Most experts agree that it’s only a matter of time before the Fusarium wilt strain known as tropical race 4 escapes from Asia and reaches Latin America and Africa. The recent development of a rapid and reliable diagnostic test that can be used on plant and soil samples should make it harder for the fungal strain to spread undetected.Embrapa, the Brazilian Agricultural Research Corporation, have developed a PCR diagnostic test specific to Fusarium oxysporum f. sp. cubense (Foc) tropical race 4 (TR4). The test provides results in less than a day, compared to four months or more for the current method of pairing isolates with nitrate nonutilizing mutants to determine their vegetative compatibility group (VCG).
Molecular methods specific to strains are needed to compensate for the fact that the Foc strains pathogenic to bananas are indistinguishable morphologically, and so are the symptoms they cause. They do, however, show a high level of host specificity, hence their classification into races based on the differential response of cultivars: race 1 for the strains that cause disease in Gros Michel, Silk, Pome and Pisang Awak cultivars, and race 2 for those that prey on Bluggoe and other cooking bananas.
The shortcomings of such a system, however, soon become apparent for strains like TR4 that have a wide host range. In addition to attacking some of the cultivars that are resistant to races 1 and 2, such as the Cavendish bananas that dominate the export trade and potentially other varieties grown by smallholders, TR4 also causes disease in cultivars susceptible to these strains. So while a Cavendish with Fusarium wilt would immediately raise alarm, a Gros Michel infected with TR4, for example, would not because the assumption would be it is infected with race 1.
Such a situation may have already occurred with a milder version of race 4, subtropical race 4 (ST4). Unlike TR4 strains, which are pathogenic in both tropical and subtropical situations, ST4 strains only cause disease in Cavendish bananas that have been exposed to an abiotic stress such as cold temperatures. ST4 on Cavendish has been reported in Australia, the Canary Islands and South Africa. But while no sign of the disease has been reported on Cavendish in Latin America, isolates collected in Brazil on cultivars susceptible to race 1 have tested positive for a VCG associated with ST4.
The ambiguity inherent in the race designation system complicates the interpretation of field reports, hence the importance of having specific and reliable diagnostic tests, not just for TR4 but for all the pathogenic strains.
Corrigendum to the article "A molecular diagnostic for tropical race 4 of the banana fusarium wilt pathogen".