Race 4 identification of Fusarium oxysporum f. sp. cubense from Cavendish cultivars in Hainan province, China

Panama disease in banana, caused by Fusarium oxysporum f. sp. cubense, is one of the diseases that has caused the greatest damage to banana crops worldwide throughout the 20th century (Stover 1972). The highly susceptible cultivar, Gros Michel, used by growers until 1960, was replaced by clones of the Cavendish Subgroup. Although Cavendish cultivars are resistant to Race 1 and Race 2 of the pathogen, Race 4 damages these cultivars in subtropical banana-growing regions (Su et al. 1977). During surveys conducted in 2005 and 2006, a high incidence of Banana fusarium wilt on Cavendish cultivars was observed in Hainan province of South China. A rapid pathogenicity test using small seeding Gros Michel had previously been developed (Stover 1959), which was useful for rapid identification of F. oxysporum f. sp. cubense, but not for races of this fungus. We used colony morphology on modified K2 medium (Sun et al. 1978) to identify Race 4 of F. oxysporum f. sp. cubense in Hainan, China.

Forty isolates of F. oxysporum f. sp. cubense obtained from Cavendish cultivars sampled at random from 20 banana farms of Hainan province, China, and 10 isolates obtained from Gros Michel cultivars sampled from five other banana farms in Hainan with the same method, and six other species of Fusarium conserved in our laboratory, were tested on modified K2 medium.

The modified K2 medium consists of a basal medium and 100 mL of solution. The basal medium contains the following compounds in 900 mL of distilled water: K₂HPO₄, 1 g; KCl, 0.5 g; MgSO₄.7H₂O, 0.5 g; FeNaEDTA, 0.01 g; L-asparagine 2 g; galactose 10 g, and 16 g of agar. After autoclaving, the basal medium was mixed with 100 mL of solution containing the following agents: PCNB (pentachloronitrobenzene, 75% WP), 0.9 g; oxgall, 0.5 g; Na₂B₄O₇ 0.10H₂O 0.5 g; streptomycin sulfate 0.3 g. The medium was adjusted to pH 3.8 ± 0.2 with 10% phosphoric acid. Six plates per treatment were used and colonies were observed after 15-day incubation at 28°C under fluorescent light. The experiment was repeated three times.

All 40 isolates of F. oxysporum f. sp. cubense obtained from Cavendish cultivars formed laciniated colonies on modified K2 medium that were distinct from those of isolates from 10 Gros Michel cultivars and six other Fusarium spp. (Fig. 1). The number of rays produced per colony ranged from 20 to 50 and the colonies appeared yellowish when observed from the bottom (Fig. 2). None of the isolates from Gros Michel cultivars and from six other Fusarium spp. formed laciniated radial colonies on the same medium. All the isolates of F. oxysporum f. sp. cubense from Cavendish cultivars, and 10 isolates from Gros Michel cultivars and six other Fusarium spp. did not produce laciniated colonies on potato-dextrose agar.

Fig. 1.  Colony morphology of F. oxysporum f. sp. cubense race 4 on modified K2 medium, showing colonies of isolates from Cavendish cultivars (upper) and isolates from isolates from Gros Michel cultivars and other Fusarium spp. (lower).

Fig. 2.  Colony morphology of F. oxysporum f. sp. cubense race 4 on modified K2 medium, showing that the colonies of isolates from Cavendish cultivars appeared yellowish when observed from the bottom. Those colonies of laciniated appearance are F. oxysporum f. sp. cubense race 4.

The Cavendish cultivars are susceptible only to race 4 (Su et al. 1977). According to the colony morphology on modified K2 medium (Sun et al. 1978), 40 isolates of F. oxysporum f. sp. cubense obtained from Cavendish cultivars from 20 banana farms of Hainan province, China, were race 4. These results further support the reliability of the method for identification of F. oxysporum f. sp. cubense race 4, at least for the isolates from Hainan. This may reflect the short history of the fungus in Hainan. It is likely that the fungus originated from a single introduction from Taiwan. However, it is possible that a clone of race 4 without laciniated colonies may be found in the future.