Emergence, spread and strategies for controlling the pandemic of cassava mosaic virus disease in east and central Africa

During the 1990s, an epidemic of an unusually severe form of cassava mosaic virus disease (CMD) has expanded to cover virtually all of Uganda, and substantial areas in the neighbouring countries of Kenya, Tanzania, Sudan and the Democratic Republic of Congo. Losses in the generally sensitive local cassava cultivars have been so great that a common farmer response to the problem has been the temporary abandonment of cassava cultivation. As a consequence, the CMD ‘pandemic’ has had a significant destabilising effect on food security in East Africa. In attempting to combat the problem, vigorous efforts have been made to identify, multiply and disseminate resistant cassava germplasm. This has included cultivars obtained from the breeding programme of the International Institute of Tropical Agriculture (IITA), based in Nigeria, as well as local selections derived from IITA parent stock. Whilst considerable success has been realised in controlling the pandemic in Uganda, addressing the problem at the regional level remains a major challenge. This article reviews progress made in recent years, both in understanding the cause and mechanisms behind the pandemic's expansion, and in developing strategies to control it. Particular attention is drawn to the likely threat the pandemic poses to cassava production in the countries of central Africa, most notably the Democratic Republic of Congo, where insecurity currently impedes an assessment of the problem and the ready implementation of control measures. Scientists and other stakeholders with an interest in CMD management are therefore urged to explore innovative mechanisms that will allow them to develop a more comprehensive approach to CMD control in the region.     

Epidemiological assessment of cassava mosaic disease in Central African Republic reveals the importance of mixed viral infection and poor health of plant cuttings

Cassava is a vital crop in Africa and represents the main food crop in Central African Republic (CAR). CAR has recently faced large reductions in cassava yields that have led to a surge in market prices. To better understand the causes of the reduction in yield, we identified biotic constraints to cassava production by means of a large-scale plant epidemiological survey conducted in 2007 and 2008. Standard protocols were used for the assessment of the major cassava pests and diseases. Cassava mosaic disease (CMD) was shown to be the most serious constraint to cassava in CAR, with symptoms observed at all localities surveyed. CMD is distributed throughout the country, with an average incidence of 85%. Importantly, 94% of diseased plants had cutting-derived CMD infection suggesting that farmers mostly use virus-infected cuttings for planting. PCR amplification and direct sequencing of partial fragments of the Rep ORF revealed that the causal agents of CMD in CAR are African cassava mosaic virus (ACMV) and the Uganda strain of East African cassava mosaic virus (EACMV-UG). We also demonstrated that 58% of CMD samples present mixed infections (ACMV and EACMV-UG) and that these samples had significantly higher symptom severities. Our results suggest that mixed infection and synergism between CMGs, could be an important feature in the yield reduction of cassava plants in CAR, similar to the other severe CMD epidemics reported in East Africa.     

Control of tobacco mosaic virus by Pseudomonas fluorescens CZ powder in greenhouses and the field

Pseudomonas fluorescens strain CZ has been reported to inhibit tobacco mosaic virus (TMV) by producing an antibiotic protein. In this study the effects of CZ powder on TMV infection were investigated on tobacco grown in greenhouses and in the field. CZ stocks were cultured in a mixture medium (5 g wheat bran, 1 g soybean flour, 1 g corn flour, 1 g peanut flour), and then mixed with silica white at a ratio of 6 ml:1 g to make CZ powder. The effective cell concentration in a 100-fold dilution of CZ powder was 49.8 × 10¹⁰ cfu ml⁻¹. In greenhouse experiments where Nicotiana tabacum cv. Samsun NN plants were challenged by mechanical inoculation with a mixture of the 100-fold diluted CZ powder and TMV, the in vitro suppression of the TMV infection was 88.3% compared to the controls in which a mixture of water and TMV was used as inoculum. Similarly when N. tabacum cv. NC89 plants were inoculated by cutting the leaves with virus-contaminated scissors, dipping the scissors in a 100-fold dilution of CZ powder before cutting showed a disinfection effect of 96.3% compared with water dipping controls. Drenching and spraying a 100-fold dilution of CZ promoted growth and inhibited virus infection by 59.2% on N. tabacum cv. NC89 plants in the greenhouse. In field trials, 100-fold dilution of CZ suppressed TMV infection by 58.2% and 47.6% in 2010 and 2011, respectively, which was similar to the effect of Ningnanmycin (antibiotic purified from Streptomyces noursei var. xichangensis), a registered antiviral agent in tobacco. All these results indicated that CZ has a potential to be used as a hand tool disinfector and an antiviral agent against TMV.     

Quantifying economic losses associated with levels of wheat streak mosaic incidence and severity in the Texas High Plains

Wheat streak mosaic (WSM), caused by the Wheat streak mosaic virus (WSMV), which is transmitted by the wheat curl mite (Aceria tosichella), is the most prevalent virus disease of wheat in the Texas High Plains. Infected plants initially exhibit mosaic symptoms, which lead to severe stunting, complete chlorosis and, in the most severe cases, eventually plant death. Wheat plants infected with WSMV have lower forage and grain yields and exhibit reduced water-use efficiency compared to non-infected plants. The disease impact on water-use efficiency raises an important issue of whether diseased fields should be irrigated as frequently as non-diseased fields. The issue becomes more relevant when energy costs and the dwindling water resources from the Ogallala Aquifer are taken into consideration. This study examined the potential economic losses due to WSM, using data collected from two fields in 2007 and 2009. A hand-held hyperspectral radiometer was used to quantify severity of WSM in multiple 1 m² plots along two transects, each stretching from the edges of the fields to their centers. Grain yield declined exponentially (R² = 0.79, P < 0.0001) with increasing disease severity, as measured by reflectance at 555 nm. For economic analysis, grain yield from each plot was used for determining cost adjustments and linked revenues in relation to WSM severity levels, which allowed calculations of potential profit reduction. The method enables one to compare losses associated with different levels of WSM severity to a baseline with little or no WSMV infection. Results show that losses from the disease are primarily an outcome of reduced revenue due to a decrease in grain yield, and, as expected, losses incurred per unit land area rapidly increase with increasing disease incidence and severity. Furthermore, producers incur additional marginal losses when irrigating fields with WSM because there is little or no return for irrigation inputs, as water-use efficiency of severely diseased wheat is drastically reduced. Results from this study are useful in estimating losses at differing levels of disease incidence and severity and represent the first step in development of an economic threshold for wheat streak mosaic.     

Evaluating sustainable and profitable cropping sequences with cassava and four legume crops: Effects on soil fertility and maize yields in the forest/savannah transitional agro-ecological zone of Ghana

Rotations are important practices for managing soil fertility on smallholder farms. Six cropping sequences (cassava, pigeonpea, mucuna–maize–mucuna, cowpea–maize–cowpea, maize–maize–maize, and speargrass fallow) were evaluated during 2003–2004 in Wenchi district of Ghana for their effects on the profitability of the different rotations and the productivity of subsequent maize. Soil chemical properties were not significantly affected by cropping sequence. On the researcher-managed and farmer-managed plots maize grain yields were significantly influenced by cropping sequence. On the researcher-managed plots maize grain yield ranged from 1.0 t ha⁻¹ after speargrass fallow to 3.0 t ha⁻¹ with cassava cropping when N fertiliser was not applied to maize and from 2.1 t ha⁻¹ with continuous maize to 4.2 t ha⁻¹ with mucuna–maize–mucuna when 60 kg N ha⁻¹ was applied to maize. On the farmer-managed plots where N fertiliser was not applied to maize, maize grain yields ranged from 0.4 t ha⁻¹ on speargrass fallow to 2.2 t ha⁻¹ on plots previously cropped to pigeonpea. High maize grain yields associated with the cropping sequences involving cassava, mucuna and pigeonpea were related to the faster decomposition and N release of the biomass compared with the slower release of N by the poorer quality materials like maize stover and speargrass. Return on investment of the different rotational sequences ranged from −22% with speargrass/maize to 235% with cassava/maize when no N application was made to maize, and from 29% with continuous maize to 196% with cassava/maize when N fertiliser was applied to maize. Cassava/maize rotation was ranked by native farmers as the most preferred rotation whereas migrant farmers ranked cowpea–maize–cowpea–maize as the most preferred rotation. Among natives, male farmers ranked rotation involving cowpea as the next most preferred rotation after cassava/maize. In contrast, female farmers ranked pigeonpea/maize rotation as the second most preferred rotation, due to low labour and external input requirements of pigeonpea compared with cowpea. The choice of a particular rotational sequence is related to access to resources and the needs of the farmer. The study therefore suggests that, in a heterogeneous farming community like Wenchi, technology development should be targeted to suit the needs and resources available to each particular group of farmers.     

以农业高质量发展提升吉林省粮食安全保障能力

在复杂多变的国际形势下,如何通过农业高质量发展,快速提升粮食有效供给能力,是吉林省农业发展面临的重大任务。在总体分析当前我国粮食安全形势基础上,系统论述吉林省粮食生产经营面临的问题与挑战,提出吉林省以农业高质量发展提升粮食有效供给能力的必要性。在此基础上,从强化市场意识,转变粮食安全观;集约高效生产,巩固粮食产能;调优粮食结构,提升种粮效益;加强技术攻关,支撑转型发展;加强产销融合,实施精准生产等方面提出系列对策建议。     

Rice spikelet rot disease in China - 1. Characterization of fungi associated with the disease

Rice spikelet rot disease (SRD) is an emerging disease of rice panicle in China, which affects both rice yield and grain quality. Four fungal pathogens were isolated from diseased rice grains. Morphological observation, biological testing and molecular characterization led to identify these fungi as Fusarium proliferatum, Bipolaris australiensis, Curvularia lunata and Alternaria tenuis. The four fungi can grow from 10 °C to 40 °C, and from pH 5 to pH 10. The most suitable temperature range is 25 °C-30 °C, however the optimal pH for sporulation of these fungi varies greatly. The four fungi can grow on media supplemented with different carbon and nitrogen sources. These differences in carbon and nitrogen requirements suggest differences in trophism, and have large effects on hyphal growth and spore production. The results suggest that rice SRD is caused by various fungi with diverse physiological characteristics.     

Suppression of spinach wilt disease by biological soil disinfestation incorporated with Brassica juncea plants in association with changes in soil bacterial communities

Biological soil disinfestation (BSD) is a method of controlling soil-borne pests and diseases through anaerobic decomposition of plant biomass incorporated in field soil with temporary irrigation and covering with sheets. In this study, effects of BSD on suppression of spinach wilt disease were investigated in two different field experiments using mainly Brassica juncea plants as plant biomass. Soil bacterial community compositions were analyzed with clone library analysis based on 16S rRNA gene sequences to determine the relationship between the bacterial composition in the treated soil and suppression of the disease. For the BSD-treated soils, oxidation–reduction potential dropped, and acetate was usually detected at high concentrations. Although the control treatment (irrigation and polythene covering without biomass) decreased the wilt disease incidence in spinach plants cultivated in the treated plot as compared with those for the non-treated plot, BSD-treatments suppressed the disease more effectively. The clone library results showed that both non-treated and control soils contained diversified bacterial communities of various phylogenetic groups, while members of the Firmicutes mainly from the class Clostridia dominated in the BSD-treated soils. The clostridial groups detected were diverse and the major clone groups were closely related to strictly anaerobic fermentative bacteria such as Clostridium saccharobutylicum, Clostridium cylindrosporum, Clostridium sufflavum, and Clostridium xylanovorans. These clostridial groups were almost eliminated from the soil bacterial community when the BSD-treated soil was treated again with irrigation and covering without biomass before the next cropping, in which the wilt disease was hardly suppressed.