Qualitative and quantitative evaluation of cassava bacterial blight resistance in F1 progeny of a cross between elite cassava clones

Deployment of resistant varieties is one major approach to controlling cassava bacterial blight (CBB), caused by Xanthomonas axonopodis pv. manihotis (Xam). To understand the genetic determinism of resistance to CBB, the use of reliable parameters measuring resistance is necessary. In order to test a relevant method for evaluation of quantitative resistance for mapping QTL (quantitative trait loci), the response of 150 F₁ individuals, inoculated with four different Xam strains (CIO-84, CIO-1, CIO-136 and CIO-295), was assessed under controlled conditions. We used two types of evaluations at different intervals after inoculation, one based on a scale of 0 to 5 and the second based on the determination of the bacterial population in the vascular system. Both evaluation types revealed interaction between strains and F₁ genotypes. Population values at 3 and 6 cm from the point of inoculation showed a high level of correlation. By performing an association analysis, at 7 and 15 days after inoculation, a significant positive correlation between both evaluation types was obtained. However, the disease rating at 30 days did not correlate with bacterial populations at either 7 or 15 days after inoculation, except for one strain, CIO-84. Evaluation of the bacterial population in stem tissues is time and labour consuming, consequently, for a rapid and reliable assessment of CBB resistance for QTL analysis, we strongly recommend evaluation based on the use of a symptom scale.     

Correlated resistance of cassava to mosaic and bacterial blight diseases

Summary The two most serious diseases of cassava (Manihot esculenta Crantz) are cassava mosaic disease (CMD) and cassava bacterial blight (CBB) (Xanthomonas manihotis Starr). Clone 58308, derived from the third backcross of the interspecific cross of cassava (M. esculenta) x ceara rubber (M. glaziovii), showed a high level of resistance to both diseases. Crosses of 58308 with several other clones which varied from susceptible to moderately susceptible to both diseases gave progenies with a significant genotypic correlation between resistance to both diseases (r=0.90), apparently due to linkage. The heritabilities of resistance to the diseases were estimated at 50–70% for CMD and 25–65% for CBB. Resistance to both diseases is assumed to be polygenic. The correlated response to selection for CMD and for CBB was estimated.     

Methods for detecting the cassava bacterial blight pathogen: A practical approach for managing the disease

Cassava bacterial blight (CBB), caused by Xanthomonas axonopodis pv. manihotis (Xam), is a particularly destructive disease in South America and Africa. The movement of infected asymptomatic stems is a major means of pathogen dispersal as well as infected seeds. The success of a cassava-seed certification program depends on the availability of reliable tests to detect the pathogen in vegetative planting materials and true seeds. We report here the different methods that permitted to detect the pathogen in cassava tissues. A polymerase chain reaction (PCR) test was developed for this pathogen. The PCR assay worked well for pathogen detection in extracts from leaf and stem lesions and the minimum number of cells that could be detected ranged from 3 × 10² to 10⁴ CFU per ml. Nested-PCR worked well for Xam detection from naturally infected seeds. This technique was specific, sensitive, and rapid for detecting Xam in cassava true seeds. The highest detection level found was 1–2 viable cells per reaction. A dot-blot assay was developed by evaluating a 898 bp DNA fragment unique to Xam strains as a diagnostic DNA probe. The probe detected Xam strains in crude extracts of leaf and stem lesions, cassava fruits and sexual seeds that were naturally infected. Overall sensitivity of the dot-blot method was about10³CFU per reaction. The dot-blot hybridization technique can be easily used for culture indexing. A monoclonal antibody (MAb) was also used for an enzyme-linked immunosorbent assay (ELISA) and tested with various infected tissues. Overall sensitivity of the method was about10³CFU per reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Leaf Waxes of Cassava (Manihot Esculenta Crantz) in Relation to Ecozone and Resistance to Xanthomonas Blight

Summary To elucidate the role of leaf surface structures as first barriers to confer resistance to bacterial blight, leaf stomata and their occlusion with leaf waxes were studied in cassava genotypes. For the first time, cassava leaf waxes were quantitatively and qualitatively analysed. Comparing the susceptible and resistant standard genotypes BEN86052 and TMS30572, respectively, the total quantity of triterpenes was significantly higher in the resistant genotype, grown in three ecozones of Benin. In cuticular leaf waxes of seven cassava genotypes the triterpenes beta amyrins, epi-taraxerol, taraxerone and taraxerol were dominant constituents across genotypes, and alkanes (C₂₅-C₃₃) and acids (C₃₀ and C₃₂) occurred in minor concentrations. Comparing seven genotypes, no clear relation between resistance or ecozones and total quantities of the major wax constituents was observed. Only the highly resistant genotype TMS30572 showed high triterpene levels irrespective of ecozone. Scanning electron-microscopy revealed a regular distribution of waxes at the abaxial leaf surface, covering and occluding stomatal pores of susceptible and resistant genotypes, while on the adaxial leaf surface waxes were in form of crystalloids and did not occlude the stomata. The number of stomata on the abaxial surfaces was about 7–11 fold higher than on the adaxial surfaces, where stomata were located along the midrib and major veins. No significant differences in stomata number were observed between genotypes varying in resistance to bacterial blight. It is suggested, that stomata on the adaxial surface might be portals of entry for the bacteria.     

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.     

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.     

Breeding cassava for resistance to cassava mosaic disease

Summary Cassava mosaic disease (CMD) is one of the most serious and widespread diseases throughout cassava growing areas in Africa, causing yield reductions of up to 90%. Early research on breeding of cassava (Manihot esculenta Crantz) for resistance to CMD in Africa is reviewed. Changes in population size and in activity of the white-fly vector to CMD (Bemisia tabaci Genn.) in relation to changes in environmental conditions such as amount and distribution of rainfall, light intensity and temperature are discussed in relation to screening for resistance to CMD. Over the past eight years, significant progress has been made at the International Institute of Tropical Agriculture (IITA). Resistance to CMD has been successfully incorporated into high yielding cultivars of acceptable quality. The CMD resistant material has been evaluated and many promising clones have been selected in various countries in tropical Africa and India. The resistance has been effective in those countries.     

A major QTL for common bacterial blight resistance derives from the common bean great northern landrace cultivar Montana No.5

Knowledge of the evolutionary origin and sources of pest resistance genes will facilitate gene deployment and development of crop cultivars with durable resistance. Our objective was to determine the source of common bacterial blight (CBB) resistance in the common bean Great Northern Nebraska #1 (GN#1) and GN#1 Selection 27 (GN#1 Sel 27). Several great northern cultivars including GN#1, GN#1 Sel 27, and Montana No.5 (the female parent of the common x tepary bean interspecific population from which GN #1 and GN # 1 Sel 27 were derived) and known susceptible checks were evaluated for CBB reaction in field and greenhouse environments. These genotypes and CBB resistant and susceptible tepary bean including Tepary #4, the male parent and presumed contributor of CBB resistance toGN#1 and GN#1 Sel 27, were assayed for presence or absence of three SCAR markers tightly linked with independent QTLs conditioning CBB resistance. The parents and F₂ of Montana No. 5/GN #1 Sel 27 and Montana No.5/Othello(CBB susceptible) were screened for CBB reaction and SCAR markers. CBB resistance in Montana No.5 was comparable to that of GN#1 and GN#1 Sel27. The SAP6 SCAR marker present in GN#1 and GN#1 Sel 27 was also present in Montana No.5, and it co-segregated (R ² =35%) with the CBB resistance in the Montana No.5/Othello F₂ population. Although a few CBB resistant and susceptible transgressive segregants were found in the F₂ of MontanaNo.5/GN #1 Sel 27 and later confirmed by F₃ progeny tests, SAP6 SCAR marker was present in all progenies. None of the tepary bean specific CBB resistance-linked SCAR markers were present in GN#1, GN#1 Sel 27, or Montana No.5. A cluster analysis of 169 polymorphic PCR-based markers across three common bean and Tepary #4 indicated that GN#1, GN#1 Sel 27, and Montana No.5 were closely related, and not related at all with Tepary #4.Thus, these results clearly indicate Montana No.5, not Tepary #4, as the source of CBB resistance in GN#1 and GN#1 Sel 27. This revised version was published online in August 2006 with corrections to the Cover Date.     

Resistance to common bacterial blight in Phaseolus vulgaris L. recombinant inbred lines under natural infection of Xanthomonas axonopodis pv. phaseoli

Among the main causes of poor yield in common beans are fungal, viral and bacterial diseases. Common bacterial blight, caused by Xanthomonas axonopodis pv. phaseoli (Xap), is one of the major bacterial diseases leading to significant losses in Brazil. Chemical control is ineffective, therefore, the use of resistant varieties becomes an interesting alternative. The objective of the present work was to evaluate disease resistance under natural infection of the pathogen in 109 recombinant inbred lines (F₇) of P. vulgaris originated from the cross HAB-52 (susceptible — snapbean) × BAC-6 (resistant — common bean) in two different environments, as well as to calculate genetic parameters to assist in the selection of promising materials to be used in the CBB resistance breeding program. The data of the genetic parameters were compared to those calculated for the F₃ generation originated from the same cross. The heritability results for DI (disease index) and VI (variation index) in F₃ were 26.85% and 0.26, respectively, whereas in F₇ they were 91.77% and 1.36, respectively. These results demonstrate a potential to be explored for this advanced population, that in the future, along with other pathogen variability studies and tests in other environments, may provide more information regarding a more precise evaluation of promising genotypes to be used in common bean breeding programs aiming to obtain CBB resistant varieties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Backcross breeding for improved resistance to common bacterial blight in pinto bean (Phaseolus vulgaris L.)

Common bacterial blight (CBB) caused by Xanthomonas campestris pv. phaseoli reduces common bean (Phaseolus vulgaris L.) yield and quality worldwide. Genetic resistance provides effective disease control; however. a high level of resistance is difficult to attain and does not exist in pinto bean, the most important dry bean market class in North America. Our objective was to determine if a backcross breeding approach with the aid of molecular markers linked to quantitative trait loci (QTL) for resistance to CBB in a donor parent could be used to attain higher levels of resistance to CBB in pinto bean. QTL conditioning CBB resistance from the donor parent XAN 159 were introgressed into the recurrent parent‘Chase’using classical backcross breeding and intermittent marker‐assisted selection.‘Chase’pinto bean is moderately resistant and the breeding line XAN 159 is highly resistant to Xanthomonas campestris. Marker assays confirmed the presence of independent QTL from GN no. 1 Sel 27 and XAN 159 in advanced backcross‐derived pinto bean lines with improved CBB resistance. Agronomic characteristics of‘Chase’were fully recovered in the backcross‐derived lines. An important QTL for CBB resistance from XAN 159 on linkage group B6 was not introgressed because tight linkage between this QTL and the dominant V allele that causes an unacceptable black‐mottled seed coat colour pattern in pinto bean could not be broken.     

RAPD-based genetic diversity study of fifty cassava (Manihot esculenta Crantz) genotypes

Fifty cassava clones were studied using RAPD technique. They included landraces from the Wenchi, Nkoranza, Dormaa Ahenkoro and Asonafo districts of the Brong Ahafo region of Ghana and three improved varieties. Genetic diversity of these genotypes was studied using four primers, OPK-01, OPR-02, OPR-09 and OPJ-14. A total of 41 different bands were detected. Levels of polymorphic fragments detected by the four primers ranged from 90% to 100%. By pooling bands from individual accessions together, mean number of fragments per accession per primer ranged from 5.50±1.04 for the Improved cultivars to 7.00±0.71 for populations of landraces from Dormaa. Mean frequencies of fragments not detected by the primers for the accessions were 0.524±0.12, 0.460±0.12, 0.561±0.12 and 0.523±0.12 for landraces from Wenchi, Nkoranza, Dormaa Ahenkro, Asonafo and the Improved varieties, respectively. The grand mean frequency of individuals showing fragments not present in populations was 0.522±0.10. Genetic diversity estimates ranged from 0.290 to 0.425 (mean 0.352±0.05) for primer OPK-01, 0.001 to 0.381 (mean 0.309±0.06) for primer OPR-02, 0.335 to 0.344 (mean 0.283±0.04) for primer OPR-09 and 0.152 to 0.352 (mean 0.261±0.07) for primer OPJ-14. Within the accessions mean gene diversity estimates were 0.316±0.03, 0.293±0.09, 0.331±0.02, 0.322±0.07 and 0.247±0.03 for accessions from Wenchi, Nkoranza, Dormaa Ahenkro, Asonafo districts and the Improved varieties, respectively. Interpopulational genetic divergence ranged from 0.069 to 0.203 (mean 0.119±0.04). Rate of nucleotide substitution among the landraces was 9.8 per cent per site per year, while that for the Improved varieties was 15 per cent. This revised version was published online in August 2006 with corrections to the Cover Date.     

Heredity of seventeen isozyme loci in cassava (Manihot esculenta Crantz)

Summary Starch gel electrophoresis was used to assess isozyme polymorphism in two Manihot species. Crude extracts were obtained from leaves and pollen. Ten enzymes were examined for their polymorphism in a germplasm collection of 365 cultivated plus 109 wild accessions, mainly from Africa. The inheritance of these enzymes was examined using 13 intra and interspecific progenies. Seventeen polymorphic loci were found for the ten enzyme systems, with 59 alleles. All the markers showed disomic heredity and three linkage groups were identified.     

Characterization and genetic distance analysis of cassava (Manihot esculenta Crantz) germplasm from Mozambique using RAPD fingerprinting

Twenty-eight cassava genotypes from Mozambique, along with seven genotypes from Angola, Madagascar, Nigeria, Togo, Columbia, and Thailand for comparison, were fingerprinted using random amplified polymorphic DNA (RAPD) analysis. The Mozambican material represented a wide range of landraces. A total of 311 scored RAPD loci were used to calculate genetic distances between the genotypes. This revealed an average genetic distance of 3.1% between all the germplasm. The average genetic distance between the Mozambiquen genotypes was 2.7%, whilst the seven accessions from the other countries showed an average distance of 3.4%. Neighbor-joining (NJ) method cluster analysis of the genetic distance yielded a tree that did not indicate a relationship between geographic distribution and genetic diversity. This revised version was published online in August 2006 with corrections to the Cover Date.     

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     

The effect of cassava mosaic disease on the genetic diversity of cassava in Uganda

Summary Cassava (Manihot esculenta) is a tropical crop that is grown in Africa, Latin America and Southeast Asia. Cassava was introduced from Latin America into West and East Africa at two independent events. In Uganda a serious threat to cassava's survival is the cassava mosaic disease (CMD). Uganda has had two notable CMD epidemics since the introduction of cassava in the 1850s causing severe losses. SSR markers were used to study the effect of CMD on the genetic diversity in five agroecologies in Uganda with high and low incidence of CMD. Surprisingly, high gene diversity was detected. Most of the diversity was found within populations, while the diversity was very small among agroecological zones and the high and low CMD incidence areas. The high genetic diversity suggests a mechanism by which diversity is maintained by the active involvement of the Ugandan farmer in continuously testing and adopting new genotypes that will serve their diverse needs. However, in spite of the high genetic diversity we found a loss of rare alleles in areas with high CMD incidence. To study the effect of the introgression history on the gene pool the genetic differentiation between East and West Africa was also studied. Genetic similarities were found between the varieties in Uganda and Tanzania in East Africa and Ghana in West Africa. Thus, there is no evidence for a differentiation of the cassava gene pool into a western and an eastern genetic lineage. However, a possible difference in the genetic constitution of the introduced cassava into East and West Africa may have been diminished by germplasm movement.     

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.     

Classification of cassava into ‘bitter’ and ‘cool’ in Malawi: From farmers' perception to characterisation by molecular markers

Cassava roots, a major food in Africa, contain cyanogenic glucosides that may cause toxic effects. Malawian women farmers considered fields of seemingly similar cassava plants to be mixes of both ‘cool’ and ‘bitter’ cultivars. They regard roots from ‘cool’ cultivars as non-toxic. Roots of ‘bitter’ were considered to require extensive traditional processing done by women to be safe for consumption. But curiously, these women farmers preferred ‘bitter’ cultivars since toxicity confers protection against theft, which was a serious threat to the food security of their families. We studied how well these farmers comprehend the effects of genetic variations in cassava when dealing with cyanogenesis in this complex system. Using molecular markers we show that most plants farmers identified as belonging to a particular named cultivar had a genotype typical of that cultivar. Farmers' ethno-classification into ‘cool’ and ‘bitter’ cultivars corresponded to a genetic sub-division of the typical genotypes of the most common cultivars, with four-fold higher cyanogenic glucoside levels in the bitter cultivars. Examining morphology, farmers distinguished genotypes better than did the investigators when using a standard botanical key. Undoubtedly, these women farmers grasp sufficiently the genetic diversity of cassava with regard to cyanogenesis to simultaneously benefit from it and avoid its dangers. Consequently, acyanogenic cassava – the breeding of which is an announced good of some cassava genetic improvement programmes – is not a priority to these farmers. Advances in molecular genetics can help improve food supply in Africa by rapid micropropagation, marker assisted breeding and introduction of transgenic varieties, but can also help to elucidate tropical small-scale farmers' needs and skills. This revised version was published online in August 2006 with corrections to the Cover Date.     

Resistance to Helminthosporium leaf blight and agronomic performance of spring wheat genotypes of diverse origins

Helminthosporium leaf blight (HLB), caused by a complex of Cochliobolus sativus (Ito & Kurib.) Drechsler ex Dastur and Pyrenophora tritici-repentis Died, is a serious disease of wheat (Triticum aestivum L.) in the warm lowlands of South Asia. Wheat cultivars grown in the area are either susceptible to HLB or possess low levels of resistance to it. A replicated field study was conducted in 1999 and 2000 at two sites in Nepal to determine the level of HLB resistance and other desirable traits in 60 wheat genotypes of diverse origin. The test genotypes were planted in main strips divided into two strips one of which was sprayed four times with Tilt (a.i. propiconazole) @ 125 g of a.i. ha^–1. Four readings of HLB were recorded to calculate the area under the disease progress curve (AUDPC). Other traits under investigation included biomass yield (BY), grain yield (GY), 1000-kernel weight (TKW), harvest index (HI), days to heading (DH) and maturity (DM), plant height (PHT), and effective tiller number (ETN). Wheat genotypes differed significantly for all traits. Mean AUDPC values ranged from 45 to 1268. A few exotic genotypes were highly resistant to HLB. Losses in GY due to HLB ranged from 2 to 26%, and TKW was reduced by up to 33%. A few genotypes showed HLB tolerance, i.e., relatively smaller GY and TKW reductions despite high levels of HLB. In general, medium to late maturity and higher levels of HLB resistance and low to high GY and TKW characterized genotypes exotic to South Asia. Biplot analysis identified several genotypes that were HLB-resistant and agronomically superior. Results suggest it is possible to improve HLB resistance of local wheat cultivars based on selective breeding using this pool of germplasm.     

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