Fusarium and Mycosphaerella Diseases of Banana

Banana is globally threatened by two diseases. Black Sigatoka or black leaf streak disease and Panama disease. Black Sigatoka is caused by the Dothideomycete fungus Pseudocercospora fijiensis and the most significant foliar disease of banana. The lack of effective host resistance reduces disease management to mere fungicide applications, which greatly increase the economic and environmental costs of banana production. Therefore, fungicide-resistant strains evolve rapidly in most banana plantations, causing disease-control failures throughout the world. Recently, two strains of P. fijiensis were sequenced and assembled. The 74-Mb genome of P. fijiensis is massively expanded by LTR retrotransposons, making it the largest genome within the Dothideomycetes. Electrophoretic karyotyping and analyses of molecular markers in P. fijiensis field populations showed chromosome-length polymorphisms and high genetic diversity, but limited gene flow. Frequencies of fungicide resistance in fungicide-treated plantations were much higher than those in untreated wild-type P. fijiensis populations. A homologue of the Cladosporium fulvum Avr4 effector, PfAvr4, was identified in the P. fijiensis genome. Infiltration of the purified PfAVR4 protein into leaves of the resistant banana cultivar Calcutta 4 resulted in a hypersensitive response, indicating that a homolog of the tomato Cf4 resistance gene is present in banana. Panama disease is caused by the soil-borne fungus Fusarium oxysporum f.sp, cubense. One particular strain, called Tropical Race 4 threatens the global banana crop and disseminates from Southeast Asia to other continents. Both fungi and strategies for long-term solutions will be discussed.