EVALUATION OF A DNA-PROBE FOR THE QUANTITATIVE DETECTION OF COMMON BACTERIAL-BLIGHT IN COMMON BEAN AND ITS APPLICATION IN A BREEDING PROGRAM

Breeding of Phaseolus vulgaris L. for resistance to common bacterial b light (CBB) can be done with visual evaluations of symptoms to disting uish broad resistance classes, but a more quantitative measure was nee ded for genetic studies of resistance. A novel method of evaluation wa s developed by quantifying Xanthomonas campestris pv. phaseoli (XCP) i n bean leaf tissue infected with CBB using a P-32-labeled probe and de nsitometric analysis of hybridization signals. Quantification of bacte rial populations using the probe was highly correlated (r = 0.98) with the number of colony forming units (CFU) from plate counts of the sam e leaf samples. The probe was used to follow XCP population dynamics o n 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 F-2/F-3 populati on segregating for resistance. Narrow sense heritability estimates wer e 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 anticipa ted response to selection for CBB resistance would be less based on ba cterial numbers than based on symptom expression in this population. I n breeding for resistance to CBB, selection based on visual symptoms c ombined with measurements of XCP populations using a DNA probe can be used to develop bean genotypes that are both resistant to symptom deve lopment and bacterial multiplication.