Plant pathogen focus: black Sigatoka as a worldwide threat to banana

Banana trees affected by Sigatoka in Malawi. Photo courtesy of APS.

Since I work in the department of plant pathology at UCD—and even though I am not a plant pathologist myself!—I decided to start a series of posts on microbes that cause plant disease, focusing on stories that are of economic and societal importance.

A couple of weeks ago, one of my colleague—him a true plant pathologist—, managed to scare me by claiming during a talk that banana could disappear in the not-so-distant future! The culprit? The fungus Mycosphaerella fijiensis: This ascomycete causes a disease (black Sigatoka) that damages the leaves of banana trees and reduces photosynthesis. Moreover, the fungus triggers a premature ripening that spoils the fruit. Together these effects of black Sigatoka provoke the loss of 50% or more of the fruit production. 

[I learnt a lot about black Sigatoka in an online article by Randy Ploetz on the website of the American Phytopathological Society (APS). When no other source is explicit, the information in this post comes from the Ploetz article. In general, the APS website is a great starting point for everything related to plant pathology!]

Now, the prospect of having to say farewell to bananas was sad enough for me, but then I learned that banana was much more than an enjoyable fruit after lunch: Not only is it the most important fruit in terms of production, but it is also an essential food crop, coming right after rice, wheat and maize (Churchill, 2011). It is worth knowing that only about 10% of the world production is exported; the vast majority of bananas (including plantains) are consumed in the countries that produce them, and it is thus an essential part of the diet of many poor inhabitants. Originally from Southeast Asia, bananas are now cultivated worldwide, including in India, Uganda, Ecuador, Brazil and Colombia (Marín, 2005). What is also interesting to know is that there is no season of banana harvest, since banana trees produce all year long! 

Leaf spots caused by M. fijiensis. Photo courtesy of APS.

Banana trees are susceptible to several diseases, most of them caused by fungi. Among these diseases, the current most threatening one is the aforementioned black Sigatoka, also known as black leaf streak. Mycosphaerella fijiensis, the causative agent of the disease, was first observed in the Sigatoka Valley of Fiji (hence its name) in the sixties. Since then, black Sigatoka has spread to America, the Caribbean and Africa… and a considerable amount of money has been spent for years in its control by fungicides. And, unsurprisingly, the appearance of resistances followed. Today, to overcome resistance, growers apply mixtures of fungicides that have different modes of action. These treatments are costly, and it is estimated that they are responsible for up to 20% of the price we pay the banana on the shelf.

Efforts have led to the development of new plant hybrids that are resistant to black Sigatoka, but at present these new banana cultivars (cultivated varieties) do not produce fruits that meet exportation standards. Scientific research on the disease is very active, but it is complicated by the fact that Mycosphaerella fijiensis does not grow well in the laboratory. Despite the fungus’ reluctance to lab cultivation, much information has been obtained on M. fijiensis’ biochemistry, and its complete genome sequence is available since 2010. It has to be noted, in addition, that the banana genome has been completed this year. These developments will help research, undoubtedly, but the solutions to the threat caused by black Sigatoka are neither simple nor immediate. As far as I know there is no attempt to produce genetically-modified banana that would be resistant to black Sigatoka (on the other hand, GM bananas are currently developed in Uganda to resist to another banana disease caused by the bacterium Xanthomonas campestris). If the fungus resistance to chemical treatment increases, however, it might boost the use of the new hybrids in the future, despite the lower quality of their fruits. 

The future is challenging for the culture of banana all over the world, but hopefully growers and researchers will meet the challenge!

References:

• Ploetz R. C. (2001). Black Sigatoka of Banana:The most important disease of a most important fruit. American Phytopathological Society.
•  Marín D. H., Romero D. A., Guzmán M., Sutton T. B. (2003). Black Sigatoka: An increasing threat to bananacultivation. Plant Disease Vol. 87, pp.208-222.
• Churchill A. L. C. (2011). Mycosphaerella fijiensis, the black leaf streak pathogen of banana: progress towards understanding pathogen biology and detection, disease development, and the challenges of control. Molecular Plant Pathology, Vol.12(4), pp.307-328.