Black Sigatoka The Pseudocercospora fijiensis causes Black Sigatoka (aka Black Streak Disease, BLSD) on and

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Pathogen Description to UV-exposure. Both spore types can occur on the leaf The fungus causing BLSD has the scientific name simultaneously. Pseudocercospora fijiensis (formerly known as Control fijiensis). It reproduces both sexually The commercial Cavendish cultivars on which the and asexually with the different phases in its life-cycle export industry is based are highly susceptible producing different spore types. It is a hemi-biotrophic to the disease, and chemical control via the regular pathogen so it requires living plant tissue for part of its application of fungicides (often weekly) is essential. life-cycle and lives on dead/dying plant material at However, the continuous reduction of fungicide other times. sensitivity in the population of the fungus is a major Mode of Infection problem in controlling the disease. The fungus infects leaf tissue, entering the plant Detection through its stomata typically on the abaxial (lower) side BLSD may be found together with related fungal of the leaf. It encircles the substomatal cavity, growing pathogens in a so-called ‘disease complex’ (particularly between cells in the mesophyll and into the pallisade in SE Asia). This makes it difficult to distinguish by layer and air cavities. It continues to live and grow this symptoms alone; DNA-based methods have been way (without damaging its host) for several weeks developed to screen for the fungus in plant tissue before causing widespread cell death. Sexual (notably using the Polymerase Chain Reaction, PCR). ascospores and asexual conidia are dispersed from out of the stomata. Interesting Facts Symptoms BLSD is considered the most damaging and costly First visible signs of infection are reddish-brown spots disease of banana & plantain. Banana is grown and streaks parallel to leaf veins. These merge to give throughout the tropical and sub-tropical regions of the larger dark-brown to black streaks, which eventually world, and is a staple crop for many developing leads to large patches of dead (necrotic) tissue. Leaf countries. It is both a vital income source for chlorosis (yellowing) and necrosis reduces the subsistence farmers and a multi-billion dollar export photosynthetic capacity of and causes a industry, in part because banana is the most widely physiological switch to (early) fruit ripening (making eaten fresh fruit in North America and Europe. the fruit unfit for export). Further reading Disease Spread Open access journal paper (2016), Combating Sigatoka http://dx.doi.org/10.1371/journal.pgen.1006234 Leaf-to-leaf, and plant-to-plant spread occurs via rain- Open access journal paper (2016) Genomics and Disease Control splash and run-off of conidia. Regional spread (<100 http://dx.doi.org/10.1371/journal.pgen.1005876 km) can occur with wind-dispersal of ascospores being Banana Knowledge Bank significant in the outbreak of epidemics. Longer- http://www.promusa.org/Black+leaf+streak distance spread is rare because the spores are sensitive