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Establishing the basis for disease resistance genetic studies on Portuguese common bean (Phaseolus vulgaris L.) germplasm

Traditional landraces are unique genetic resources, highly valuable for yield, disease resistance and quality modern crop improvement. Common bean (Phaseolus vulgaris L.) was introduced in Portugal more than 5 centuries ago and due to its high organoleptic and nutritional value, it became a popular crop all over the country. Very diverse landraces are still widely grown at the farmersʼ fields where they are commonly found in intercropping associations with other crops, such as maize (Zea mays L.). Nevertheless, several constraints to bean yield stability exist, such as susceptibility to several major fungal diseases. Rusts and powdery mildew fungi, foliar biotrophic pathogens, are among the limiting factors of common bean production. Identification and characterization of sources and mechanisms of resistance to use in plant breeding is regarded as a cost efficient method. Understanding the detected resistance genetic basis will facilitate the resistance handling by the breeders. The study of the genetic diversity existing among the different sources of resistance will be fundamental to select the parental lines to use on the development of contrasting populations needed for the genetic resistance studies. During a field expedition to the Central region of Portugal, members of our team collected several bean landraces adapted to intercropping and low-input farming systems. From those, 45 were screened for genetic diversity with 6 microsatellite markers. Parameters of genetic variation were analyzed, and a Neighbor-joining tree generated, suggesting the existence of two genetically distinct clusters. Additionally, a collection of 92 common bean landraces (7 in common with the described molecular screening) was evaluated for resistance to rust and powdery mildew. Rust infection was analyzed 15 days after inoculation (DAI) with results revealing different types of infection and, although several accessions showed intermediate infection patterns, some landraces presented small necrotic flecks, indicative of resistance. Moreover, regarding the severity of damage caused by both pathogens, the evaluation done 15 DAI suggested that some accessions among the collection have potential to be chosen for genetic resistance studies as they presented low levels of disease severity. The high disease resistance and genetic diversity present among these traditional bean landraces foresee great potential for the improvement of this important national germplasm resources.

Phaseolus vulgaris; landraces; Portugal; resistance; microsatellites; diversity

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