Mycosphaerella fijiensis v2.0

M. fijiensis

Photos: Micrograph by Cecilia M. Rodriguez-Garcia (top),
Gert Kema (bottom)

The fungus Mycosphaerella fijiensis causes black leaf streak disease (BLSD) or black Sigatoka, the most economically important disease of bananas.  BLSD was first described during 1963 from samples collected in the Sigatoka Valley of Viti Levu, the largest island in the Republic of Fiji, and has spread since to most banana-producing regions worldwide.  This fungus is more aggressive than its close relative M. musicola, causal agent of yellow Sigatoka, which it has largely replaced during its global spread over the past 20 years.  Infection by M. fijiensis can reduce banana yields by more than 50%, and the disease is controlled only by extensive use of fungicides with up to 50 sprays per annum.  These sprays can comprise 40% of overall production costs and, globally, total more than $520 million per year.  In addition to the monetary cost, the extensive aerial application of fungicides extracts a toll on the environment and on the health of farm workers.

Mycosphaerella fijiensis is a haploid, hemibiotrophic ascomycete with filamentous growth in both liquid and solid media.  It has a bipolar, heterothallic mating system and crosses can be made under laboratory conditions.  The current genetic linkage map of M. fijiensis contains 10 large and 13 smaller linkage groups with markers spaced approximately every 7.0 cM.  The mating-type locus has been sequenced recently, revealing two idiomorphs – alleles that code for different proteins – that are similar to those in other ascomycete fungi.  Annotation of the genome will be aided by 40,000 EST sequences that were generated from three in vitro libraries.

This pathogen is related to another fungus with a sequenced genome, the Septoria tritici blotch pathogen of wheat, M. graminicola.  These two together represent the Mycosphaerella branch of the fungal evolutionary tree, the largest genus of plant pathogenic fungi.  Hosts for species of Mycosphaerella and related asexual genera include virtually every major crop plant in temperate, subtropical and tropical environments with a huge overall economic cost.  The genomic sequence of M. fijiensis will be valuable for comparative genomics with M. graminicola to identify the genetic factors involved in host specialization and aggressiveness, and hopefully also to find additional genes for fungicide targets that can be exploited for better control of disease epidemics.

Bananas are grown in more than 100 countries in tropical and subtropical regions, where they are the fourth most important food crop after rice, wheat and maize.  More than 80% of global banana production is consumed locally, providing a nutritious, staple food for millions of subsistence farmers as well as a source of income.  About 13% of the global crop is exported.  Banana leaves and stalks left over from harvesting, plus peels discarded during processing, provide a cheap source of biomass that can be fermented into ethanol for potentially economical energy production in developing countries.  Black leaf streak disease is devastating to subsistence farmers who depend on banana crops for food, and could have a negative effect on energy production from banana biomass.  The genomic sequence of M. fijiensis will be essential to unraveling the secrets of its pathogenic abilities for better control of the disease with reduced fungicide input and concomitantly higher yields of the developing world’s fourth most important crop.