Proceedings of the 13th ISTRC Symposium, 2007 pp. 307 - 311 Characteristics and control of a new basidiomycetous root rot of cassava (Mannihot esculents) in Ghana Moses E.1, Akrofi S.2 and Mensah G.A.1 1CSIR-Crops Research Institute, P.O. Box 3785, Kumasi, Ghana 2Plant Genetic Resource Centre, Bunsu ER, Ghana Abstract. Cassava (Manihot esculenta) is than 500 million people in the developing one of the most important food crops in the world (Lozano, 1986; Farquet and Fargette, tropical world. Diseases such as African 1990; Thresh et al., 1997). Diseases such as cassava mosaic virus (ACMV), cassava African cassava mosaic virus (ACMV), bacterial blight (CBB) and cassava cassava bacterial blight (CBB) and cassava anthracnose (CAD) constitute a major anthracnose (CAD) constitute a major constraint to cassava production in parts of constraint to cassava production in parts of sub-saharan Africa. A mushroom type of sub-saharan Africa where the crop is fungus previously known to be parasitic on cultivated. ACMV alone potentially causes forest tree species has in recent years been yield losses of about 50% in susceptible found attacking cassava and causing high cultivars (Farquet and Fargette, 1990; Lozano yield losses in some of the major cassava and Nolt, 1989). Farmers in most parts of growing districts in Ghana. Surveys have Africa do very little or practically nothing to established presence of the disease in the control diseases of cassava and are often Volta, Central and Ashanti regions of the satisfied with the reduced yields they obtain country. Major symptoms of the disease from susceptible varieties they cultivate include wilting and defoliation and eventually (Thresh et al., 1994). plant death. Storage roots of infected cassava In addition to the major diseases (ACMV, plants are often rotten and result in yield CBB and CAD) cassava is also affected by losses as high as 100%. Local and improved root rot infections. Root rot diseases of genotypes of cassava available to farmers in cassava are common in poorly drained soils endemic areas are susceptible to attacks by especially in periods of excessive rainfall or this fungus. The large, bright yellow fruiting when cassava is planted in areas where forest body produced by the fungus is characteristic trees previously grew. Rots of cassava in of the basidiomycete Polyporus sulphureus poorly drained soils could be due to (Laetiporous sulphureus). Citrus and a Phytophthora species (Cook, 1978). Soft rots number of important timber tree species are caused by Phytophthora spp. are also hosts of this fungus. Results from field characterized by exudation of liquids with foul screening of cassava genotypes indicate that smell often from swollen storage roots (Lozana materials with high levels of resistance to this and Sequira, 1974; Oliveros et al., 1974). This fungus are available. rot is prevalent in waterlogged soils. Botryodiplodia throbromae has been Introduction isolated frequently from rotten cassava roots harvested from poorly drained soils in Ghana. Cassava (Manihot esculenta) is one of the Control of these rots lie in avoiding poorly most important food crops in the tropical world drained soils. and the principal carbohydrate source for more Proceedings of the 13th ISTRC Symposium, Arusha, Tanzania, 2007 In the last decade however, few new general surveys, localized surveys were diseases of cassava have been identified in conducted in areas where Polyporus Ghana. One of these new diseases is caused sulphureus attacks on cassava were found or by a basidiomycete previously known to be a reported. fungal parasite of woody forest tree species. In the last decade this new parasite of cassava Screening of cultivars for resistance. has been observed to be causing significant Experiments to test varieties for resistance to yield losses in important cassava growing the fungus were set up in disease hot spot districts of the country. The basidiomycete areas of Sabadu and Avemedra in the Kpando produces a bright yellow fruiting body which district of the Volta region. Disease incidence is characteristic of Polyporus sulphureus in these two adjacent farming communities (Laetiporus sulphureus). The parasite which was 80% in 2000. Ten elite cultivars in the was first observed in one of the ten regions advanced stages of the cassava breeding of the country in the early 1990s but is now programme of Crops Research Institute present in five important cassava growing (Kumasi, Ghana) were tested for resistance regions of southern Ghana. Severe yield in these areas in a randomized complete block losses ranging between 50 and 100% have design (RCBD). A popular improved cassava been recorded in disease hot spot areas of variety, ‘Afisiafi’ and a local farmer’s variety Central and Volta regions of the country were included in the experiments as checks. (Moses 2001). Symptoms of the disease Disease progress was monitored as soon include wilting, defoliation and root rot as as a small fruiting body of the fungus was observed in most root rot diseases of cassava seen at the base of the stem of a healthy (Msikita et al., 2000). In areas where this root growing cassava plant. Morphological rot disease has been reported, it is more changes in both parasite and attacked cassava important than any other known cassava plants were documented throughout the 18 disease. months crop cycle. Features of storage roots Effective control measures for this disease were recorded at harvest and losses due to must be developed and promoted to reduce rots quantified. In addition to the root rot its effects. If uncontrolled, this new root rot disease, the reaction of the genotypes to all of cassava is likely to disrupt food security in the major diseases of cassava were assessed the region. The host range of P. sulphureus on a 1-5 scale (where 1= no visible disease is wide. A number of important tree species symptom observed; 5= symptoms of disease are hosts to this parasite. depicting severe damage to tissues organs). This paper reports research results aimed Disease development was also monitored on at obtaining information regarding this new farmers’ fields. Local farmers were involved root rot disease of cassava and the efforts in the evaluation of the genotypes for disease being made towards its control. resistance. Materials and Methods Host range. Host range of the pathogen was determined in endemic areas through Distribution of the disease. Annual surveys surveys. The roots of attacked secondary were conducted in the dry and wet seasons host plants were exposed to establish the type of 1999, 2000 and 2001 under the Integrated of association or linkage that exist between Pest Management Component of the National host and parasite. Root and Tuber Improvement Programme (RTIP) of Ghana to quantify incidence and Cultural Practices. Cultural practices that severity of the major diseases of cassava and promoted spread and persistence of the root provide information on new diseases and their rot disease in the endemic areas were spread in the country. In addition to these identified. Farmer practices like methods of 308 Proceedings of the 13th ISTRC Symposium, Arusha, Tanzania, 2007 land preparation, source of planting materials, in the Kpando district had the highest handling of harvested storage roots and incidence of the root rot disease (80%) in 2000. handling of plant debris after harvest were A survey of the same area in the wet season recorded. Selection of planting materials for of 2002 showed a drop in disease incidence cuttings was also investigated. from 80 to 50%. Workshops and Farmer Field Days. Disease development and yield loss. The first Workshops and Farmer Field Days were sign of the disease is the appearance of the organized for farmers and Agriculture fruiting body of the basidiomycete on the Extension Agents (AEAs) in disease hot spot distal end of the stem of an attacked cassava areas in the Kpando District of the Volta plant. In endemic areas new attacks on farms region. These activities were used to increase are observed soon after the first rains farmer s aware of cassava diseases. In these following the prolonged dry season. The activities, farmers and AEAs were trained in bright yellow fruiting body develops rapidly disease identification and their control. A and can expand to 30 cm in diameter in less television documentary on the root rot disease than 4 weeks. The fruiting body persists on was prepared from workshops and field days the attacked plant for weeks. It may shrink in for awareness creation. size and finally become a dry brown leathery tissue attached to infected cassava plant. Results Depending on the variety of cassava, wilting and defoliation may be observed several Distribution of the disease. Results of weeks after the initial attack. Harvested roots surveys indicate the presence of the disease from the defoliated plants are often completely in Ashanti, Volta, Central, Greater Accra and rotten. Eastern regions of Ghana. Districts with significant presence of the disease identified Screening for resistance. Results from the in the surveys were Kpando, Ho, Ejura- resistance screening experiments are shown Sekyere Dumase, Awutu, Swedru and Gomoa in Table 1.The genotypes TME 1, TME 6 and districts. The highest incidence of 33% was 91/02324 were not affected by the root rot recorded in the Kpando district of the Volta organism. All other genotypes suffered yield region. Avemedra and Sabadu farming area losses due to the root rot disease. Yield losses Table 1: Reaction of elite cultivars of cassava to tuber rot (caused by P. sulphureus) and the other major diseases of cassava (figures represent disease severity scores on a 1-5 scale). Genotype ACMV CBB CAD BLS Root rot TME 1 1.0 1.0 4.0 3.0 1.0 TME 5 1.0 1.0 3.0 3.0 3.0 TME 6 1.0 1.0 4.0 3.0 1.0 TME 11 1.0 1.0 4.0 3.0 3.0 TME 12 1.0 1.0 4.0 3.0 2.0 TME 14 1.0 1.0 4.0 3.0 2.0 91/03227 1.0 1.0 4.0 3.0 2.0 91/02324 1.0 1.0 5.0 3.0 3.0 182/0328 1.0 1.0 4.0 3.0 2.0 92/0398 1.0 2.0 3.0 3.0 3.0 ‘Afisiafi’ 3.0 1.0 3.0 3.0 3.0 Local 4.0 1.0 3.0 3.0 3.0 309 Proceedings of the 13th ISTRC Symposium, Arusha, Tanzania, 2007 ranged between 2 and 50% among the not contained, it may affect the exportation of susceptible genotypes.