Prevalence and Incidence of Cassava (Manihot esculenta) Brown Leaf Spot Disease Caused by Cercospora heningsii in Macuata Province, Vanua Levu, Fiji

Cassava (Manihot esculenta Crantz) is a crop of many values in the tropical and subtropical regions of the globe. In Fiji, cassava is cultivated on vast acres of land but, the yield obtained is relatively lower because of many constraints, including the prevalence of diseases caused by the different pathogens. Among various pathogens responsible for a lower yield, the cassava brown leaf spot disease caused by Cercospora heningsii is responsible for causing enormous annual losses of cassava in tropical and subtropical regions. Because there is very little information regarding the association of the brown leaf spot disease and cassava in the country, the present study using survey as research instrument endeavors to determine the disease incidence and prevalence of brown leaf spot disease in the cassava fields of three villages (Mani Road, Boca, and Anuve) in the Bulileka area of the Macuata province in Vanua Levu, Fiji. The study found that brown leaf spot disease prevailed (100%) in all three villages. The percentage of disease incidence ranged from 36.4% to 42.9%. The maximum incidence (42.9%) of cassava brown leaf spot disease was found in Anuve village, followed by Mani Road village (38.2%), with the lowest disease incidence recorded for Boca village (36.4%).


Introduction
Cassava (Manihot esculenta Crantz) is a shrubby perennial plant of the family Euphorbiaceae that typically grows from one to three meters (3-10 feet) in height (Thresh et al., 1998). Among the 28 known species of the Euphorbiaceae family, cassava is the only edible crop, and its tuberous roots are a good source of carbohydrates (Katz & Weaver, 2003). Cassava is a key staple food in several countries (African, South American, Asian, and the Pacific) and has the highest production potential calories per hectare per day among tropical crops (Alicai et al., 2007). In Fiji, cassava is grown by small-scale farmers for subsistence use and its cultivation often also constitutes an essential source of income in rural and marginal areas.
However, cassava growth and yield are affected by biotic constraints (Hahn et al., 1989), among which pathogenic diseases are of critical importance. In Fiji, a primary fungal disease that affects cassava production is the brown leaf spot disease caused by Cercospora heningsii (Tsatsia & Jackson, 2010). The symptoms of this disease appear as small brown spots with dark borders on the upper surfaces of the leaves (Msikita et al., 2000). On the underside of the leaf, the disease spot displays greyish color with less distinct borders, with minor veins crossing the disease spots and appearing as black necrotic lines. The center of the disease spot is dry, looks cracked, and appears as if it will suddenly fall off (Tsatsia & Jackson, 2010).
The cassava leaves are the core source of assimilation for dry matter production. An increase in brown leaf spot disease reduces the photosynthetic area and capabilities of the plant (Hahn & Hozyo, 1984). A severe brown leaf spot infection results in total defoliation and hence failure (Alabi & Waliyar, 2004). Similarly, many of the leaf spot-causing organisms can kill the host partially or fully, not only by direct destruction of the tissues but also by systemic dispersal of toxic substances far beyond the original areas of infection (Bilgrami & Dube, 1976

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Disease in calculation followed b (Table 1).  Vol. 13, No. 8; influenced the disease incidence in these three villages to vary. Banito et al. (2007) reports that environmental conditions such as temperature, humidity, soil type, soil moisture, and cropping patterns are important parameters that influence the incidence of this disease.
Although we have observed higher disease incidence in all three villages, the percentage of disease incidence is below 50%, meaning that more than 50% of the cassava plants are unaffected by the brown leaf spot disease, which explains why the Crop Farmer's Guide for Farmers formulated by the Ministry of Agriculture, Government of Fiji indicated that there is no significant disease that threatens cassava in Fiji (Fiji Government, 2015).

Prevalence of Cassava Brown Leaf Spot Disease
The prevalence of cassava brown leaf spot disease for the three villages was calculated using Equation 2. Results reveal a 100% disease prevalence for all the three villages (Table 2). High disease prevalence can be attributed to the susceptibility of cassava plants to the disease and the availability of a conducive climate for the disease to proliferate. For example, its prevalence in the study areas can be attributed to weather parameters such as the wind (moderate to high), which all three villages experience every day. In their study, Pelczar et al. (2020) and Tsatsia and Jackson (2010) found that wind was responsible for carrying disease spores across fields and/or from one field to another, and these spores infected cassava plants. In addition, because of tropical climate that Fiji enjoys, the study areas are also exposed to high humidity. Garcia-Guzman et al. (2016) write that humidity also increases the prevalence of the disease in the fields. Therefore, a study as such helps validate the many reports that say that the brown leaf spot disease could be prevalent around the globe where cassava grows (Pei et al., 2014;Powbunthorn et al., 2012;Banito et al., 2007), as we have seen in the case of Fiji for the villages understudy.

Implication of Study, Limitation, and Further Studies
The impacts of the cassava brown leaf spot disease on cassava yield is widely reported (Elegba et al., 2013;Tsatsia & Jackson, 2010;Otim-Nape et al., 1997;Terry & Hahn, 1980;Terry & Oyekan, 1976). High disease incidence and prevalence and as well improper controls and management by the farmers can lead to yield losses if left unattended. Loss of yield will not only threaten food security but also income security in the villages of Mani Road, Boca, and Anuve, and the same can be said for other villages in Fiji where cassava is cultivated and may also be victims of the disease but are unaware of it. Furthermore, since the current climate of Fiji (Australia Bureau of Meteorology and CSIRO, 2014) is conducive for its proliferation (Tsatsia & Jackson, 2010), we suspect that other parts of Fiji may be facing similar issues.
The limitation of this study is that it does not measure the climatic conditions of the study area for supplementary explanations of the variation in incidence, and the prevalence of the disease at the Mani Road, Boca, and Anuve villages. Also, the number of study sites and the total number of plants selected are too small to determine the overall incidence and prevalence of cassava brown leaf spot disease for the whole of Fiji. However, this can be done by increasing the number of study sites and the selection of reasonable representative sample of plants.
Further studies can be undertaken to determine disease severity, impacts on yield, and its management. Also, it will be worthy to evaluate the current and future effects of climate change on disease incidence, severity, prevalence, and its implications on food and income security of local cassava farmers in Fiji.

Conclusion
Cassava brown leaf spot disease was prevalent in this study, 100 percent for all three villages, while the disease incidence varied across the three villages. Cassava plants showed extremely severe brown leaf spot symptoms in the cassava fields, raising the fear of high yield losses. Therefore, it is essential to conduct regular inspection and monitoring of cassava brown leaf spot disease to ensure the application of correct disease management strategies, and for successfully increasing crop yields.

Disclaimer
This paper represents the views of the authors and not that of their employers.