In the initial stage of the disease, small reddish-brown flecks (1mm) appear on the underside of the 3rd or 4th open leaf. Later on, these flecks enlarge and gradually become dark brown to black, elliptical or elongated sunken spots (4 to 12 mm) with sometimes a gray center and a characteristic yellow halo. The growth occurs parallel to the veins and is by then visible on both leaf surfaces. As the disease progresses, the spots coalesce and form long narrow brown streaks surrounded by large yellow areas, always following the tracing of the veins. Leaf margins might become water-soaked and black. In susceptible varieties or in cases of heavy infections, large areas of the leaf surface can become necrotic and die, reducing fruit yield or causing uneven ripening of fruit bunches.
Black Sigatoka is a fungal disease of banana caused by the organism Mycosphaerella fijiensis. It is sometimes referred to as black leaf streak because of the characteristic symptoms. It survives in dead or living plant tissues, and produces spores that are be spread by wind or rain splashes. Another way of disease transmission is via the movement of infected living plant material, plant trash or contaminated fruits. Black Sigatoka is more frequent in environments with high humidity and elevated temperatures. The optimal growth temperature for the fungus is around 27°C and young leaves are the most susceptible. It is one of the most severe banana diseases worldwide.
Bio-fungicides based on Trichoderma atroviride have the potential to control the disease and are being tested for possible field applications. Bordeaux spray applied on pruning sites can hinder the propagation of the disease on these plant parts.
Always consider an integrated approach with preventive measures together with biological treatments if available. Fungicides containing mancozeb or chlorothalonil can be applied as foliar spray when the disease is not widespread. Rotations of systemic fungicides such as propiconazole, fenbuconazole or azoxystrobin also work fine. The rotation is important to prevent the fungus from building up resistance.