Assessment of quantitative resistance of rice cultivars to Xanthomonas campestris pv. Oryzae: a comparison of inoculation methods

Summary Lesion size and lesion number were measured on cultivars of rice inoculated by clipping or spraying with virulent isolates of Xanthomonas campestris pv. oryzae. Mean percentage diseased leaf area (%DLA) gave a similar ranking for the two inoculation methods but differences in lesion size among cultivars were much more evident after clip than after spray inoculation. Correlation between the methods was high (r=0.82^**), but some cultivars responded differently with the two techniques. Cultivars which had low scores following spray inoculation showed low disease progress during the first nine weeks after transplanting into a screen-house experiment. Assessment after clip inoculation measures resistance due to spread of bacteria within the leaf xylem system, an important component of quantitative resistance. Assessment after spray inoculation measures all resistance, including resistance to entrance of bacteria into the leaf. In order to select rice entries with improved quantitative resistance to X. c. oryzae based on both components, a screening based on lesion length after clip inoculation, followed by a test for lesion number after spray inoculation, is advised.     

Effect of leaf age on components of partial resistance in rice to leaf blast

Summary Ten rice genotypes were inoculated with a virulent isolate of Magnaporthe grisea to study the effect of leaf age on components of partial resistance, and evaluate associations between these components. The number of sporulating lesions per cm² leaf declined with increase of leaf age in all genotypes. The number of lesions per cm² leaf area in one week old leaves was about 25% that of the number in very young leaves in the susceptible cultivar CO39, but less than 2% in the more resistant cultivars IR36, IR60 and IR62. Large differences between genotypes were found for the number of sporulating lesions that developed, and this factor was closely related to the period that leaves remained susceptible after appearance. The number of lesions in the most susceptible cultivar CO39 was about 7 times that in the cultivars IR60 and IR64. Differences between genotypes were also found for lesion size. The effect of aging on average lesion size was less pronounced than on lesion density. Lesion size and lesion density were positively correlated, but a rapid decline of density was not necessarily accompanied by a rapid decline of size. No apparent differences between genotypes were observed for latent period. Genotypes with leaves that became highly resistant soon after appearance expressed higher levels of partial resistance in the field.     

Effect on leaf photosynthetic rate by leaf blast for rice cultivars with different types and levels of resistance

Summary The effect of an inoculation with Pyricularia oryzae (isolate P06-6) on net leaf photosynthetic rate of rice (Oryza sativa) was studied with four cultivars. Measurements were taken on the sixth leaf of the main culm of plants in the early tillering stage. On cultivars CO39, IR50 and IR64 a susceptible infection type developed, but a clear difference in relative infection efficiency of the cultivars was observed. The highest number of lesions developed on leaves of CO39, whereas the lowest number was found on leaves of IR64. For all three cultivars the effect of a single lesion on the reduction in net leaf photosynthetic rate was found to be equal to a reduction in leaf area of three times the area occupied by the visible lesion. On IR68, a cultivar with complete resistance, brown specks of pinpoint size appeared without any effect on net leaf photosynthetic rate.     

Inheritance of resistance to angular leaf spot in Nicotiana tabacum L. cultivars burley 21 and kentucky 14

Summary The genetics of resistance to angular leaf spot caused by Pseudomonas syringae pv. tabaci in Nicotiana tabacum cultivars Burley 21 and Kentucky 14 was investigated by studying disease reactions to three isolates of parental, F₁, F₂ and backcross generations derived from crosses between the resistant cultivars and the susceptible cultivar Judy's Pride. Studies were conducted in the greenhouse and in field plant beds. Chi-square values were computed to determine whether the observed ratios for disease reactions deviated from expected Mendelian ratios for a single, dominant gene controlling resistance to angular leaf spot in tobacco. Based on the resistance of the F₁ and the backcross generation to the resistant parent (BC-R), a 3 resistant: 1 susceptible segregation ratio in the F₂, and a 1 resistant: 1 susceptible segregation ratio in the backcross generation to the susceptible parent (BC-S), it was concluded that resistance to three isolates of Pseudomonas syringae pv. tabaci is governed by a single, dominant gene.     

Resistance of wheat to Fusarium spp. in an in vitro seed germination assay and preliminary investigations into the relationship with Fusarium head blight resistance

Resistance of wheat cultivars from the UK, 2003 recommended list were evaluated against Fusarium culmorum, F. graminearum, Microdochium nivale var. majus and var. nivale, F. avenaceum and F. poae using a seed germination assay and related to previously published data on detached leaf assays, using M. nivale, and UK recommended list Fusarium head blight (FHB) resistance ratings. Significant cultivar differences were found after inoculation with all fungal species with the exception of F. poae where the decline in the percentage of seed germinating relative to the controls was small. Correlations of the percentage seed germinating inoculated with the remaining Fusarium spp. and M. nivale were high (r = 0.68; P < 0.01 to r = 0.94; P < 0.001). Overall, F. graminearum caused the greatest reduction with a mean seed germination of 61.7% relative to the controls followed by F. avenaceum (65.5%), M. nivale var. majus (67.2%), F. culmorum (76.6%), M. nivale var. nivale (89.2%) and was least for F. poae (92.5%). The resistance detected in the germination assay was significantly correlated to whole plant FHB resistance ratings (r_s = 0.45; P < 0.05) but was not correlated to partial disease resistance (PDR) components detected using the detached leaf assay, namely, incubation period, latent period and lesion length. The results suggest that while resistances detected in the seed germination and detached leaf assays may in part share a common genetic basis to FHB resistance, resistances detected in both in vitro assays are independent of each other. Resistances detected in the seed germination assay had a lower correlation with FHB resistance ratings against F. culmorum, than those detected by latent period in the detached leaf assay.     

Residual effects of the Xa-4 resistance gene in three rice cultivars when exposed to a virulent isolate of Xanthomonas campestris pv. oryzae

Summary F₂ plants of five, and F₃ plants of three, crosses between genotypes carrying the race-specific resistance gene Xa-4 and genotypes not carrying this gene were inoculated with two isolates of Xanthomonas campestris pv. oryzae. Half the tillers of each plant received isolate PX061, avirulent on the Xa-4 gene, the other half of the tillers received isolate PX099, virulent for the Xa-4 gene. The F₂ and F₃ populations segregated for a single dominant resistance gene, Xa-4.The parental, F₂ and F₃ genotypes not carrying Xa-4 had mean lesion lengths between 28 and 29 cm for both isolates. The Xa-4 carrying parents showed a mean lesion length of 2.7 cm with the avirulent isolate and of 12.4 cm with the virulent isolate. The Xa-4 carrying F₂ and F₃ genotypes had mean lesion lengths of 5.2 and 20.1 cm for the two isolates, respectively. These observations strongly indicate that the Xa-4 gene, carried by the rice genotypes studied (IR28, Cisadane and BR51-282-8), had a considerable residual effect when exposed to virulent isolate PXO99.     

Genetic analysis of resistance to bacterial leaf streak caused by Xanthomonas campestris pv. undulosa in bread wheat

Summary The inheritance of resistance to bacterial leaf streak or black chaff of wheat was studied under field conditions, with an artificial epidemic of Xanthomonas campestris pv. undulosa. A complete series of crosses between five parents, differing in reaction to X. c. pv. undulosa, was generated. Disease was recorded at two different stages of growth. No evidence of cytoplasmic effect was found from the comparison between reciprocal F₁ crosses. The study of the F₃ generations attested that five genes were involved in resistance to bacterial leaf streak. Separate analyses carried out for the two scoring dates were mutually consistent: genotypes, number of genes, and their action and relative importance were verified. The genes differed in strength of expression of resistance. One of the two strongest genes, Bls1, is present in all three superior parents, Pavon 76, Mochis T88 and Angostura F88. Resistance was not complete, and proved to be stable over the season.     

Testing inoculation methods and sources of resistance to the halo blight bacterium (Pseudomonas syringae pv. phaseolicola) in Phaseolus vulgaris

Summary A comparative test of six inoculation methods was conducted using 2 halo blight race 2 virulent strains, Nebr. HB 16 and HB 21 (Pseudomonas syringae pv. phaseolicola), on five dry bean cultivars/lines (Phaseolus vulgaris L.) of known resistance and susceptibility. The water-soaking of leaves method caused the most severe reaction among the leaf inoculation methods, followed by the carborundum, spraying and multiple needle methods, respectively. The seed soaking method was considered too severe to be useful, since entries identified as resistant by the other methods, were susceptible with the former method. Great Northern Nebraska # 1 sel. 27 and PI 150414 had the highest level of leaf resistance, but the former developed systemic chlorosis with the stem stabbing method, but not the latter line. No systemic chlorosis was seen in either line with the other methods of inoculation. This suggests that there may be a different genetic mechanism conferring resistance/susceptibility to the toxin in these two lines when the stabbing method is used. No interaction occurred between method by genotype and isolate by method but significant interactions occurred between genotype by isolate and method by isolate by genotype. The leaf and pod reaction of forty cultivars/lines to the new halo blight Nebr. Charlevoix strain was also determined. Different combinations of degrees of resistance and susceptibility of leaves and pods were observed. GN Tara, GN Harris, and PI 150414 had the highest combination of leaf and pod resistance.Published as paper No. 7094, Journal Series, Nebraska Agricultural Experiment Station. Research was conducted under Project No. 20-036.     

Heritability and gain from selection for quantitative resistance to Xanthomonas campestris pv. vesicatoria in Capsicum annuum L.

Summary Breeding for disease resistance in peppers (Capsicum spp.) to the bacterial spot pathogen (Xanthomonas campestris pv. vesicatoria (Doidge) Dye) has been based on either qualitative or quantitative evaluation methods. Quantitative evaluation of components of resistance, lesion number and lesion diameter, has been useful for determining quantitative resistance, but few breeders have applied these methods in routine selection programs. This study was aimed at determining the heritability and gain from selection for resistance to the bacterial spot pathogen based on three components of resistance. Random selections from a diverse intermated population of Capsicum annuum L. were self-pollinated for two generations to create S₁ and S₂ families. Thirty S₁ families, corresponding S₂ families and four homozygous check lines were evaluated. At forty-two days after seeding, two different leaves of each plant were inoculated by leaf infiltration with low concentrations (5×10³ colony forming units) of Group 2 (XCV PT, race 1) and 4 (XCV P, race 1) of the bacterium, respectively. After 15 days, lesion number cm^-2 and lesion diameter were measured. Total lesion area was calculated. Narrow-sense heritabilities for lesion number, lesion diameter, and total lesion area were 0, 43, and 31%, respectively, with Group 2, and 26, 43, and 33%, respectively, with Group 4. Actual S₂ gain from 20% selection pressure in the S₁ was approximately 50%, when selection towards resistance was based on total lesion area.     

Enhanced leaf rust resistance in wheat conditioned by resistance gene pairs with Lr13

Summary Leaf rust resistance gene Lr13 is present in many North American hard red spring wheat cultivars that have shown durable resistance to leaf rust. Fifteen pair-wise combinations of Lr13 and seedling leaf rust resistance genes were developed by intercrossing near isogenic Thatcher lines. In both seedling and adult plant tests, homozygous paired combinations of specific resistance genes with Lr13 had enhanced resistance relative to either parent to rust isolates that had intermediate avirulent infection types to the additional genes. In field tests, homozygous lines were more resistant than either parent if the additional leaf rust gene conditioned an effective level of resistance when present singly.     

Development of a seedling screening test for predicting relative grain protein content in oats

Summary Growth room experiments were conducted to study associations of grain protein content with properties of seedling leaf sections of oats (Avena sativa L.) using (1) 10 cultivars differing genetically in grain % protein, and (2) 10 populations of a single high protien cultivar (Hinoat) differing phenotypically in grain % protein. These populations, which were derived from a nitrogen fertilizer experiment, had grain protein concentrations which varied over the whole range displayed by the high and low protein cultivars when the latter were tested in a conventional field trial.Seedling leaf % protein was closely associated with grain % protein in both (1) and (2). Chlorophyll content per unit leaf area and leaf dry weight per unit leaf area were significantly higher in the high than in the low protein cultivars, and were significantly higher in all the Hinoat populations than in the low protein cultivars.Excised seedling leaf sections were placed on filter paper moistened with 1 ppm kinetin solution and kept in the dark at 25°C. After 96 h chlorophyll content per unit leaf area was again significantly higher in the high protein cultivars and in all the Hinoat populations than in the low protein cultivars, and the consequent differences in leaf colour were then readily visible. Absolute amounts of chlorophyll lost per unit leaf area were similar in all cultivars and populations, but the low protein cultivars showed a greater proportional loss (as % of initial content). A colour scale was used to visually rate the senesced leaf sections. The visual rating allowed the rapid separation of the high and low protein cultivars, and there was no significant variation in the ratings of the Hinoat populations.It is suggested that this procedure may be useful in the early selection phases of protein breeding programs for screening large populations rapidly at the seedling stage to detect genetic differences in potential grain protein content.Contribution No. 438 from Ottawa Research Station.     

The effect of pre- and post-inoculation temperature on resistance in certain cultivars of poplar to races of Melampsora larici-populina kleb.

Summary Four cultivars of Populus spp., compatible to varying degrees with four races of M. larici-populina Kleb., were raised in a controlled environment on a high (28°/20°C, day/night) and low (20°/10°C) temperature regime. Leaf discs cut from the plants were inoculated separately with four individual races of M. laricipopulina and subsequently incubated at either low (20°C) or high (25°C) temperature for 14 days when disease development on the discs was assessed using three parameters (Incubation period to flecking, uredia per leaf disc and uredospores per mm²). The degree of resistance in all cultivar/race combinations was high on cultivars cultured at a high temperature regime compared to those cultured on a low temperature regime. Analysis of variance demonstrated that the major components: pre-inoculation temperature regime, post-inoculation temperature regime, race and cultivar, and most second and third order interactions between these were highly significant (P<0.001) for most disease parameters. The variance of the temperature components and all interactions involving these were usually higher than those for the cultivar and race components and those interactions lacking temperature components.These results emphasize the importance of the temperature regime at which plants are raised and the temperature of incubation, following the inoculation in determining the relative degree of resistance of these cultivars of poplar to races of M. larici-populina. The implications of these results in the epidemiology of leaf rust and the stability of the host-parasite relationship are discussed.     

Evaluation of seedling and adult plant resistance to leaf rust in European wheat cultivars

Summary A set of 105 European wheat cultivars, comprising 68 cultivars with known seedling resistance genes and 37 cultivars that had not been tested previously, was tested for resistance to selected Australian pathotypes of P. triticina in seedling greenhouse tests and adult plant field tests. Only 4% of the cultivars were susceptible at all growth stages. Twelve cultivars lacked detectable seedling resistance to leaf rust, and among the remaining cultivars, 10 designated genes were present either singly or in combination. Lr13 was the most frequently detected gene, present in 67 cultivars, followed by the rye-derived gene Lr26, present in 19 cultivars. Other genes present were Lr1, Lr3a, Lr3ka, Lr10, Lr14a, Lr17b, Lr20 and Lr37. There was evidence for unidentified seedling resistance in addition to known resistance genes in 11 cultivars. Field tests with known pathotypes of P. triticina demonstrated that 57% of the cultivars carried adult plant resistance (APR) to P. triticina. The genetic identity of the APR is largely unknown. Genetic studies on selected cultivars with unidentified seedling resistances as well as all of those identified to carry APR are required to determine the number and inheritance of the genes involved, to determine their relationships with previously designated rust resistance genes, and to assess their potential value in breeding for resistance to leaf rust.     

Identification of soybean strains resistant to Xanthomonas campestris pv. glycines

Summary Soybean germplasm was screened for resistance to bacterial pustule disease. The etiological agent, Xanthomonas campestris pv. glycines, was isolated from the leaves of field grown soybean in Maharashtra, India. The screening of soybean stocks was carried out by excised leaf inoculation method. A differential susceptibility to the pathogen was observed in the tested stocks. Two stocks P-4-2 and P-169-3 were found to be completely resistant to the pathogen and displayed an incompatible reaction. Four cultivars, EC-34160, Bragg, Kalitur and PK-472 displayed moderate resistance and the remaining stocks were susceptible to the attack of the pathogen. The stocks P-4-2 and P-169-3 remained resistant even to a high concentration of 10⁹ colony forming units (cfu)/ml of the pathogen.     

Field and greenhouse evaluation of the components of partial resistance to early leafspot in peanut

Summary Ten peanut (Arachis hypogaea L.) genotypes were evaluated fro components of partial resistance to Cercospora arachidicola Hori in the field and in two detached leaf tests in the greenhouse. The relationships among the resistance components in the two environments were examined by computing rank correlations among the components measured in the greenhouse and among those measured in the field. A comparison of field and greenhouse testing for partial resistance was made by determining the correlations between components measured in the two testing methods. The genotypes were significantly different for all the components of resistant measured in the field and for most of the components measured in the greenhouse. In the field study necrotic area/10 cm² leaf area was moderately correlated (r=0.58) with lesion number/10 cm² leaf area and highly correlated (r=0.71–0.76) with total lesion number, predicted number of days after planting (X) to reach a standard lesion count, and defoliation. In the greenhouse only the correlation between necrotic area (mm²)/10 cm² leaf area and sporulation/leaf was highly significant (r=0.71–0.83) in both greenhouse tests. Necrotic area (mm²)/10 cm² leaf area measured in the field was significantly correlated with that measured in the greenhouse (r=0.66). Sporulation/leaf measured in the greennhouse was significantly correlated (r=0.66) with lesion increase in the field (measured as predicted X). It should thus be possible to evaluate and select for components of partial resistance in the greenhouse to develop resistant lines for the field.     

Stability of bacterial leaf spot resistance in peach regenerants under in vitro,greenhouse and field conditions

Summary Phenotypic stability of bacterial leaf spot resistance in peach (Prunus persica (L.) Batsch) regenerants, either selected at the cellular level for insensitivity to a toxic culture filtrate of Xanthomonas campestris pv. pruni or screened at the whole plant level for resistance to X. campestris pv. pruni, was investigated. A detached-leaf bioassay was used to evaluate the original regenerants again after three years in the greenhouse and also after a two to three year cycle of tissue culture propagation. Peach trees derived through micropropagation from the original regenerants were also evaluated after one to three years growth in the field. Although leaf spot resistance was retained in some regenerants over time in the greenhouse, following in vitro propagation, and under field conditions, resistance was either lost or not expressed in others. Regenerants # 19-1 and #156-6, derived from embryo callus of bacterial spot susceptible Sunhigh, were significantly more resistant than Sunhigh. High levels of resistance were exhibited in greenhouse plants and field-grown trees of regenerant #122-1, derived from embryo callus of moderately resistant Redhaven. This research provides additional evidence that selecting or screening for somaclonal variants with disease resistance is a feasible approach to obtaining peach trees with increased levels of bacterial spot resistance.Abbreviations TC Tissue-Cultured - TF Toxic culture Filtrate     

Genes conferring low seedling reaction to Mexican pathotypes of Puccinia recondita f. sp. tritici,and adult-plant responses of recent wheat cultivars from the former USSR

Fifty-five spring bread wheat (Triticum aestivum L.) cultivars, mostly released between 1975 and 1991 in eight leaf rust-prone spring wheat growing regions of the former USSR, were tested in the seedling growth stage for reaction to 15 Mexican pathotypes of Puccinia recondita f. sp. tritici. In total, seven known and at least two unknown genes were identified, either singly or in combinations: Lr3 (7 cultivars), Lr10 (14), Lr13 (5), Lr14a (1), Lr16 (1), Lr23 (3); the unknown genes were identified in 14 cultivars. The first unknown gene could be either Lr9, Lr19, or Lr25; however, the second unknown gene in 9 cultivars was different from any named gene. Twelve of the 15 pathotypes are virulent for this gene, hence its use in breeding for resistance will be limited. The cultivars were also evaluated at two field locations in Mexico with two pathotypes in separate experiments. The area under the disease progress curve and the final disease rating of the cultivars indicated genetic diversity for genes conferring adult plant resistance. based on the symptoms of the leaf tip necrosis in adult plants, resistance gene Lr34 could be present in at least 20 cultivars. More than half of the cultivars carry high to moderate levels of adult plant resistance and were distributed in each region.     

Rooting of bean leaves and use in germplasm evaluation for common bacterial blight resistance

Summary A simple protocol for leaf rooting in beans (Phaseolus vulgaris L.) was developed and used to investigate the reaction of Xanthomonas campestris pv. phaseoli (Xcp), causal pathogen of common bacterial blight disease, in detached versus attached bean leaves. Trifoliate leaves of different sizes (one-third, two-thirds, and fully expanded), either with or without the pulvinus attached to the petioles, were excised from 20 day-old plants of six bean cultivars/lines. Leaf cuttings were cultured in potting medium and then incubated for 5 to 10 days under transparent polyethylene plastic cover in the greenhouse. Roots were readily initiated along the petioles of the leaf cuttings, whether the pulvinus was present or absent. All leaves which were two-thirds expanded and fully expanded developed roots 5 to 7 days after culture. Eighty to 90 percent of the leaves which were one-third expanded formed roots 8 to 10 days after incubation. Laminae of the rooted leaf cuttings were viable and green during the 2 to 3 months period in culture after removing the plastic cover. The common bacterial blight reactions were similar for inoculated attached leaves, detached rooted leaves (inoculated either after or prior to rooting), and moistened detached leaves incubated without rooting. The latter were only usable for evaluation of the Xcp reaction in growth chamber experiments but not under greenhouse conditions. The rooted leaves would be useful for screening bean lines for multiple disease resistance, especially if the pathogens require different environments for disease expression.Abbreviations CBB Common Bacterial Blight - Xcp Xanthomonas campestris pv. phaseoli (Smith) Dye     

Inheritance of resistance to Xanthomonas campestris pv. translucens in triticale

Summary Three triticale lines, Siskiyou, M2A-Beagle, and OK 77842 have been reported to possess resistance to bacterial leaf streak caused by Xanthomonas campestris, pv. translucens (Xct.). The three resistant lines were crossed to susceptible lines and crossed with each other. F₂, BC₁-F₁, BC₂-F₁ plants were inoculated with a mixture of two Xct strains. The segregation data indicate the presence of a single dominant gene in each of the three resistant lines to bacterial leaf streak. These three genes are either the same or closely linked herein designated as Xct₁.     

Components of resistance to late leaf spot and rust among interspecific derivatives and their significance in a foliar disease resistance breeding in groundnut (Arachis hypogaea L.)

Late leaf spot (LLS) and rust cause substantial yield losses and reduce the fodder and seed quality in groundnut (Arachis hypogaea L.). Adoption of resistant cultivars by the semi-arid tropic farmers is the best option to overcome yield losses. Knowledge on components of resistance to these diseases should facilitate the development of groundnut cultivars with enhanced resistance to LLS and rust. The objectives of the experiments were to study the genetic variability and relationships among components of resistance to LLS and rust, and assess their significance in disease resistance breeding. Fifteen interspecific derivatives for LLS and 14 for rust and a susceptible control, TMV 2, were evaluated in a randomised complete block design with two or three replications under greenhouse conditions. The experiments were repeated twice. Genotypic differences were highly significant for all the traits studied. Resistance to LLS is due to longer incubation and latent periods, lesser lesions per leaf, smaller lesion diameter, lower sporulation index, and lesser leaf area damage and disease score. Selection based on components of resistance to LLS may not lead to plants with higher retained green leaf area. The remaining green leaf area on the plant should, therefore, be the major selection criteria for resistance to LLS in breeding programs. Resistance to rust is due to longer incubation and latent periods, fewer pustules per leaf, smaller pustule diameter, lower sporulation index, and lesser leaf area damage and disease score. Rust resistant components appear to work additively, therefore, selection based on resistance components together with green leaf area retained on the plant should be the basis of selecting for resistance to rust in breeding programs. ICGV 99005, 99003, 99012, and 99015 for rust and ICGV 99006, 99013, 99004, 99003, and 99001 for LLS are the better parents for use in resistance breeding programs. This revised version was published online in July 2006 with corrections to the Cover Date.