First symptoms can be found on the 3rd and 4th open leaf for both Sigatoka fungi. Tiny, light yellow specks (1-2 mm long) appear on the upper leaf blade, parallel to the secondary veins (Yellow Sigatoka) and reddish-brown flecks on the underside (Black Sigatoka). These specks later develop into narrow, brown or dark green spots with a spindle shape. These lesions expand further parallel to the veins and form oblong rusty red streaks with water-soaked centers and yellow halos (4 to 12 mm in length). The centers of the streaks gradually turn gray brown to brown, a sign of necrosis. Along the leaf margins, they coalesce to form large, black or brown necrotic lesions surrounded by yellow halos. The cracking of the leaves gives them a ragged appearance.
The symptoms of Yellow and Black Sigatoka are caused by the fungi Mycosphaerrela sp. and occur throughout the world. It is one of the most destructive diseases of banana. The fungus survives in dead or living plant tissues, and produces spores that are 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. It is more frequent at higher altitudes and cooler temperatures, or during rainy seasons in subtropical growing regions with warm environments and high relative humidity. The optimal growth temperature for the fungus is around 27°C and young leaves are the most susceptible. The disease reduces plant productivity, which in turn affects the size of the bunch and shortens the ripening time of the fruit.
Biological control with bio-fungicides based on Trichoderma atroviride have the potential to hinder 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, calixin or chlorothalonil can be applied as foliar sprays when disease is not widespread. Rotations of systemic fungicides such as propiconazole, fenbuconazole or azoxystrobin also work fine. The rotation is important to prevent the build up of resistance in the fungus.