Black Sigatoka or Black leaf spot disease of banana (scientific name : Pseudocercospora fijiensis) is one of the most economically important threat for banana growers. Sigatoka disease is caused by an ascomycete fungus which affects leaves thereby decreasing the photosynthetic surface of the plant.
In addition, fruits produced on heavily diseased plants can show a significant reduction in « green life » span. For export, this constitutes a problem as fruits ripen prematurely during shipment and are lost before being sold. Most areas of the tropics and subtropics are affected, with reported losses of up to 30-50%.
This relatively recent disease is now responsible of intensive fungicide application in producing countries. As the most exported banana cultivar in the world « Cavendish » is very susceptible to Sigatoka, this disease understandably represents a major threat. The risk is two-fold: a poor disease management leads to yield losses and increases production costs, and non-optimal product application can have negative impacts on human and environmental health. In both cases poor disease management leads to economic loss and impacts producing countries.
Black sigatoka symptoms
Black sigatoka is identified through the presence of streaks or leaf spots. They are first tiny and yellow, appearing on the lower face of leaves. They then become reddish to rusty-brown forming streaks running parallel to the veins. Within a few days, the spots become a few centimetres in size and turn brown with light grey centres. Plants with advanced black Sigatoka disease may have large areas of dead leaf which have lost their photosynthetic capacity, leading to a decrease in production. The leaf blade edges, which are often the most affected parts, tend to fold on themselves. The fruits are smaller and fewer and tend to have premature ripening.
Black Sigatoka transmission and cycle
Black sigatoka transmission occurs via two different types of aerial spores, called ascospores and conidia. Conidia are dispersed when water droplets are splashed on to other leaves within the same plant or neighboring plants, during rain events. Ascospores are transported by the wind over longer distances. They enter the leaf via the stomata and develop into colonies. After infection, hyphae emerge from the stomata and either develop into conidiophores with new conidia or grow across the surface and infect adjacent stomata. Both types of spores can germinate within two or three hours under humid conditions, but infection takes a minimum of two or three days under optimal conditions (high humidity and temperature >20 °C).
Black Sigatoka Control
In export plantations, Black Sigatoka is controlled with frequent applications of phytosanitary products and cultural practices, such as the removal of affected leaves, adequate spacing of plants and efficient drainage within plantation. The most used active ingredients in fungicides against Black sigatoka are chlorothalonil and mancozeb. Paraffinic oils are often used as adjuvants for fungicides during applications.
Biological control represents an alternative to the use of chemicals that may be feasible when disease pressure is limited. For example, fungi of the genus Trichoderma are extensively employed as biological control agents of many plant pathogens and have shown some success for Black sigatoka disease control . Among the advantages of using biological agents is the low risk of adverse impacts on the environmental and human health associated with the lower cost of this technology.
The use of resistant cultivars is another control method that might be used; however, this strategy is hampered by the long time necessary to develop new banana cultivars which meet consumers demands, and the variability of the pathogen, that adapt fast against new resistance genes.
All these practices turn out to be expensive. For example, fungicide application requires the use of airplanes or specialized machinery, facilities for mixing and loading the fungicides, man work, and the high expense of phytosanitary products. In effect, the costs of control might be as high as 15-20% of the final retail price of bananas.
Black sigatoka indicators
Given the elevated costs of Black sigatoka disease control and the danger of new resistances against currently used fungicides, it is important to develop an efficient treatment plan maximizing efficacy and avoiding unnecessary applications.
Among the tools that can assist this task is the use of indicators of disease severity.
What is an indicator ?
Indicators are statistics or values used to measure current conditions or to forecast future trends. They offer a synthetic view of a situation and give useful and quick indications that can help decision making. In agriculture, indicators are usually regarded as tools for assessment and management. As an example, a well-known indicator of agricultural perfomance is the rate « yield/cultivated land surface ».
In order to help growers to fight black Sigatoka, different indicators have been developed since the seventies in the Antilles and Cameroon. These indicators have been integrated into a warning system against yellow and black Sigatoka by the researchers of CIRAD (the French institution research leader in tropical agronomy) . This system is based on an early detection of attacks, which are monitored regularly and used to quantify the development of the epidemic. Observed symptoms are evaluated, classified and then transformed into scores that can be used to decide phytosanitary application when the risk is high.
Collecting observations for the calculation of the indicators of Black sigatoka
How to make the most of Black Sigatoka indicators: Sigatocare
ITK has recently responded to CIRAD’s request and digitalized their warning system: a new application for desktop and smartphone has automated the calculation of indicators and made observation recording easier.
Moreover, Sigatocare app provides a new type of indicator, which integrates those classically calculated by CIRAD, to evaluate the necessity of phytosanitary applications in the coming days. It is expressed according to a color key (red : high risk, yellow : medium risk, green : low risk). In order to develop it, ITK has formalized CIRAD’s expert reasoning behind the issuing of phytosanitary warnings.