Delayed upward colonization of xylem vessels is associated with resistance of common bean to Fusarium oxysporum f. sp. phaseoli

This work was aimed at comparing the spatial-temporal colonization of F. oxysporum f. sp. phaseoli (Fop) in the resistant line UFSC-01 with that in the progenitor susceptible cv. Uirapuru. Disease incidence and severity, vascular discoloration in hypocotyls, colony forming units (cfu) and ergosterol were quantified on root, hypocotyl and epicotyl tissues at 5, 10, 15, 20, 25 and 30 days after inoculation (dai). Fungal colonization was also monitored by light microscopy of cross sections from the mentioned plant parts as well as root crown at 1, 3, 5 and 25 dai. Cfu and ergosterol determinations lead to similar results showing that fungus colonized more efficiently susceptible plants. Fop grew intercellularly until reaching the xylem vessels of taproots. Thereafter, it started to colonize upper parts by producing a large amount of microconidia inside plant vessels. Earlier and stronger symptoms on susceptible plants were associated with both faster colonization and collapse of xylem vessels in aerial tissues. In contrast, vessels of resistant plants remained unaffected, and delayed colonization was associated with weak formation/transport of conidia in vessels of taproot and root crown. The possible role for microconidia in the upward colonization of bean plants is discussed.