Rapid screening of Musa species for resistance to Fusarium wilt in an in vitro bioassay

In order to accelerate breeding and selection for disease resistance to Fusarium wilt, it is important to develop bioassays which can differentiate between resistant and susceptible cultivars efficiently. Currently, the most commonly used early bioassay for screening Musa genotypes against Fusarium oxysporum f. sp. cubense (Foc) is a pot system, followed by a hydroponic system. This paper investigated the utility of in vitro inoculation of rooted banana plantlets grown on modified medium as a reliable and rapid bioassay for resistance to Foc. Using a scale of 0 to 6 for disease severity measurement, the mean final disease severities of cultivars expressing different levels of disease reaction were significantly different (P a parts per thousand currency signaEuro parts per thousand 0.05). Twenty-four days after inoculation with Foc tropical race 4 at 10(6) conidia ml(-1), the plantlets of two susceptible cultivars had higher final disease severities than that of four resistant cultivars. Compared with 'Guangfen No.1', 'Brazil Xiangjiao' is highly susceptible to tropical race 4 and its mean final disease severity was the highest (5.27). The plantlets of moderately resistant cultivar 'Formosana' had a mean final disease severity (3.53) lower than that of 'Guangfen No.1' (4.33) but higher than that of resistant cultivars: 'Nongke No.1', GCTCV-119, and 'Dongguan Dajiao' (1.87, 1.73, and1.53, respectively). Promising resistant clones acquired through non-conventional breeding techniques such as in vitro selection, genetic transformation, and protoplast fusion could be screened by the in vitro bioassay directly. Since there is no acclimatization stage for plantlets used in the bioassay, it helps to improve banana breeding efficiency.