plants, the oxidative burst was short and transient. The percentage of infection units with H₂O₂ accumulation reached its highest level (60–70%) 2 dpi, and decreased thereafter. Four days later, the low percentage and weak DAB staining indicated very low H₂O₂ accumulation. The localization and the time‐course changes of the oxidative burst correlated with the profiles of the micronecrotic response, haustorium mother cell formation and pathogen development termination. An early and localized induction of oxidative burst followed by its rapid quenching correlated with high resistance and almost no disease symptoms. The possible correlation of the oxidative burst and pathogen development patterns with the level and durability of resistance conferred by Lr genes are discussed.     

Inheritance of resistance to a Bacillus thuringiensis toxin in a field population of diamondback moth (Plutella xylostella)

Inheritance of resistance to the Bacillus thuringiensis Berl. CryIA(b) crystal protein was studied in Plutella xylostella L. (diamondback moth). A field population 50-fold more resistant to CryIA(b) than a control susceptible strain was used. Dose-mortality curves of the resistant population, the susceptible strain and the F₁ from the two reciprocal crosses were compared. Resistance transmission to the F₁ was dependent on the sex of the resistant progenitor. Sex ratio of the survivors to high doses of CryIA(b) in the F₁ of the two reciprocal crosses did not corroborate the preliminary hypothesis of resistance being due to a recessive sex-linked allele. Since, in a previous work, the loss of CryIA(b) binding capacity of resistant insects had been demonstrated, binding to midgut tissue sections from F₁ individuals was also analysed. The presence of binding in all of the F₁ preparations showed that, at least, a recessive autosomal allele was responsible for the loss of binding capacity in the resistant population.     

Temporary partial breakdown of mlo-resistance in spring barley by sudden relief of soil water-stress under field conditions: the effects of genetic background and mlo allele

This study, carried out under field conditions, assessed the extent to which temporary breakdown of mlo- resistance, following relief of water-stress, was determined by genetic background and mlo allele. Commercial barley cultivars expressing the mlo gene for resistance to powdery mildew ( Erysiphe graminis ( Blumeria graminis ) f.sp. hordei ) were tested as well as doubled haploid progeny from spring barley genotypes, a proportion of which were sown in the field in two successive years. Plants were protected from natural rain by a mobile rain shelter and either watered by trickle-irrigation or allowed to dry. Percentage mildew infection resulting from natural inoculum was recorded and the doubled haploid genotypes were classified as resistant, intermediate or susceptible on the basis of their control (watered) treatment scores. In each of the three designated classes, particular genotypes developed infection levels, following relief of water-stress, that were higher than those observed on the well-watered controls. This was found not to be related to the mlo allele as there was no significant difference between the increases observed on resistant plants carrying mlo9 and resistant plants carrying mlo11 . Differences in the degree of breakdown of resistance were attributed to genetic background rather than to the specific mlo allele.     

Effects of initial nematode population density and water regime on resistance and tolerance to the rice root-knot nematode Meloidogyne graminicola in African and Asian rice genotypes

The rice root-knot nematode (RKN), Meloidogyne graminicola, is an important pathogen affecting rice production in South and Southeast Asia. Efficacy of resistance and tolerance in selected M. graminicola-resistant African rice genotypes TOG5674, TOG5675 and CG14 and -susceptible Asian rice genotypes IR64 and UPLRi-5 were examined under a range of initial population densities (Pi) and water regimes. Resistance to M. graminicola in resistant rice genotypes was not broken with increasing pathogen pressure (Pi = 15,000 to 60,000 J2/plant). Resistant rice genotypes were even tolerant to the damage and yield loss caused by high pathogen pressure. On the other hand, increasing Pi levels caused more damage on susceptible rice genotypes. Final nematode population densities in the root systems of resistant and susceptible rice genotypes were significantly lower under flooded conditions than under upland and drought conditions. TOG5674, TOG5675 and CG14 were more tolerant to M. graminicola infection even when grown under upland and drought conditions while IR64 and UPLRi-5 were highly sensitive.     

Secondary Ambrosia Beetles in Apparently Healthy Trees: Adaptations, Potential Causes and Suggested Research

Some ambrosia beetles are primary attackers of healthy, living trees, but in recent years normally secondary species have been increasingly observed attacking living trees, either as exotics or in their native geographic ranges. We identified five factors that could underlie an increasing prevalence of attack by secondary ambrosia beetles on living trees: (1) early flight before the host tree has recovered the ability to resist attack in the spring, possibly associated with climate change; (2) nutritional independence from the host that may enable ambrosia beetles to feed on ambrosia fungi that live on dead tissue in a living tree; (3) potentially pathogenic fungi that could become more pathogenic with climate change or through hybridization with exotic strains; (4) cryptic behavior that facilitates international transport and the establishment of exotic species and genotypes; and (5) a complex chemical ecology that enables secondary ambrosia beetles to locate stressed living trees that may temporarily appear to be suitable hosts for secondary beetles. We propose four avenues of research that will lead to an increased understanding of attack of living trees by ambrosia beetles, and may facilitate the implementation of effective pest management strategies and tactics: (1) intensive surveys, particularly for exotic beetle species and associated fungal strains; (2) molecular genetics studies that would facilitate the identification of known and new strains and genotypes, particularly of ambrosia fungi; (3) studies of the pathogenicity of ambrosia fungi as well as other fungi that could predispose trees to attack; and (4) investigations of the chemical ecology of tree-attacking species that could lead to new pest management tools and tactics.     

Critical disease components of common bacterial blight to effectively evaluate resistant genotypes of snap bean

Common bacterial blight (CBB) caused by Xanthomonas axonopodis pv. phaseoli (Xap) is the main bacterial disease of snap bean. The present work aimed to select snap bean genotypes that are resistant to CBB based on three components of resistance: area under the disease progress curve, latent period, and lesion diameter on pods (DL). A completely randomized two-factor factorial design with six replications was used to evaluate leaves and pods of 14 snap bean and three common bean genotypes (PI 207262, BAC-6 and A-794) with high resistance to CBB. Two Xap isolates, 1394-98 and 775-90, were used to inoculate leaves and pods. Data were analyzed using nonparametric statistics. Significant differences were observed among the evaluated genotypes for all of the components of resistance, except for DL. The snap bean UENF 1482 demonstrated good performance in two disease resistance components. For this reason, this genotype can be recommended for use in breeding programs that aim to generate snap bean genotypes resistant to Xanthomonas axonopodis pv. phaseoli.