Black Sigatoka

What you are looking at

Small, pale yellow to light green streaks appearing on the underside of a banana plant’s younger leaves, running parallel to the leaf veins. Over the following days and weeks each streak lengthens and darkens, passing through brown to black and developing a yellow halo around it, and in warm humid conditions individual streaks coalesce into large, irregular dead patches that can destroy most of a leaf’s functioning surface within weeks.

Because the disease kills leaves from the oldest outward, a heavily affected plant can be left with only its youngest few leaves still working by the time its fruit is filling out, and a plant photosynthesising through a fraction of its normal leaf area produces a smaller bunch, ripens it more slowly and unevenly, and often continues ripening unevenly after harvest.

The most damaging leaf disease banana growers face

This fungus is endemic across nearly all of the world’s major banana and plantain-growing regions, having spread from its origin in South-East Asia and Fiji, the source of the disease’s common name, through repeated introductions since the 1960s. It reached the Americas in Honduras in 1972 and arrived in the Caribbean through two separate introductions, in 1990 and 2003, and continues moving along banana-growing corridors wherever infected leaf material travels. Global yield losses attributed to this single disease commonly exceed a third of production where it is present and uncontrolled, and the cost of controlling it, mostly through frequent fungicide spraying, can account for more than a quarter of a commercial plantation’s total production cost.

That cost structure is the disease’s real difficulty. Fungicide rotation works, but weekly or near-weekly application is only realistic for large commercial operations, and resistant fungal strains have already emerged in several regions under sustained spraying pressure. For the smallholder growers who produce a large share of the world’s banana and plantain crop, cultural measures and, where a suitable variety exists, resistance bred into the plant itself matter more than any spray programme. It has not been found in mainland Europe or North America, where banana is not grown commercially at meaningful scale, and Australia and New Zealand both maintain strict quarantine on banana planting material specifically because of diseases like this one.

What to do about it

Remove and destroy heavily infected leaves regularly, which reduces local spore load and is valuable wherever fungicide is unaffordable or unavailable.

Space plants at recommended density rather than crowding them, and improve field drainage, both of which reduce the humidity that favours infection and spread.

Use certified, disease-free planting material, since infected suckers moved between farms and regions are one of the main ways this disease has spread internationally.

Grow a resistant or tolerant variety where one suits the local market, particularly on a smallholding that cannot sustain a commercial spray programme.

Favour a resistant variety and leaf pruning over spraying wherever the market allows it, particularly on a smallholding. Reserve fungicide rotation on a monitored schedule for a commercial planting where those measures alone are not enough, checking current local resistance management guidance given how widely resistant strains have already developed under heavy spraying.