Induction and selection of mutations for resistance against bacterial leaf blight in rice

Summary Dry seed lots of a rice cultivar, Harebare, susceptible for bacterial leaf blight (BLB), were exposed to thermal neutrons with and without pre-treatment of seed for boron-enrichment, and to gamma-rays. M₁ plants of each of the treatments were grown and their seeds were separately harvested. M₂ populations were raised in rice fields of a farmer in a region where BLB is epidemic every year, and M₂ and control plants which expressed resistant reactions to BLB were selected. M₃ and control lines as plant progenies of the selected M₂ plants were raised in the same rice fields as in M₂ generation in order to investigate their BLB resistance and other agronomically important traits. Variances for disease severity of M₂ populations were significantly larger than those of the control population. whereas their means were not singificantly different from that of the control population, suggesting induction of mutations toward both resistance and susceptibility to BLB. Mean disease severity values of the M₃ lines from selected M₂ plants were significantly smaller than those of the control lines, indicating gains of selection in M₂ for quantitative resistance against BLB. Thermal neutrons, especially with boron-enrichment pre-treatment were effective to induce mutations for resistance against BLB. Some M₃ mutant lines with quantitatively enhanced resistance against BLB were not modified in other agronomic traits from those of the original cultivar. Significance of the induced quantitative resistance in breeding programmes for BLB resistance is discussed.     

Time-related behaviour of phenotypic variation in Lotus corniculatus regenerants under field conditions

Summary Plants of Lotus corniculatus regenerated from single genotype-derived calli showed for several agronomic and morphological traits a variation similar to that present in the original population. Comparisons of the results obtained under field conditions during the 2nd and 3rd year from plant regeneration show: a) the maintenance of significant variance within callus-derived population for several traits examined; b) the total disappearance of minus variants indicating a time-related decrease of depressive in vitro effects.Research work supported by C.N.R., Italy-Special grant I.P.R.A.-Subproject 1. Paper n. 553.     

Application of cell and tissue culture and in vitro selection for disease resistance breeding — a review

Summary Somaclonal variation, i.e. the variation induced by cell and tissue culture, offers an opportunity to broaden the genetic variation of crops. As a result of somaclonal variation a wide range of plant characteristics can be altered. However, the selection of agronomically important traits, e.g. disease resistance, has many limitations. The efficiency of selection can be increased by the application of in vitro selection procedures. Selection strategies that may be applied to obtain disease resistant somaclonal variants are described. Their merits and limitations, in relation to the efficiency of the procedures, the frequency of disease resistant variants and the genetics of the resistance obtained, are discussed.     

Pollen retention following natural self pollination in peach,almond, and peach × almond hybrids

Summary Retention of pollen grains following natural self-pollination was evaluated in 15 cultivars (cvs.) of almond, 4 peach and 2 nectarine cvs., and 37 interspecific peach × almond hybrids compared to 7 almond seedlings. The level of pollen retention was presumed to reflect and integrate the degree of homogamy, the amount of pollen produced by the flower, the extent of anther-stigma contact during anthesis, and the level of pollen germination. Pollen retention averaged 5 times greater in the peach and nectarine cvs. than in the almond cvs. The greater pollen retention, characteristic of the peach, was dominant in expression in the interspecific F₁ hybrids over the lower levels of pollen retention, characteristic of the almond. Thus, gametophytic self-incompatibility is not the only trait supporting outcrossing in the almond. Our data are consistent with the concept of co-evolution of floral traits relating to different breeding strategies. The level of pollen retention could often be anticipated at anthesis on the basis of blossom phenotype. That is, stigma-anther contact was observed frequently in the blossoms of peach, nectarine, and the peach × almond F₁ hybrids, but only infrequently in almond.     

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 bacterial blight disease of rice

Summary Inheritance of resistance to the Punjab isolate of Xanthomonas campestris pv. oryzae of bacterial blight disease of rice was studied in seven breeding lines resistant to the disease. The results revealed that resistance in breeding lines PAU 122-73-1-4-1, PAU 164-102-1-2-1-1-1, KJT 24, IR 5657-33-2-1-2 and IR 22082-41-2-2 was controlled by single dominant genes allelic to the dominant gene which confers resistance to the Punjab isolate in Patong 32. Resistance to the Punjab isolate in breeding lines IET 7172 and RP 2151-40-1 was found to be controlled by single recessive resistance genes allelic to one of the recessive resistance genes present in BJ 1. The two genes are independently inherited and are being used to develop bacterial blight resistant varieties.     

Inheritance of resistance to wildfire and angular leaf spot derived from Nicotiana rustica var. Brasilea

Summary The introgression of wildfire (races 0 and 1) and angular leaf spot (ALS) resistance from N. rustica var. Brasilea into N. tabacum has proved economically useful in Zimbabwe although the mode of inheritance of, and genetic relationships between the resistance are unknown. This study was undertaken to (1) examine the mode of inheritance of the resistance to races 0 and 1 of wildfire, and ALS, (2) determine the genetic relationship between the resistances and (3) establish whether the N. rustica-derived wildfire race 0 resistance is allelic to that obtained from N. longiflora. Inheritance was examined under greenhouse and field conditions by studying disease reactions in the parental, F1, F2 and backcross generations derived from crosses of three susceptible lines to a resistant line Nr-7. Three-point backcrosses to the susceptible parent were examined for linkage and segregating generations from a cross of Nr-7 to Burley 21 which carries the N. longiflora race 0 resistance were used to test for allelism. In general, we observed that all resistances are determined by a single dominant gene although some incosistent ratios were obtained likely due to misclassification of disease reactions and erratic transmission. All resistances showed linkage although pleiotropism cannot be ruled out. Allelism tests demonstrated that the N. rustica race 0 resistance is not allelic to that obtained from N. longiflora. Our findings are examined in relation to the efficacy of indirect selection for resistance.     

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.     

Evaluation of field resistance of Daucus carota cultivars to Cercospora carotae (carrot leaf spot)

Summary A world collection of 142 cultivars of carrot was tested for resistance to Cercospora carotae in the field. C. carotae infection was observed in all the cultivars. Field resistance was observed in only about 30% of the cultivars, the remainder being highly susceptible.     

Measuring field resistance of cabbage cultivars to black rot

Summary Disease progress and gradient curves of black rot on cabbage were evaluated in field plots of the cultivars Bartolo, Erdeno, Perfect Ball, and Roxy in The Netherlands during 1991 and 1992. Plots were inoculated by single sources in the centre of each plot. Individual plants were examined for disease incidence and severity. Disease progress was described by the Gompertz model. The overall measure of absolute rate (disease progress rate r multiplied with maximum disease intensity K) was used to compare cultivar effects on disease progress. Disease gradients were described by the negative exponential model. The percentile distance (distance from the source at which disease intensity reached 1% of the empirical maximum disease intensity) was used to compare cultivar effects on disease spread. Disease severity is more sensitive than disease incidence to calculate the disease progress and spread of black rot. Measures of progress and gradient were about equally effective to screen cultivars for field resistance to black rot. Perfect Ball was the most susceptible, Erdeno and Bartolo were intermediate and Roxy was the most resistant for incidence and severity measures. Increased levels of field resistance reduced the development of black rot in time and in space. Field resistance of black rot is thought to be composed of several mechanisms. Microplots provide a good instrument for the assessment of small differences in field resistance, expressed equally well in disease progress as in disease gradient curves.     

Characterization of resistance of cassava genotypes to bacterial blight by evaluation of leaf and systemic symptoms in relation to yield in different ecozones

Twenty-two improved and local cassava genotypes were evaluated for their bacterial blight symptom types in reaction to infection by Xanthomonas axonopodis pv. manihotis under field conditions in the forest, forest savanna transition and wet savanna zones of Togo. High genotype × environment interactions in development of each symptom type were observed. Combining data on environments and genotypes, spot, blight and wilt symptoms were positively correlated. Analysing genotype reactions across environments, indications for independent mechanisms of resistance on leaf and stem level, varying by genotype, were found. Genotypes Main27 with resistance to spot and blight symptoms and TMS4(2)1425 with resistance to wilt symptoms are recommended to breeders to introgress their resistance characteristics. Significant negative correlations were generally observed between blight and wilt symptom development and root yield across ecozones, with blight being more important under lower, and wilt under higher inoculum pressure. Genotypes TMS30572, CVTM4, TMS92/0429 and TMS91/02316 showed low spot, blight and wilt symptoms combined with high root yield across ecozones.     

Sources and inheritance of resistance to stemphylium leaf spot of lettuce

Summary None of the tested cultivars of lettuce was found resistant to Stemphylium leaf spot, a common disease in Israel. Within a Lactuca saligna population collected in wild lettuce in Israel, resistance was traced. Interspecific crosses of L. saligna x L. sativa were made and the mode of inheritance of resistance to this disease was studied. Resistance is apparently controlled by two genes: one dominant (Sm₁) and one recessive (sm₂).Contribution No. 1176-E 1984 series, from the ARO.     

Independence of self-compatibility and potentiality for self-pollination in peach x almond hybrids

Summary The almond of commerce (Prunus dulcis (Mill.) D.A. Webb) is self-incompatible (SI) and requires honey-bees to effect the transfer of pollen among cultivars that flower simultaneously. Four year old trees from the F₂ generation of several peach x almond hybrids were studied to determine whether self-compatibility (SC) and the potentiality for natural, i.e., abiotic, self pollination (NSP) are genetically related or are inherited independently. Both SC and the high potentiality for NSP are characteristic of peach (Prunus persica (L.) Batsch) but not almond. Forty percent of SC genotypes exhibited adequate NSP (SC, +NSP) for good fertility i.e., without insect-mediated pollination. The remaining 60% of SC genotypes (SC,-NSP) exhibited an average 61% reduction in fruit set on limbs bagged to exclude honeybees during anthesis relative to fruit set on open pollinated limbs. Our data are consistent with the concept that fertility is dependent upon the load of compatible pollen deposited on the stigma. Fruit set reduction on bagged limbs, compared with bagged and self-pollinated limbs, was presumably due to a) lack of/insufficient pollination for fertilization and/or b) post-zygotic abortion of genetically inferior recombinants. Selection following manual self-pollination may result in SC genotypes with or without the capacity for NSP. In contrast, significant fruit set on limbs enclosed during pistil receptivity necessitates that the genotype selected express both SC and the potentiality for NSP.     

Salt-tolerance in Brassica juncea L. I. In vitro selection,agronomic evaluation and genetic stability

Summary In vitro selection of salt tolerant plants of Brassica juncea L. (Indian mustard) cv. Prakash has been accomplished by screening highly morphogenic cotyledon explant cultures on high NaCl media. Out of a total of 2,620 cotyledons cultured on high salt medium, 3 survived, showed sustained growth and regenerated shoots. They were multiplied by axillary bud culture on NaCl free medium. The salt-selected shoots retained salt tolerance following 3 month of growth and multiplication on control medium. While two of these somaclones flowered and set seeds, third one grew slowly, had abnormal leaf morphology and was sterile. The seed of the two fertile plants were sown in the field to raise R₁ segregating generation. Data were recorded for field, other agronomic components and oil content. The somaclonal lines, both selected salt-tolerant and non-selected, showed tremendous amount of variation for all the characters studied. One of the two tolerant somaclones invariably showed reduced height, longer reproductive phase and higher 1000 seed weight. Based on the agronomic performance of R₁ plants of these somaclones, some plants were selected and their progeny were evaluated for agronomic performance under standard field conditions and salt-tolerance in the greenhouse using sand pot culture method. Most of the lines bred true for their specific characteristics. In the greenhouse, selected salt-tolerant somaclones (SR-2 and SR-3) performed better for plant growth, yield and other agronomic traits at higher salt treatments, indicating thereby that salt-tolerance trait selected in vitro was expressed in the whole plants and is genetically stable and transmitted onto the progeny. The two tolerant lines, however, differed in their salt-tolerance during vegetative and reproductive phases as indicated by their salt-tolerance and stress susceptibility indices. The mechanism of salt-tolerance is not clear and needs to be further investigated.     

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.     

Genetic resources in apple,pear and vineyard peach populations in former Yugoslavia

Summary During the period 1988–90, several germplasm collecting trips were made to all republics of former Yugoslavia. A total of 56 old apple cultivars, many of which are represented in up to 5 types, 38 old pear cultivars and 367 genotypes of vineyard peaches were collected. The availability of so much genetic and genotypic wealth made it possible to start apple disease resistance breeding and peach cultivar and rootstock breeding programmes.     

Genetical relationships between reactions to bacterial leaf spot,yellow mosaic and Cercospora leaf spot diseases in mungbean (Vigna radiata)

Summary Studies on the inheritance pattern of bacterial leaf spot (BLS), yellow mosaic (YM) and Cercospora leaf spot (CLS) reactions in crosses of BLS and YM resistant/tolerant but CLS susceptible × CLS resistant but BLS and YM susceptible parents indicated that resistances to BLS and CLS were governed by single dominant genes, whereas YM tolerance was a monogenic recessive character. The studies also indicated that these three genes were inherited independently. The simple inheritance pattern and independent assortment of the genes governing resistance/tolerance to these diseases suggest that the usual breeding methods will be adequate to develop multi-disease resistant mungbean cultivars.Paper XII in the series Studies on resistance in crops to bacterial diseases in India.     

Wheat somaclonal variants showing earliness, improved spot blotch resistance and higher yield

Somaclones (R₂, R₃ and R₄generations) were regenerated from immature embryos of two spring wheat varieties,HUW-206 and HUW-234. Many somaclones displayed improved earliness, enhanced resistance to spot blotch disease and increased yield over the parent. The superiority of variants for yield traits and disease resistance was established in R₄ generation, confirming the possibility of wheat improvement through somaclonal variation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetics of host reactions to three races of the bacterial pustule pathogen in cowpea

Summary Inheritance of the brown hypersensitive resistant (BHR), non-hypersensitive resistant (R) and susceptible (S) host reactions produced by three races of the bacterial pustule pathogen (Xanthomonas campestris pv. vignaeunguiculatae) was studied in 45 F₁, F₂ and testcross progenies, using the infiltration inoculation method BHR reaction was dominant over R and S reactions, and R was recessive to S reaction. Two genes appeared to be involved in BHR reaction; one governing BHR reaction to race 1 and the other to races 1 and 2. Both were ineffective against race 3. R reaction, effective against all the races, appeared to be controlled by one, two or three recessive genes. One cowpea line had one BHR gene and two duplicate recessive R genes. Reaction expression in the segregants was clear and as expected with races 2 and 3 but was modified with race 1, possibly due to interactions between dominant or recessive alleles of the BHR genes and the homozygous recessive allele of the R genes. Gene symbols Bp-1 and Bp-2 are proposed for the BHR genes and bp-3, bp-4 and bp-5 for the recessive R genes. The genes present in each of the differential cowpea line are suggested.Contribution from the International Institute of Tropical Agriculture, Ibadan, Nigeria and Crop Development Division, Ministry of Agriculture, P.O. Box 9071, Dar es Salaam Tanzania.     

Evaluation of a DNA probe for the quantitative detection of common bacterial blight in common bean and its application in a breeding program

Summary Breeding of Phaseolus vulgaris L. for resistance to common bacterial blight (CBB) can be done with visual evaluations of symptoms to distinguish broad resistance classes, but a more quantitative measure was needed for genetic studies of resistance. A novel method of evaluation was developed by quantifying Xanthomonas campestris pv. phaseoli (XCP) in bean leaf tissue infected with CBB using a ³²P-labeled probe and densitometric analysis of hybridization signals. Quantification of bacterial populations using the probe was highly correlated (r=0.98) with the number of colony forming units (CFU) from plate counts of the same leaf samples. The probe was used to follow XCP population dynamics on susceptible (BAT 41) and resistant (OAC 88-1) bean genotypes. OAC 88-1 supported a maximum XCP population which was approximately tenfold less than BAT 41. The probe was also used to study an F₂/F₃ population segregating for resistance. Narrow sense heritability estimates were less for resistance measured on the basis of bacterial populations (0.18–0.26) than on visual scores of symptoms (0.29–0.38). The anticipated response to selection for CBB resistance would be less based on bacterial numbers than based on symptom expression in this population. In breeding for resistance to CBB, selection based on visual symptoms combined with measurements of XCP populations using a DNA probe can be used to develop bean genotypes that are both resistant to symptom development and bacterial multiplication.Abbreviations CBB common bacterial blight - CFU colony forming units - XCP Xanthomonas campestris pv. phaseoli