The wind-borne fungal disease commonly known as black Sigatoka is continuing its steady progression through the Caribbean region.Martinique was one of the few islands still free of black leaf streak disease (BLSD) when the air-borne fungus that causes it was detected in September 2010 on a crop of plantain in Ducos, in the southwestern part of the island. The disease was confirmed both by observation of the anamorph form of the pathogen and by molecular analysis.
Symptoms have since been observed in several other locations on the island and on other susceptible varieties such as Grande Naine and Figue Sucrée. Scientists believe that ascospores were carried to the island by southerly winds over a 2-week period in August and that the heavy rains that followed favoured disease development.
Originally from Southeast Asia, genetic studies of the pathogen suggest that the fungus was carried to other continents in infected plant material. The first observation in Latin America dates back to 1972 in Honduras (see figure). The fungus quickly colonized all of central America before entering the North and South American continents. It then jumped to the Caribbean, arriving in Cuba in the early 1990s. Its arrival in Trinidad in 2003 opened up the front that is now threatening Dominica and Guadeloupe.
BLSD is dreaded by commercial banana producers because of the ever increasing cost of controlling it. Being a foliar disease, BLSD interferes with photosynthesis and, as such, affects bunch weight, but it also affects fruit quality by reducing the green life of the fruit after harvest.
Growers manage the problem by spraying with fungicides so that the plants retain at least 3-5 standing leaves. Bunches that come from plants with less leaves at harvest are usually not exportable, as they would ripen too quickly.
Compounding the problem, history shows that the number of chemical treatments needed to keep the disease in check increases with time, as the fungus evolves resistance to fungicides. In Costa Rica, for example, the number of fungicide applications went from 30 a year at the beginning of the 1990s to 50 in 2007.
In Martinique, where the disease is still spreading and disease severity is still low, scientists at the French international agricultural research institute CIRAD are confident that the impact on the banana industry can be minimized, especially if growers are careful to regularly remove necrotic leaves.
For more information contact Catherine Abadie