Differential expression of resistance to powdery mildew incited by race 1 or 2 ofsphaerotheca fuliginea inCucumis melo genotypes at various stages of plant development

Thirty-nine genotypes ofCucumis melo (plant introduction entries, open-pollinated cultivars and F₁ hybrids) were evaluated for resistance to powdery mildew under either natural field conditions or artificial inoculation in growth chambers at the cotyledonary stage and the 2-true-leaf stage. Results confirmed that susceptibility in cotyledons was not necessarily associated with susceptibility in either true leaves in growth chambers or adult plants in the field. However, resistance at the 2-true-leaf stage in growth chambers was highly correlated with resistance of field-grown plants. Results also showed that 20 muskmelon genotypes resistant to race 1 at the cotyledonary stage were also resistant at the 2-leaf-stage and as adult plants in the field. The same was true for ten genotypes with race 2 inoculations. Because muskmelon genotypes expressing resistance in cotyledons were also resistant in true leaves in growth chambers or the field, the use of plants at the cotyledonary stage is recommended for screening for powdery mildew resistance caused by race 1 or race 2 ofS. fuliginea. When cotyledons are susceptible, screening should be done at the 2-true-leaf stage.     

Genetic analysis of abamectin resistance in Tetranychus cinnabarinus

The carmine spider mite is the most serious crop mite pests in China. Abamectin has been used to control insects and mites worldwide but carmine spider mites, Tetranychus cinnabarinus, had developed resistance to it. Genetic research on insecticide resistance has been fundamental for understanding the resistance development, studying resistance mechanisms, and designing appropriate resistance management strategies to control insect pests. A resistant colony of T. cinnabarinus, RRG42, was established to examine the inheritance of abamectin resistance in T. cinnabarinus. The females of T. cinnabarinus were selected for bioassay using a slide dip method. After 42 generations of selection, the RRG42 strain was 8.7-fold resistant to abamectin compared with the susceptible strain (SS). The logarithm (log) concentration–probit response curve for F₁s from reciprocal crosses, of F₁RS and F₁SR, were inclined to that for SS and the degree of dominance (D) values for F₁s were −0.81 and −0.17. There was a significant difference in values of LC₅₀ and slope of log concentration–probit lines between F₁RS and F₁SR. The observed mortalities of BC1 (F₁RS♀ × RRG42♂) and BC1′ (F₁SR♀ × SS♂) were significantly different from the expected mortalities based on a monogenic resistance in the chi-square tests. The inheritance of abamectin resistance in T. cinnabarinus is incompletely recessive and may be controlled by more than one gene. The maternal or cytoplasmic effect may exist in the inheritance of resistance to abamectin in T. cinnabarinus.     

In‐field classification of herbicide‐resistant Papaver rhoeas and Stellaria media using an imaging sensor of the maximum quantum efficiency of photosystem II

Reliable in‐season and in‐field tools for rapidly quantifying herbicide efficacy in dicotyledonous weeds are missing. In this study, the maximum quantum efficiency of photosystem II (F_v/F_m) of susceptible and resistant Papaver rhoeas and Stellaria media populations in response to treatments with acetolactate synthase (ALS) inhibitors were examined. Seedlings (4–6 leafs) were transplanted into the field immediately after the application of the ALS inhibitors florasulam, metsulfuron‐methyl and tribenuron‐methyl. The F_v/F_m values were assessed 1–7, 9 and 14 days after treatment (DAT). Based on the F_v/F_m values of all fluorescing pixels in the images of herbicide‐treated plants, discriminant maximum‐likelihood classifiers were created. Based on this classifier, an independent set of images were classified into ‘susceptible’ or ‘resistant’ plants. The classifiers’ accuracy, false‐positive rate and false‐negative rate were calculated. The F_v/F_m values of sensitive P. rhoeas and S. media plants decreased within 3 DAT by 28–43%. The F_v/F_m values of the resistant plants of both species were 20% higher than those of the sensitive plants in all herbicide treatments. The classifier separated sensitive and resistant plants 3 DAT with accuracies of 62–100%. False‐positive and false‐negative classifications decreased with increasing DAT. We conclude that by the assessment of the F_v/F_m value in combination with the classification sensitive and resistant P. rhoeas and S. media populations could be separated 3 DAT. This technique can help to select effective control methods and speed up the monitoring process of susceptible and resistant weeds.     

Genetic control of sorghum resistance to leaf anthracnose

A study was carried out to investigate the inheritance of resistance to anthracnose, caused by Colletotrichum sublineolum, in sorghum. Crosses between resistant and susceptible parents and backcrosses between F₁ plants and the susceptible parents were carried out under field conditions. The F₁ generations and the segregant populations were evaluated under artificial inoculation conditions in the greenhouse. In the F₁ generation of all crosses with the respective isolates, all of the plants presented a resistance reaction except for the F₁ plants resulting from the BR009 × SC283 cross. In the F₂ generation, the frequencies of resistant and susceptible plants conformed to the hypothesis that one gene with two alleles controls host resistance, except in one cross. Out of the eight backcrosses, six presented segregation that corresponded to the hypothesis formulated. For most crosses, resistance was dominant, and the proportions of resistant and susceptible plants in the segregant populations conformed to the frequencies expected under the hypothesis of gene‐for‐gene resistance and dominant gene action.     

Genetics of resistance to downy mildew in Brassica oleracea and breeding towards durable disease control for UK vegetable production

Downy mildew resistance was previously identified from screening a Brassica oleracea collection against two standard UK isolates of Hyaloperonospora parasitica. Sources of resistance were chosen from this material and developed further in this study by generating doubled haploid (DH) and inbred lines. Seedlings from the new lines were tested for resistance to a larger collection of H. parasitica isolates collected in 2001–2002 and 2007–2008 from the main broccoli and cauliflower production regions of the UK. Three lines (derived from borecole or summer cabbage) were broadly resistant to the pathogen isolates. Three of the remaining lines exhibited strong isolate‐specific resistance; several examples of weak or basal level of resistance to some isolates were observed. A new H. parasitica variant collected in 2008 was virulent in the broadly resistant lines, but was avirulent in a line with narrow specificity of resistance. The F₂ and BC₁ seedlings derived from outcrossing each of the three broadly resistant lines to susceptible broccoli and cauliflower lines segregated in a manner indicating that the resistance was controlled by a single dominant gene. No susceptibility was observed amongst F₂ seedlings derived from intercrossing the three resistant lines, indicating that they all share the same or closely linked broad‐spectrum resistance gene(s). DH lines were produced from F₁ plants, and resistant plants were further backcrossed to produce broccoli and cauliflower‐like lines that could be useful pre‐breeding material. A combination of resistance from lines with broad and narrow specificity is recommended for controlling downy mildew in UK brassica production.     

Identification of allele specific AFLP markers linked with bacterial wilt [Ralstonia solanacearum (Smith) Yabuuchi et al.] resistance in hot peppers (Capsicum annuum L.)

Even though the bacterial wilt is identified as the most destructive disease in hot peppers world-wide, robust molecular markers that facilitate marker assisted selection are absent till date. Kerala Agricultural University (India) has released two hot peppers named Ujwala and Anugraha which show high level field resistance to this pathogen. The variety Anugraha was developed through backcross breeding between a high yielding but highly susceptible variety Pusa Jwala with the highly resistant Ujwala, using Pusa Jwala as a recurrent parent. Thus, Pusa Jwala and Anugraha are near isogenic lines (NILs) differing for the resistance to bacterial wilt only and the resistance is governed by a homozygous recessive (rr) gene action. The F₁s of Anugraha × Pusa Jwala were selfed to generate the segregating F₂ population. The F₂ population has been field screened, 10 highly susceptible and 10 most resistant plants were identified and DNA from these plants were bulked separately. Bulked segregant analysis with AFLP primer combination EcoACT + MseCAC was done using the DNA from donor parent Ujwala, susceptible parent Pusa Jwala, resistant parent Anugraha, bulked susceptible F₂ and bulked resistant F₂ plants. On resolution using capillary electrophoresis system in genetic analyzer, the AFLP products have yielded three polymorphic bands (103, 117, and 161 bp) which were linked with the resistant recessive allele and three polymorphic bands (183, 296, 319 bp) linked with the dominant susceptible allele of the bacterial wilt resistance gene. The results were confirmed through co-segregation analysis in most resistant and susceptible plants of F₂ segregating population.     

Leaf chlorophyll fluorescence discriminates herbicide resistance in Echinochloa species

Timely detection of herbicide resistance at an early stage of crop cultivation is essential to help farmers find alternative solutions to manage herbicide resistance in their fields. In this study, maximum quantum yield of PS II [F_v/F_m = (F_m – F_o)/F_m] was measured at the 4–5 leaf stage to discriminate between herbicide‐resistant and susceptible biotypes of Echinochloa species. The differences in F_v/F_m between herbicide‐resistant and susceptible Echinochloa spp. were consistent with the whole‐plant assay based on I₅₀ (herbicide doses causing a 50% inhibition of F_v/F_m) and GR₅₀ (herbicide doses causing a 50% reduction in plant fresh weight) values and R/S ratios (herbicide resistance index), regardless of the mode of action of the tested herbicides. A PS II inhibitor caused the fastest inhibition of F_v/F_m, compared with ACCase and ALS inhibitors, after herbicide treatment. The required time for discrimination between herbicide‐resistant and susceptible Echinochloa spp. was 64 h after PS II inhibitor treatment, much shorter than those of ACCase and ALS inhibitor‐treated plants, which required 168 and 192 h respectively. The leaf chlorophyll fluorescence assay provided reliable diagnostics of herbicide resistance in Echinochloa spp. with significant time savings and convenient measurement in field conditions compared with the conventional whole‐plant assay.     

Genetic analysis of lethal tip necrosis induced by <Emphasis Type="Italic">Clover yellow vein virus</Emphasis> infection in pea

Clover yellow vein virus (ClYVV) elicits lethal tip necrosis in the pea line PI 118501. Pea line PI 118501 develops necrotic lesions and veinal necrosis on inoculated leaves, followed by systemic necrosis, leading to plant death. To understand the genetic basis of this lethal tip necrosis, we crossed lines PI 226564 and PI 250438, which develop mosaic symptoms in response to ClYVV inoculation. In reciprocal crosses of PI 118501 with PI 226564, all F₁ plants had mosaic symptoms with slight stem necrosis and early yellowing of upper leaves. Essentially the same symptom was manifested in PI 118501 × PI 250438 F₁ plants. In F₂ populations from the cross between PI 118501 and PI 226564, the observed ratios of necrosis, mosaic with slight stem necrosis, and mosaic fit the expected 1 : 2 : 1 ratio. These results indicate that a single incompletely dominant gene confers the induction of necrosis in PI 118501. This locus in pea, conferring necrosis induction to ClYVV infection, was designated Cyn1 (Clover yellow vein virus-induced necrosis). A linkage analysis using 100 recombinant inbred lines derived from a cross of PI 118501 and PI 226564 demonstrated that Cyn1 was located 7.5 cM from the SSR marker AD174 on linkage group III.     

Resistance to acylalanines in Pseudoperonospora cubensis1

Resistance to acylalanine fungicides in Pseudoperonospora cubensis, the downy mildew of cucurbits, is reviewed. Insensitive strains of the fungus predominated during the second year of metalaxyl application in cucumber glasshouses in Greece and Israel. Resistance was detected either on detached cucumber leaves floated on metalaxyl solutions or on potted plants treated (sprayed or drenched) with this fungicide. Metalaxyl-resistant strains of P. cubensis showed good pathogenicity and fitness, and competed favorably with the sensitive strains in the absence of the fungicide. A synergism between these two biotypes resulted in an increased virulence of the resistant forms on metalaxyl-treated plants. Metalaxyl-resistant strains of the pathogen exhibited cross resistance to other acylalanine fungicides. Strains resistant to acylalanines and to the chemically unrelated oomycete-fungicides fosetyl-Al and propamocarb were isolated in Israel, as well. In Greece, however, protective sprays with fosetyl-Al, cymoxanil and mancozeb gave good control of cucumber downy mildew when the pathogen was resistant to metalaxyl. P. cubensis is the first case of fungus which developed resistance to acylalanines in the field.     

Inheritance of resistance to paraquat in barley grass Hordeum glaucum Steud.

Two biotypes of the grass weed barley grass (Hordeum glaucum), one resistant and the other susceptible to the herbicide paraquat, were studied along with their F₁, F₂ and F₃ progeny to determine the inheritance of paraquat resistance. The plants were sprayed with 50–200 g a.i. paraquat ha^?1. These concentrations killed the susceptible type. The data obtained from segregating populations indicated that paraquat resistance in H. glaucum is controlled by a single nuclear gene with incomplete dominance.     

Genetic Analysis of the Relationship between the Browning Reaction and Bacterial Blight Resistance Gene Xa3 in Rice

The relationship between bacterial blight resistance gene Xa3 and browning reaction was genetically analyzed using F₂ plants from the cross of rice cultivar Kuntulan with Kinmaze. Kuntulan harbors resistance Xa3 and developes a browning reaction to avirulent races of Xanthomonas oryzae pv. oryzae, whereas Kinmaze has a typical susceptible reaction to all known Japanese races of X. o. pv. oryzae. The F₂ plants were tested for their resistance to avirulent race II strain of X. o. pv. oryzae and the development of browning. Of 337 F₂ plants tested, 251 had resistance to the strain. In all the resistant plants, a browning reaction developed around the point of inoculation. The remaining 86 had a susceptible reaction to the strain without a browning reaction. The F₂ population of Kuntulan × Kinmaze had a clear-cut segregation ratio of 3R : 1S. These facts led to the conclusion that the browning reaction is a pleiotropic effect of Xa3. Received 29 January 2001/ Accepted in revised form 24 April 2001     

Molecular basis of resistance to acetolactate synthase-inhibiting herbicides in Sisymbrium orientale and Brassica tournefortii

Three Australian Sisymbrium orientale and one Brassica tournefortii biotypes are resistant to acetolactate synthase (ALS)-inhibiting herbicides due to their possession of an ALS enzyme with decreased sensitivity to these herbicides. Enzyme kinetic studies revealed no interbiotypic differences within species in K_m (pyruvate) (the substrate concentration at which the reaction rate is half maximal) but a greater V_max (the rate when the enzyme is fully saturated with substrate) for two of the resistant S orientale biotypes over susceptible levels. F₁ hybrids from reciprocal crosses between resistant and susceptible biotypes of S orientale showed an intermediate response to chlorsulfuron compared to the parental plants. ALS herbicide resistance in S orientale segregated in a 3:1 (resistant:susceptible) ratio in F₂ plants with a single rate of chlorsulfuron, indicating that resistance is inherited as a single, incompletely dominant nuclear gene. Two regions of the ALS structural gene known to vary in ALS-resistant biotypes were amplified and sequenced. Resistant S orientale biotypes NS01 and SS03 contained a single nucleotide substitution in Domain B, predicting a Trp (in susceptible) to Leu (in resistant) amino acid change. Two adjacent nucleotide substitutions (CC T to AT T) predicting a Pro (in susceptible) to Ile (in resistant) change in the primary amino acid sequence were identified in Domain A of resistant S orientale biotype SS01. Likewise, a single nucleotide substitution at the same site in the resistant B tournefortii biotype predicts a Pro (in susceptible) to Ala (in resistant) substitution. No other interbiotypic nucleotide differences predicted amino acid changes in the sequenced regions, suggesting that the amino acid substitutions reported above are responsible for resistance to ALS-inhibiting herbicides in the respective biotypes. © 1999 Society of Chemical Industry     

玉米自交系心叶期对亚洲玉米螟抗性遗传规律的研究初报

不同抗螟性亲本所组配的F_1代在玉米心叶期,对亚洲玉米螟 Ostrinis furnacalis(Guenée)抗性研究表明,玉米抗螟牲为数量遗传,亲本抗性在杂种后代中具有累加效应,杂种的抗性与双亲平均抗性呈高度正相关。中等以上抗性亲本组配的 F_1代为抗螟型。而以中抗系组配杂种时,不论父本或母本,只要一个亲本为感螟系,则 F_1代均为感螟型。不同水平抗性亲本组配的正反交组合 F_1代差异极显著。     

Inheritance of resistance in <Emphasis Type="Italic">Solanum nigrum</Emphasis> to <Emphasis Type="Italic">Phytophthora infestans</Emphasis>

Solanum nigrum, black nightshade, is a wild non-tuber bearing hexaploid species with a high level of resistance to Phytophthora infestans (Colon et al. 1993), the causal agent of potato late blight, the most devastating disease in potato production. However, the genetic mode of resistance in S. nigrum is still poorly understood. In the present study, two S. nigrum accessions, 984750019 (N19) and #13, resistant (R) and susceptible (S), respectively, to three different isolates of P. infestans, were sexually crossed. The various kinds of progeny including F1, F2, F3, and backcross populations (BC₁; F1 × S), as well as two populations produced by self-pollinating the R parent and S parent, were each screened for susceptibility to P. infestans isolate MP 324 using detached leaf assays. Fifty seedling plant individuals of the F1 progeny were each resistant to this specific isolate, similarly to the seedling plants resulting from self-pollination of the resistant R parent. Thirty seedling plants obtained from self-pollination of the S parent were susceptible. Among a total of 180 F2 plants, the segregation ratio between resistant and susceptible plants was approximately 3: 1. Among the 66 seedling plants of the BC₁ progeny originating from crossing an F1 plant with the susceptible S parent, there were 26 susceptible and 40 resistant plants to P. infestans. The segregation patterns obtained indicated monogenic dominant inheritance of resistance to P. infestans isolate MP 324 in S. nigrum acc. 984750019. This gene, conferring resistance to P. infestans, may be useful for the transformation of potato cultivars susceptible to late blight.     

小偃6号成株期高温抗条锈性遗传分析

为揭示小偃6号抗病机制和培育持久抗病品种,采用常规杂交分析方法,在小麦抽穗期利用小麦条锈菌小种CYR30、CYR32和Su11-4对小偃6号、铭贤169及其杂交F1、F2、F2∶3接种,平均气温达到21℃时对小偃6号进行了抗条锈性调查和遗传分析。结果显示,接种CYR30、CYR32时,F1代表现高感,F2代群体中抗感分离比例符合1 R∶15 S的理论比例。接种Su11-4时,F1代表现高抗,F2代群体中抗感分离比例符合3R∶1S的理论比例。研究表明小偃6号对CYR30、CYR32的抗病性均由2对隐性基因累加作用控制,对Su11-4的抗病性由1对显性基因控制。     

Differences in the responses of melon accessions to fusarium root and stem rot and their colonization by Fusarium oxysporum f. sp. radicis‐cucumerinum

Fusarium root and stem rot caused by the fungus Fusarium oxysporum f. sp. radicis‐cucumerinum is a major disease in greenhouse cucumbers. Over the past decade, the disease has been documented in melon greenhouses in Greece, and recently it has been sporadically recorded in greenhouse melons in Israel. Variations in disease response were found among 41 melon accessions artificially inoculated with the pathogen: 10 accessions were highly susceptible (90–100% mortality), 23 exhibited an intermediate response (20–86%) and eight were resistant (0–4%). Two melon accessions – HEM (highly resistant) and TAD (partially resistant) – were crossed with the susceptible accession DUL. The responses of the three accessions and F₁ crosses between the resistant and susceptible parents were evaluated. HEM contributed higher resistance to the F₁ hybrid than TAD. Roots of susceptible and resistant accessions were 100 and 79% colonized, respectively, following artificial inoculation. However, only susceptible plants showed colonization of the upper plant tissues. Microscopic evaluation of cross sections taken from the crown region of the susceptible DUL revealed profuse fungal growth in the intercellular spaces of the parenchyma and in xylem vessels. In the resistant cultivar HEM, very little fungal growth was detected in the intercellular spaces of the parenchyma, and none in the xylem or any other vascular tissue. Finding resistant accessions may create an opportunity to study the genetics of resistance inheritance and to develop molecular markers that will facilitate breeding resistant melon cultivars.     

Evaluation of triticale accessions for resistance to wheat bacterial leaf streak caused by Xanthomonas translucens pv. undulosa

The bacterium Xanthomonas translucens pv. undulosa (Xtu) causes bacterial leaf streak (BLS) on wheat and other small grains. Several triticale accessions were reported to possess high levels of resistance to wheat Xtu strains. In this study, a worldwide collection of triticale accessions as well as the major North Dakota hard red spring and durum wheat cultivars were evaluated for reaction to two local Xtu strains. All wheat cultivars showed a susceptible reaction but a wide range of reactions was observed among triticale accessions. Out of the 502 accessions tested, 45 and 10 accessions were resistant to the two virulent strains BLS‐LB10 and BLS‐P3, respectively, with five accessions, PI 428736, PI 428854, PI 428913, PI 542545 and PI 587229, being highly resistant to both strains. Statistical analysis showed significant difference among the accessions, strains, and the accession by strain interaction (P < 0.001). Bacterial population growth in resistant triticale was significantly slower than that in susceptible triticale. Molecular cytogenetic characterization in four representative triticale accessions confirmed the hexaploid level of the species and the presence of 12 or 14 rye chromosomes. The triticale accessions identified are valuable materials for developing wheat germplasm with high levels of BLS resistance.     

EPSPS gene amplification in glyphosate-resistant Italian ryegrass (Lolium perenne ssp. multiflorum) from Arkansas

BACKGROUND: Resistance to glyphosate in weed species is a major challenge for the sustainability of glyphosate use in crop and non‐crop systems. A glyphosate‐resistant Italian ryegrass population has been identified in Arkansas. This research was conducted to elucidate its resistance mechanism. RESULTS: The investigation was conducted on resistant and susceptible plants from a population in Desha County, Arkansas (Des03). The amounts of glyphosate that caused 50% overall visual injury were 7 to 13 times greater than those for susceptible plants from the same population. The EPSPS gene did not contain any point mutation that has previously been associated with resistance to glyphosate, nor were there any other mutations on the EPSPS gene unique to the Des03 resistant plants. The resistant plants had 6‐fold higher basal EPSPS enzyme activities than the susceptible plants, but their I₅₀ values in response to glyphosate were similar. The resistant plants contained up to 25 more copies of EPSPS gene than the susceptible plants. The level of resistance to glyphosate correlated with increases in EPSPS enzyme activity and EPSPS copy number. CONCLUSION: Increased EPSPS gene amplification and EPSPS enzyme activity confer resistance to glyphosate in the Des03 population. This is the first report of EPSPS gene amplification in glyphosate‐resistant Italian ryegrass. Other resistance mechanism(s) may also be involved. Copyright © 2012 Society of Chemical Industry     

Examination of resistance to clubroot in accessions of Brassica oleracea using a glasshouse seedling test

A glasshouse test was elaborated for assessing large numbers of seedlings ofBrassica oleracea for resistance to clubroot, a disease caused by the fungusPlasmodiophora brassicae. The method offers good control of inoculum density per plant, and requires 6–7 weeks from sowing. The results from the glasshouse test correlated well with field test results. With this method, 71 accessions ofB. oleracea reported to carry resistance to clubroot, and one susceptible control cultivar were tested with a Dutch clubroot isolate. High levels of resistance were found in several accessions of cabbage, broccoli and curly kale. F₁-populations of resistant cabbage or curly kale × susceptible cabbage were fully susceptible, indicating recessive inheritance of resistance in all cases.