While the physiological basis of cassava drought tolerance has been characterized, evaluation of the molecular responses to drought stress remains largely unexplored. This study provides an initial characterization of the molecular response of cassava to drought stress resembling field conditions. The candidate drought tolerance genes in cassava identified in this study can be used as expression-based markers of drought tolerance in cassava or be tested in the context of breeding and engineering drought tolerance in transgenics.

Bacterial wilt caused by Xanthomonas vasicola pv. musacearum (Xvm) is an important disease of enset and banana in south and south-western Ethiopia where, the diversity of the insect fauna on banana inflorescences was unknown and the role of insects as vectors of the disease had not been studied. The objectives of this study were to assess the occurrence of bacterial wilt and male bud infection, the diversity of insect families in banana plantations and the presence of the bacteria on insects collected from diseased inflorescences in south and southwestern Ethiopia. Surveys were carried out and insects were collected from three different zones in 2005. The diversity and richness of the insect families was assessed across sites and genotypes and comparisons were made using the Shannon Diversity Index and the Jack knife estimator, respectively. Correlations were made between the abundance and incidence of insects with the incidence of male bud infection on 'Pisang Awak' plants. A wide range of insect families were recorded and they varied according to banana genotype and altitude. The Drosophilidae and Apinae families were most frequently recorded across sites and genotypes. The 'Wendo' variety (AAA Cavendish group) had the highest diversity and richness of insect families within and across sites. In contrast to the Kembata Tembaro and Bench Maji zones, severe and widespread male bud infection of banana was found in Kaffa, where there was a high diversity of insects on the 'Pisang Awak' and 'Abesha muz' plants. The incidence of male bud infection on 'Pisang Awak' plants was highly correlated with the incidence of insects (R 2 = 0.964). The incidence of male bud infection however depends on the floral morphology and altitude. Artificial inoculation with Xvm ooze on fresh male bract and flower scars resulted in infections on 'Pisang Awak' and 'Abesha muz' plants, but the 'Dwarf Cavendish' plants with persistent bracts and flowers remained healthy. Few male bud infections were observed at altitudes above 1,700 masl. Xvm was isolated from Apinae, Lonchaeidae, Muscidae, Tephritidae and Vespidae insect families. Lonchaeidae (Silba spp.) were frequently observed on banana bract and flower scars and could thus be an important insect vector of Xvm in Ethiopia.

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Xanthomonas wilt caused by Xanthomonas campestris pv. musacearum has been an important constraint to enset (Ensete ventricosum) and banana (Musa spp.) in Ethiopia. It was postulated that Xanthomonas wilt has a similar epidemiology as other banana bacterial wilts, which are known to be transmitted by insect vectors and garden tools. A study to determine the role of garden tools in the transmission of Xanthomonas wilt was carried out on enset in a greenhouse at the Southern Agricultural Research Institute, Awassa, Ethiopia and on 'Pisang Awak' (AABB genome) in the field at Amaro, Southern Ethiopia. A contaminated knife was used to infect plants. The treatments in the greenhouse trials comprised of cutting: green leaves; broken green leaves; dry leaves; the pseudostem; and roots. Similar treatments were carried out on the field-grown 'Pisang Awak' plants with two additional treatments: desuckering and debudding. Debudding was done by cutting off the male bud with a contaminated machete, while a forked stick was used for control samples. In addition, bacterial ooze was smeared on fresh and dry flower and bract scars at the male part of inflorescences. All plants treated with a contaminated machete: enset and banana when cut in the pseudostem; and all banana plants when debudded got infected. Similarly, cutting green leaves and cutting broken green leaves resulted in a high disease transmission of respectively 67 and 62% in banana and 58 and 54% in enset. Similar high transmission rates (90%) were obtained for desuckering on banana. On the other hand, cutting roots with a contaminated machete resulted in low transmission levels, with 20% in bananas and 25% in enset suggesting that tool infections mainly occur above ground. This calls for rigorous tool disinfection, while desuckering and deleafing in highly infected fields should be avoided. Debudding should be carried out with a forked stick.

Early detection followed by the destruction of infected mats is one of the recommended and effective methods of controlling banana Xanthomonas wilt. As manual destruction is very tedious and time consuming, herbicide injections in the pseudostem or the stump were evaluated as a suitable alternative. The study was carried out in Luwero district in central Uganda. Herbicide injections were made on 'Pisang Awak' plants (Musa ABB group) using different concentrations of Roundup and 2,4-D at a pseudostem height of 30 and 100 cm. In addition, the herbicide was injected in the centre (diameter-wise) and in the peripheral part of the pseudostem (tangentialwise). Herbicide applications into the stump were also evaluated. All plants (100%) injected with 2,4-D had fallen and started rotting by three weeks after application. However, at three weeks, none of the plants treated with Roundup, regardless of the concentration and the height of injection had died. Similarly plants injected with 2,4-D had less re-sprouting compared to plants treated with Roundup. Therefore, 2,4-D is more effective than Roundup in destroying 'Pisang Awak' plants and in suppressing the re-sprouting of lateral shoots. Since 2,4-D is also cheaper than Roundup it is recommended for use on small-scale plantations. At least 1.6 ml of the original concentrate of 2,4-D and 2.0 ml of the original concentrate of Roundup needs to be applied per mature plant. The most effective pseudostem injection height was found to be 100 cm. The angle of injection whether peripherally or central and the plant growth stage did not affect the herbicides' efficiency. The results also showed that pseudostem injections are more effective than stump herbicide applications. périphérique ou central et l'étape de croissance de la plante n'ont pas affecté l'efficacité des herbicides ›. Les résultats ont montré aussi que les injections de pseudostem sont plus efficaces que d'applications d'herbicide à la souche.

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East African highland bananas and enset belong to the genera Musa and Ensete (Ensete ventricosum), respectively. The root, corm and shoot growth of field-established enset and East African highland banana cultivars was assessed. Banana and wild enset plants were assessed in Uganda, while domesticated enset plants were assessed in Ethiopia. Growth traits of the enset and banana mats in Uganda were recorded at flower emergence of the banana plant crop, while enset plants in Ethiopia were assessed at 4 and 5 years after planting. The results obtained in Uganda indicate that enset had a significantly (P<0.05) shorter and thicker pseudostem compared to banana. In addition, enset had a larger root system with thicker cord roots. Results further showed that young enset plants had a significantly lower shoot-root ratio compared to the bananas. The shoot-root ratio in enset is, however, clone dependent and increases with an increase in age. The results also showed that root density in enset sharply decreases with increasing soil depth. Par ailleurs, enset a un plus grand système fondamental avec les racines de corde plus épaisses. Les résultats poussés ont montré que ces jeunes plantes d'enset ont une grande proportion des rejetons en comparaison avec celle des bananes. La proportion des rejetons de enset est cependant un clone dépendant des augmentations de l'âge. Les résultats ont montré aussi que la densité des racines de enset diminue avec l'augmentation de la profondeur du sol.

Abstract. Tropical forests contribute significantly to the emission and uptake of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). However, studies on the soil environmental controls of greenhouse gases (GHGs) from African tropical forest ecosystems are still rare. The aim of this study was to disentangle the regulation effect of soil nutrients on soil GHG fluxes in a tropical forest in northwestern Uganda. Therefore, a large-scale nutrient manipulation experiment (NME) based on 40 m × 40 m plots with different nutrient addition treatments (nitrogen (N), phosphorus (P), N + P, and control) was established. Soil CO2, CH4, and N2O fluxes were measured monthly using permanently installed static chambers for 14 months. Total soil CO2 fluxes were partitioned into autotrophic and heterotrophic components through a root trenching treatment. In addition, soil temperature, soil water content, and mineral N were measured in parallel to GHG fluxes. N addition (N, N + P) resulted in significantly higher N2O fluxes in the transitory phase (0–28 days after fertilization, p