E. Arnaud-Santana

Contributions of the Bean/Cowpea CRSP to management of bean diseases

Abstract Diseases are a major constraint to production of beans in developing countries, reducing yields and seed quality. Contributing factors include poor disease management, lack of resistant cultivars, and the limited availability of certified disease-free seed. From the outset the Bean/Cowpea Collaborative Research Support Program (CRSP) has emphasized integrated disease management, and breeding for resistance to bean rust ( Uromyces appendiculatus ), common bacterial blight ( Xanthomonas campestris (= X. axonopodis ) pv. phaseoli ) (CBB) and web blight ( Thanatephorus cucumeris (anamorph Rhizoctonia solani )) (WB). Later in the 1990s, angular leaf spot ( Phaeoisariopsis griseola ) (ALS), and virus induced bean common mosaic (BCM) and bean common mosaic necrosis (BCMN) became more widespread and epidemic. The research included development of disease-resistant germplasm and studies of pathogenic variation and epidemiology. During the early 1990s bean golden yellow mosaic (BGYM), became a major disease of common bean in the Caribbean and Central America. No cultivar resistant to the gemini virus causing BGYM was available in the Dominican Republic (DR) and pesticides were used to reduce transmission of the virus by white flies ( Bremisia tabaci ) biotype A. A change in the bean-production system to limit reproduction of the vector through a host-free period and concentrate the bean growing season to a four month period reduced BGYM significantly. Improved resistant cultivars and disease management resulted in high yields and self-sufficiency of beans in the DR. Evidence for co-evolution of the pathogens causing ALS, CBB, and rust in the two major bean gene pools (Andean and Middle American) was found. Variation in the WB pathogen on bean indicated independent genetic populations and the presence of different anastosmosis subgroups. WB disease management practices should be designed for the sub-group of the pathogen. Seed transmission was found to be a significant source of R. solani inoculum. Specific ( Ur-9 gene) and adult-plant resistance ( Ur-12 gene) to rust was identified for PC-50 (Andean origin) in the DR, and the genes were mapped. A mobile rust nursery was developed to monitor races of rust in a region and assist in resistance-gene deployment in bean germplasm and varieties. DNA based diagnostic methods were developed to differentiate X. campestris pv. phaseoli from X. campestris pv. phaseoli var fuscans (prevalent in East Africa) and P. griseola isolates. DNA hybridization methods also were developed to identify BGYM and bean golden mosaic viruses. Research in USA and East Africa has helped define bean common mosaic virus (BCMV) and bean common mosaic necrotic virus (BCMNV) as separate viruses and has demonstrated that BCMNV appears to be indigenous to Africa. Serological tools were developed that assist in the detection and identification of potyviruses, BCMV and BCMNV. These tools are now used worldwide.

New sources of nonspecific resistance to rust and common bacterial blight in dry bean landrace Pompadour

SummaryThirty four lines derived from a Pompadour landrace and three other lines from the Dominican Republic were co-inoculated with three cultures of Uromyces appendiculatus and three cultures of Xanthomonas campestris pv. phaseoli on separate leaves. The majority of the lines had dense trichomes (1–2 mm long) on the abaxial surfaces of upper leaves and developed small uredinia of moderate resistant rust reaction on the pubescent trifoliolate leaves and larger uredinia of moderate susceptible reaction on the primary glabrous leaves. Lines that lacked dense abaxial pubescence developed susceptible or moderate susceptible reactions on both primary and trifoliolate leaf stages. Differential reactions between strains of X. c phaseoli were observed on both leaf and pod infections and reactions on the leaf and on the pods were often different. Pompadour lines K, AE, R, and M had overall moderate resistance to three strains of X. c phaseoli and resistant or moderate resistance to three cultures of U. appendiculatus. These lines also have indeterminate growth habit associated with favorable and more stable seed yields and can now serve as valuable new germplasm sources for multiple disease resistance.

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

SummaryA 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.