How resistant are the Portuguese common bean accessions to powdery mildew and rust fungi?

Abstract

Common bean (Phaseolus vulgaris L.) was introduced in Portugal more than 5 centuries ago, becoming a popular crop throughout the country due to its high organoleptic and nutritional properties. Nowadays it represents 80% of the National grain legume income (INE). However, common bean is vulnerable to several diseases, such as rust and powdery mildew, which leads to significant yield losses, limiting its cultivation. The very diverse bean landraces that are still grown at farmers’ fields represent unique and valuable genetic resource for breeding and disease resistance improvement. From the Portuguese Germplasm Bank (BPGV) common bean collection, 89 traditional landraces were evaluated for resistance to powdery mildew and 92 to rust fungi in seedling tests, under controlled conditions. Powdery mildew disease severity (DS) was assessed 7 and 15 days after inoculation (DAI) as the percentage of leaf tissue covered by mildew colonies. Many landraces showed intermediate levels of DS (6-25%) when compared to the susceptible check, 3 had no visible symptoms and 12 showed low levels of DS (0-5%) with hypersensitive resistant reaction. R ust infection type (IT) was analyzed 15 DAI using a 0-4 scale and a range of patterns of disease reaction was observed in the collection. DS was also estimated, with 42 landraces showing low values (DS<5%) and IT=4, indicative of partial resistance. The high levels of resistance here described and the associated high genetic diversity detected in these landraces in previous studies anticipate great potential of the traditional national germplasm for the enhance ment of this important crop.