Mechanism discovered that makes Black Sigatoka fungus less sensitive to pesticides

An international team of researchers, led by Wageningen University and Research, has discovered a new genetic mechanism that makes the infamous Black Sigatoka fungus less sensitive to the most important pesticides used against the disease. The discovery is an explanation for the trend of the ever-decreasing sensitivity of the fungus, and emphasises the importance of the development of banana varieties that are resistant to the fungus causing Black Sigatoka.

The fungus that causes the feared Black Sigatoka disease in the banana cultivation, Pseudocerospora fijiensis, has to be chemically combatted all over the world. In practice, this means that farmers have to spray against the disease between 25 and 70 times per year. A certain group of pesticides, the demethylase inhibitors (DMIs), are the backbone of controlling the disease. However, all over the world, the fungus is becoming less sensitive to these substances.

Gert Kema, professor of tropical phytopathology at the WUR, coordinated the research. Kema: “Until now, it was thought the decreased sensitivity of the fungus was only caused by changes in the proteins affected by the pesticides, a demethylase enzyme. We were therefore only looking at mutations in that part of the gene that codes for the enzyme. But changes in the promotor, the switch guiding the gene, also turned out to cause decreased sensitivity. We discovered a bit of DNA in the promotor that is present in the promotor up to eight times in a row. The larger the number of repeats of that bit of DNA in the promotor, the more insensitive to the fungus.”

The less sensitive versions of the Black Sigatoka fungus found in the cultivation of bananas and researched by the team regularly had a combination of mutations in the for the enzyme coding part of the gene, as well as DNA repeats in the promotor. Kema: “The pesticides can’t latch unto the enzymes as well due to the mutations in the coding part of the gene. The DNA repeats in the promotor cause the gene to become even more active. Those two things combined apparently mean the fungus has enough good-working enzymes in its cells, so it’s much less affected by pesticides, and this results in the banana plants even becoming ill if the farmers uses pesticides.”

The findings of the research team emphasise the importance of well thought-out pesticides, that don’t just use DMIs but also pesticides that work completely differently. That way, the fungus will become less sensitive to DMIs less quickly. Kema: “Our results also emphasise the importance of the development of banana varieties that are resistant to the Black Sigatoka fungus. Only then will we be able to globally grow bananas much more sustainably.”