Severe outbreak of Fusarium wilt on common beans (Phaseolus vulgaris) caused by Fusarium oxysporum in the Maule Region, central Chile

Common bean (Phaseolus vulgaris L.) is an important grain legume cultivated worldwide as food for humans and livestock (Schwartz et al., 2005). Common beans in central Chile reach up to 3,893 ha from which 1,069 ha are located in the Maule region. Common bean is produced by small farmers who have limited access to fertilization, technical irrigation, and crop protection. In spring 2018, bean plants initially showed a slight yellowing and premature senescence 50 days after sowing (das) until showing wilting symptoms (70 -100 das) in Curepto fields (35 05'S; 72 01'W), Maule region. The basal part of affected plants displayed internal reddish-brown discoloration of the vascular tissues. Based on the plant external symptoms, we estimated an incidence between 15% and 45% in bean fields. Nine symptomatic plants were collected, and surface washed with sterile water and disinfested with 75% ethanol (v/v). Then small fragments (5-mm) from damage vascular tissue from each plant were cut and placed on Petri dishes containing PDA acidified with 0.5 ml/l of 92% lactic acid (APDA, 2%). The isolations were incubated for seven days at 25°C. Nine Fusarium-like isolates from single-spore on APDA (2%) became pale vinaceous, floccose with abundant aerial mycelium and dark vinaceous reverse colony, with a growing rate of 10.8 to 11.6 mm/d at 25°C (Lombard et al., 2019). Phialides were short, singular growing laterally on the mycelium. Macroconidia were hyaline, fusiform with basal foot cells shaped to pointed and apical cells tapered, 2-5 septate, and 28.6 to 47.6 (av. 38.1) μm long x 2.2 to 3.6 (av. 3.1) μm wide. Microconidia were hyaline, oval to ellipsoid, one-celled, and 4.5 to 10.9 (av. 6.1) μm long and 2.2 to 3.3 (av. 2.7) μm wide (n=50 spore). For molecular identification, three isolates (Curi-3.1, Be-8.1, and Be-11.3) were sequenced using PCR amplification of the partial sequences of beta-tubulin (BT) and translation elongation factor 1-α gene (TEF) (Lombard et al., 2019). NCBI BLAST analysis showed 99 to 100% similarity with sequences (TEF; BT) of strain CPC 25822 of Fusarium oxysporum. The maximum-likelihood phylogenetic analysis placed the Chilean isolates in the F. oxysporum complex clade. Chilean sequences were deposited into GenBank under accession numbers MW419125, MW419126, MW419127 (TEF) and MW419128, MW419129, MW419130 (BT). Pathogenicity tests (isolates Curi-3.1, Be-8.1, and Be-11.3) were conducted under greenhouse (15-28°C, 85%RH) on healthy bean plants (n=