First Report of Fusarium oxysporum f. sp. cubense Tropical Race 4 Causing Fusarium Wilt of Cavendish Bananas in Israel

Abstract

Fusarium wilt, caused by the soilborne pathogen Fusarium oxysporum f. sp. cubense (Foc), is a major fungal pathogen of banana worldwide (Ploetz et al. 2015). By 1960, race 1 of Foc severely affected the cultivar ‘Gros Michel’, which was replaced by resistant Cavendish group cultivars. However, a new race of Foc, tropical race 4 (TR4), causing mortality to Cavendish cultivars, was identified in the early 1990s in southeast Asia (Ploetz 2006). TR4 has subsequently spread to Australia, Africa (Mozambique), and the Middle East (Jordan and Lebanon) (Ploetz et al. 2015). In Israel, Cavendish bananas are cultivated in the lower Carmel coastal plain (1,200 ha), western Galilee (500 ha), and Jordan valley (800 ha). During July 2016, leaf-yellowing and wilting, accompanied by internal vascular discolorations of rhizomes and pseudostems, were observed in mature ‘Grande Naine’ Cavendish plants from Shfeya, Carmel coastal plain (32°35′5.6″N; 34°56′27.6″E), and 2 months later in plants from Kibbutz Ein Gev, eastern shore of Lake Galilee (32°46′29.6″N; 35°38′23.8″E). Vascular tissues from affected plants were sampled, surface sterilized, and plated on potato dextrose agar (PDA) medium amended with 250 ppm of chloramphenicol. White-colored colonies with floccose mycelia developed consistently from the tissues. Single conidia cultures from five of these colonies (Foc TR4-1, Foc TR4-2, and Foc TR4-3 from Shfeya, and Foc TR4-5 and Foc TR4-6 from Ein Gev) from different plants resembled F. oxysporum morphologically, producing three- to five-celled macroconidia, aseptate microconidia on monophialides, and terminal and intercalary chlamydospores, after 1 to 2 weeks (Leslie and Summerell 2006). DNA was extracted from the isolates, and PCR analyses confirmed their identity as Foc TR4 (O’Neill et al. 2016). TR4 was also confirmed in symptomatic tissues by specific PCR analysis (Dita et al. 2010) and sequencing of the amplified band. All five TR4 isolates were tested for pathogenicity. Inoculum was produced from 1-week-old cultures grown in PDA Petri dishes. Inoculation was conducted by dipping (30 min) root-wounded ‘Grande Naine’ plants with four leaves in 400 ml (10⁶ conidia/ml) of sterile water. Plants treated with a strain of F. oxysporum f. sp. melonis race 1.2 (Fom) specific to melon and water served as separate controls. Inoculated plants were then planted in a soil mix (Green90; Evenari.co.il) in 750-ml pots under 30°C and 16/8-h light/darkness photoperiod conditions. Ten replicate plants were treated each with the TR4 and Fom isolates, and water control. After 2 weeks, TR4-treated plants expressed typical Fusarium wilt symptoms. After 4 weeks, TR4-treated plants were dead, whereas controls remained healthy. TR4 identification from symptomatic plants was reconfirmed by PCR (O’Neill et al. 2016). TR4 was not isolated from asymptomatic control plants. Experiments were repeated twice with similar results. This is the first report of TR4 affecting Cavendish banana in Israel, in two confined and contained areas that are under strict supervision and surveillance by the Plant Protection and Inspection Services. All affected plants, approximately eight in Shfeya and 20 in Ein Gev, were destroyed (by injecting plants with 200 ml of 50% petroleum and 50% Roundup, drip irrigating with 500 liters/ha of metam sodium), irrigation terminated, trenches dug, and the area fenced and cordoned off to avoid entry of humans and stray animals, to prevent further spread. Regular surveys in adjacent banana fields have not revealed any new cases of disease to date.