Post-harvest impact of cassava brown streak disease in four countries in eastern Africa

Cassava brown streak disease (CBSD) is endemic to the coast of East Africa and the disease continues to spread to new areas in the Great Lakes region. In both these areas CBSD leaf symptoms occur at high incidences. However, it is the associated symptom of necrosis in the starch-bearing tissues that renders the root unfit for human consumption. Where root necrosis occurs, the economic viability of processing into flour is adversely affected. Surveys undertaken in Tanzania, Kenya, Uganda, and Malawi showed that CBSD leaf symptoms were present at high incidences but root necrosis incidence was lower than would be expected. It appears that a form of tolerance to CBSD occurs in which the plants are susceptible to infection by the virus but are less affected by the root symptom. Farmers practise selection for cassava varieties less prone to CBSD root necrosis leading to dependence on a decreasing number of varieties

Identification of whitefly (Bemisia tabaci) proteins interacting with Tomato leaf curl Bangalore virus coat protein gene using Y2H system

Tomato leaf curl Bangalore virus (ToLCBV) (Geminiviridae) causes the economically important tomato leaf curl virus disease (ToLCVD), and is transmitted by the whitefy, Bemisia tabaci (Gennadius) (Aleyrodidae: Hemiptera). Successful transmission of the virus by the insect requires safe translocation of the virus through diferent barriers inside the vector. During the translocation, the viral coat protein will interact with many whitefy proteins, while a few of them are benefcial to virus (e.g. heat shock proteins) but some may be harmful (e.g. whitefy immune proteins). We carried out the yeast two hybrid (Y2H) assays to identify the proteins of the B.tabaci Asia 1 genetic group interacting with ToLCBV coat protein (CP). The Y2H assay initially identifed a total of 425 putative interacting whitefy proteins on a low stringent selection media, and they were reduced to 324 when the yeast colonies were grown on a high stringent media, and of which about 274 colonies produced single bands in colony PCR experiments while the remaining colonies produced multiple bands. Further, high selection pressure assays confrmed a total of 102 whitefy proteins interacting with ToLCBV CP and these included the heat shock proteins (HSPs) 70 kDa, GroEL, nucleoproteins, vitellogenins, apolipophorins, 40 s ribosomal proteins, sorbitol dehydrogenase, dipeptidyl peptidase, E3 ubiquitin, annexin, GTP cyclohydrolase, tropomyosin, salivary secreted proteins, succinate dehydrogenase, lachesins, enolase and others. The identifed proteins could be potential targets for novel whitefy control strategies such as using RNAi or insecticide target sites for developing future disease and whitefy control strategies

Effect of time of harvesting and disease resistance in reducing Cassava (Manihot esculenta Crantz) yield losses by two viral diseases

Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are two important biotic constraints for cassava (Manihot esculenta Crantz) production in Eastern and Southern Africa. CMD causes a general decline in yield in affected plants of susceptible cassava varieties but CBSD causes rotting of edible roots. Delayed harvesting can increase rotting of roots and making them unfit for consumption or marketing, and thus affecting the livelihoods of poor farmers. This study investigated the effect of interaction between time of harvesting and levels of disease resistance to identify ideal harvesting times for reducing yield losses. The resistant cassava variety Namikonga remained in the field for the duration of the study, up to 24 months after planting without incurring significant yield losses, while the tolerant varieties Kiroba and Kizimbani could only be maintained up to 21 months. Susceptible varieties Mreteta and Albert suffered significant yield losses beyond 15 months. Among the varieties, Kizimbani had the least CBSD and CMD foliar symptoms as well as farmer desirable traits including high root weight, quantity of marketable roots and dry matter content. Harvesting of cassava can depend on the resistance or susceptibility of the varieties grown. Therefore, the above harvesting times for different varieties were recommended for minimizing yield losses due to the diseases and thus maximizing yields to the farmers

Occurrence and estimated losses caused by cassava brown streak disease in Migori county, Kenya

Cassava is the second most important staple crop after maize in Migori County. A farm survey was conducted in Kuria East and Suna West sub-counties to determine the incidence, severity and estimated losses of cassava brown streak disease (CBSD) on cassava crops in farmers’ fields. A total of 70 farms (Kuria East = 40; Suna West = 30), with cassava crop aged 7-12 months after planning (MAP), were sampled using stratified random sampling procedure. Data were collected on CBSD incidence (foliar and root), root necrosis score, percent loss (roots) and yield traits (number and weight of fresh roots). Results showed mean foliar incidences of 58.5% and 53.9% in Kuria East and Suna West sub-counties, respectively. High foliar CBSD incidences were recorded in Kuria East sub-county with range of 54.0-74.0% observed in five local varieties; Manchoberi, Amakuria, Mwitamajera, Weite and Nyakohanda. A similar result trend was recorded in Suna West sub-county with a range of 52.1-77.5% in six varieties; Mary go round, MH95/0183 (improved), Ondielo, Nyakasanya, Nyakasamuel and Obarodak. Highest root necrosis scores and percent root loss were observed in two local varieties, Amakuria (2.4; 36.7%) and Agric IV (3.7; 40%), in Kuria East and Suna West sub-counties, respectively. Correlation analysis revealed strong positive correlation between root necrosis, root necrosis incidence and percent root loss; which implies that susceptible varieties suffer greatest loss. The estimated mean root yield loss caused by CBSD stood at 10.5% (equivalent: US 405 ha-1) and 7.2% (equivalent: US 384.30 ha-1) in Kuria East and Suna West sub-counties, respectively. The findings of this study are expected to provide impetus for the development and promotion of new high yielding, locally adapted and CBSD resistant cassava varieties

Developing dual-resistant cassava to the two major viral diseases

Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are the two important biotic constraints affecting cassava production in sub-Saharan Africa (SSA). Deployment of cassava varieties dually resistant to both diseases is the most effective and realistic way of reducing losses to African farmers. Crosses were carried out between the Tanzanian local cassava variety Namikonga (CBSD resistant/CMD susceptible) with an introduced cassava germplasm AR37-80 (CBSD susceptible/CMD resistant) from South America to develop dually resistant F1 progenies and they were evaluated for 2 seasons at Naliendele in Southern Tanzania which is a CMD and CBSD hotspot area. CMD-resistant progenies had low foliar severities (≤ 1.8 on a five-point scale) similar to CMD resistant parent. CBSD resistant progenies had minimal foliar severity (≤2.0) and root necrosis (≤1.2) similar to the CBSD resistant parent while CBSD tolerant progenies had severe foliar severity of up to 3.3 but minimal root severity (≤ 1.2). Traits with minimal environmental influence also had high heritability (≥0.65) and high selection accuracy (≥0.70) and they included CMD foliar symptoms, CBSD foliar symptoms at 6 MAP, root necrosis, root necrosis incidence, root weight, root number per plant, and harvest index. Correlation analysis showed that the presence of diseases reduces usable roots, root weight, root number per plant, and harvest index. Dual resistance can improve yield as observed in the progenies, Namar 050 and Namar 371 which had high root weights of 27.5 t/ha and 28.2 t/ha with high genetic gains of 56.1% and 58.5%, respectively. Dual resistant progenies identified were Namar 050, Namar 100, Namar 130, Namar 200, Namar 334, Namar 371, and Namar 479 as they had minimal CMD and CBSD symptoms severity (≤ 2.0) and could be used for breeding cassava varieties with superior characteristics