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

Disparity between leaf and root symptoms and crop losses associated with cassava brown streak disease in four countries in eastern Africa

Cassava brown streak disease is endemic to the coastal regions of East Africa, and from around 2004, the disease resurged and became epidemic in the Great Lakes Region, where it continues to spread. In both these areas, cassava brown streak disease (CBSD) leaf symptoms occur at high incidences. However, it is the associated symptom of root rot (necrosis) in the starch-bearing tissues that renders the root unfit for human consumption. Because the extent of root necrosis is not known until the crop is harvested and surveys require destructive sampling, root symptoms are much less frequently assessed than are the above-ground symptoms on the leaves and stems. Surveys were undertaken in selected villages in Tanzania, Kenya, Uganda and Malawi to assess the incidence of CBSD leaf symptoms and the incidence and severity of root symptoms, to estimate the impact of the disease on household food security and on cassava processing. CBSD leaf symptoms were recorded at high incidences (40–90% in individual fields) in all fields visited throughout East Africa, but root necrosis incidence was lower than would be expected from the high incidence of leaf symptoms. Severe root necrosis at high incidence was found only on a few varieties, usually grown to a limited extent. It appears that varieties that are prone to root necrosis are being abandoned in favour of those with a lower propensity to develop root necrosis after infection by the virus

Genotype by environment interactions in identifying cassava (Manihot esculenta Crantz) resistant to cassava brown streak disease

Cassava landraces were evaluated for resistance to cassava brown streak disease (CBSD) for two cropping seasons at a disease hotspot area in Naliendele, Tanzania. Based on reactions to CBSD, several landraces including Chimaje, Mfaransa and Supa B were considered to be resistant to the disease while Kikwada, Mbuyu, and Nyoka were tolerant. ANOVA revealed that the largest sum of squares (SS) (41.9–86.7%) was attributed to the genotype of the cassava landraces, while a smaller proportion of SS (8.1–38.2%) was due to genotype by environment interactions for all traits tested, which included disease symptoms, root weight, number of roots per plant and dry matter content. Environment accounted for the smallest effect (0.01–26.3%), however, the mean squares was nonetheless significant for a few genotypes, which indicated that their disease expression was indeed influenced by the environment. Increased CBSD severity was associated with low temperatures and rainfall. Increased rainfall towards harvesting led to higher root weight but lower dry matter content in the first cropping season. Correlation analysis showed that the presence of CBSD symptoms reduces the amount of usable roots, total root weight, and root dry matter content. Many resistant/tolerant landraces also had high root weight and dry matter content, and they can be used by farmers to reduce CBSD losses. The landraces described here form novel sources of CBSD resistance that can be used for breeding disease-resistant cassava varieties with superior agronomic characteristics

Flowering margins support natural enemies between cropping seasons

Here we investigated how agricultural landscapes support natural pest regulating insects between cropping seasons; an important challenge in Pest Management by Small-Holder Farmers so aligns with this special issue. Smallholders in East Africa clear margins to provide fodder for cattle or livestock or simply to reduce the ingression of weeds into crop fields. We show that there was a significant seasonal variation in plant species richness and diversity around crops and the abundance of margin plants was strongly linked to abundance of natural enemies in the off season. The time since harvesting was also a significant factor influencing the overall abundance of natural enemies. Our paper reinforces our understanding of the importance flowering plants in agricultural systems as a refuge for natural enemies and other beneficial insects but uniquely emphasising the time between cropping seasons. Improving agricultural landscapes between crops to better support invertebrates will lead to more effective natural pest regulation early in the following crop with positive outcomes for the farmers and their families

Flowering margins support natural enemies between cropping seasons

IntroductionPopulations of natural enemies of insect pests are declining owing to agricultural intensification and indiscriminate use of pesticides, and this may be exacerbated in agricultural systems that clear all margin plants after the cropping season for other uses such as fodder. Retaining a diversity of non-crop flowering vegetation outside the cropping season may support more resilient and effective natural pest regulation.MethodsWe tested the potential for non-crop vegetation to support natural enemies in fields across two locations after harvesting the primary crops of lablab and maize.ResultsA total of 54 plant species were recorded across the sites in Kenya with 59% of them being annuals and 41% perennials. There was a significant seasonal variation in plant species richness (ANOVA: F1, 16 = 33. 45; P< 0.0001) and diversity (ANOVA: F1, 16 = 7.20; P = 0.0511). While time since harvesting was a significant factor influencing the overall abundance of natural enemies (ANOVA: F2, 1,133 = 8.11; P< 0.0001), they were generally higher in abundance in locations with margin plants or where a diversity of margin plants was observed.DiscussionThese findings demonstrate that flowering plants in agricultural systems offer refuge and alternative food for natural enemies and potentially other beneficial insects between cropping seasons. The conservation of natural enemies between crops may lead to more effective natural pest regulation early in the following crop, thus reducing reliance on insecticides application

Response of Common bean to Rhizobium inoculation and fertilizers.

Common bean (Phaseolus vulgaris) yields in western Kenya are low and this has been attributed to low soil fertility. Field trials were conducted in farmers fields in Ukwala Division of Siaya District in Kenya during the long rains of 1998 and 1999 to determine the potential for improving bean yields through Rhizobium inoculation, and fertilizer N and P applications. The three factors: Rhizobium inoculation at two levels and fertilizers-N and P each at three levels were factorially combined to give 18 treatments. The treatments were laid out in a randomised complete block design with a split-plot structure and three replications. A popular local bean variety Okuodo was used as the test crop. During the 1998 long rains season, P significantly (P=O.05) increased the stand count after emergence, pod number per plant and the bean grain yields. Seed number per plant alone was significantly increased by fertilizer-N application. Significant interaction effects (P=O.05) were observed for NXP on the stand count after emergence and for Rhizobium inoculation XP on stand count after emergence, stand count at harvest and the bean grain yields. Rhizobium inoculation alone did not significantly affect any of the measured variables in both seasons. In 1999, only the bean grain yields were determined and were significantly (P=O.05) increased only by fertilizer P application

Response of Common Bean to Rhizobium Inoculation and Fertilizers

Common bean (Phaseolus vulgaris) yields in Western Kenya are low and this has been attributed to low soil fertility. Field trials were conducted in farmers fields in Ukwala Division of Siaya District in Kenya during the long rains of 1998 and 1999 to determine the potential for improving bean yields through Rhizobium inoculation, and fertilizer N and P applications. The three factors: Rhizobium inoculation at two levels and fertilizers-N and P each at three levels were factorially combined to give 18 treatments. The treatments were laid out in a randomised complete block design with a split-plot structure and three replications. A popular local bean variety Okuodo was used as the test crop. During the 1998 long rains season, P significantly (P=O.05) increased the stand count after emergence, pod number per plant and the bean grain yields. Seed number per plant alone was significantly increased by fertilizer-N application. Significant interaction effects (P=O.05) were observed for NXP on the stand count after emergence and for Rhizobium inoculation XP on stand count after emergence, stand count at harvest and the bean grain yields. Rhizobium inoculation alone did not significantly affect any of the measured variables in both seasons. In 1999, only the bean grain yields were determined and were significantly (P=O.05) increased only by fertilizer P applications. The Journal of Food Technology in Africa Volume 6 No.4, 2001, pp. 121-12

Multi-Pathotype Testing of Selected Kenyan Wheat Germplasm and Watkin Landraces for Resistance to Wheat Stripe Rust (Puccinia striiformis f. sp tritici) Races

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the key diseases of economic importance in wheat worldwide. Host resistance, which follows the gene-for-gene hypothesis between the host and pathogen, has been used in wheat lines to resolve resistance specificities and postulate resistant genes. The objective of this study was to elucidate stripe rust resistance in a collection of Kenyan wheat lines and Watkin landraces to identify new sources of stripe rust (Yr) resistance. In this study, the resistance in twenty wheat lines was determined by comparing their infection type with those of twenty differential lines using isolates representing twelve Puccinia striiformis races from Kenya, Denmark, U.K., Sweden, and Eritrea at the seedling stage. Among the twenty wheat lines, none was resistant to all the twelve Pst races and isolate DK02d/12 (“Kranich” race) was virulent on all the genotypes except wheat genotype “Kenya Tai.” This genotype (“Kenya Tai”) had the highest resistance as it was resistant to all the twelve stripe rust races used in this study. From this study, the introduction and utilization of wheat genotypes with adult plant resistant (APR) stripe rust genes, such as Yr15, are important in breeding wheat genotypes with effective resistance to wheat stripe rust in Kenya and worldwide