Fusarium oxysporum f. sp. cubense is responsible for Fusarium wilt of banana, also known as Panama disease. The fungus does not affect the fruit, but causes the infected plant to wilt and the leaves to turn yellow, leading to its death. There is currently no treatment against this fungus, which is easily spread by the planting of asymptomatic infected plants or by objects in contact with contaminated soil, such as agricultural machinery or boots. The only effective means of control is the destruction of infected plants and a ban on access to contaminated areas. This fungus has several races, which can attack different varieties of banana plants. Tropical race 4 (TR4) is the only one capable of infecting the Cavendish banana, the world’s most widely grown variety. It first appeared in the early 2000s in Australia and Southeast Asia and has since spread to several countries with tropical or subtropical climates.
Identification of the first cases in Mayotte
The Mycology Unit of the ANSES Plant Health Laboratory was first consulted in September 2019, following the discovery of a banana plant in Mayotte showing the symptoms of Fusarium oxysporum infection. This was the first contamination by tropical race 4 on the island. It was officially confirmed by three successive tests: a molecular PCR test, using a method developed by the ANSES team; a vegetative compatibility test, which verified that the fungus specimen was specifically compatible with strains of tropical race 4; and a pathogenicity test, which confirmed that it was capable of attacking the Cavendish banana plant. This work was carried out in collaboration with Stellenbosch University, South Africa, and was published in the journal Plant Disease last November. An intensive search for other contaminated banana plants was conducted on the island. Three other cases of TR4 infection were detected and the plants concerned were destroyed.
A global threat
After a period during which no further cases were reported, eleven new samples were sent to ANSES in December. According to the PCR analyses carried out, it was not TR4 this time. The tests carried out were unable to determine the causes of the symptoms observed. So far, no cases of infection by Foc TR4 have been reported in the other French overseas départements, but the fungus is progressing around the world and has reached South America, with the first case identified in Colombia in 2019. Its spread is a threat to banana crops intended for export, but also to local populations in Asia and Africa, for whom this fruit – together with plantains, which are also affected by TR4 – is a staple food.