W. Ocimati

Fine-tuning banana Xanthomonas wilt control options over the past decade in East and Central Africa

Xanthomonas wilt, caused by Xanthomonas campestris pv. musacearum has, since 2001, become the most important and widespread disease of Musa in East and Central Africa. Over the past decade, new research findings and especially feedback from small-scale farmers have helped in fine-tuning Xanthomonas wilt control options. During the initial years of the Xanthomonas wilt epidemic in East Africa, the complete uprooting of diseased mats and the burning or burying of plant debris was advocated as part of a control package which included the use of clean garden tools and early removal of male buds to prevent insect vector transmission. Uprooting a complete mat (i.e. the mother plant and a varying number of lateral shoots) is understandably time-consuming and labour intensive and becomes very cumbersome when a large number of diseased mats have to be removed. Recent research findings suggest that Xcm bacteria do not colonize all lateral shoots (i.e. incomplete systemicity occurs) and even when present that this does not necessarily lead to symptom expression and disease. This led to a new control method whereby only the visibly diseased plants within a mat are cut at soil level. The underlying idea is that the continued removal of only the diseased plants in a field will reduce the inoculum level and will bring down disease incidence to an acceptable level. This method is less labour intensive and takes a short time compared to the removal of a complete mat. However, single diseased stem removal needs to go hand in hand with prevention of new infections that can occur through the use of contaminated garden tools or through insect vector transmission. Novel transgenic approaches are also discussed. This paper presents an overview of past and ongoing research towards the development of a more practical and less demanding control strategy for Xanthomonas wilt.

Comparative importance of infection routes for banana Xanthomonas wilt and implications on disease epidemiology and management

Abstract Banana Xanthomonas wilt (XW), caused by Xanthomonas campestris pv. musacearum (Xcm), results in up to 100% yield loss. The efficiency of XW infection through the corm, pseudostem inoculation at bunch harvest, leaf, female and male bud bracts was evaluated in banana plants. The male and female bud bract inoculations caused the highest incidence (81% and 93%) compared with 0–44% for harvest and corm inoculations. Naturally mediated insect transmission in ‘Pisang Awak’ resulted in up to 99% disease incidence. Floral inoculations and natural insect-mediated infections only resulted in floral symptoms. Symptom development in insect-transmitted infections simulated artificial male bud inoculations, confirming the male bud bract wounds as the main entry points for insect vector-mediated infections, thus reaffirming the importance of continuous and timely debudding to limit insect spread. Leaf and harvest inoculations resulted only in leaf symptoms, while corm inoculations resulted in late floral symptoms. Floral inoculations were the main mode of infection. Single leaf inoculations resulted in 30% plant mortality despite 100% incidence, with 70% of plants recovering and bearing visibly healthy bunches and suckers. Thus, detection of a diseased plant in a mat shouldn’t warrant the destruction of the whole mat. A significant difference in Xcm cfu g−1 was observed between symptomatic and symptomless leaves, suggesting that practices that keep the bacterial load below the disease-causing threshold could benefit the plant. This could explain the observed success of XW control through removal of single diseased stems in farms.

Bacterial Diseases of Bananas and Enset: Current State of Knowledge and Integrated Approaches Toward Sustainable Management

Bacterial diseases of bananas and enset have not received, until recently, an equal amount of attention compared to other major threats to banana production such as the fungal diseases black leaf streak (Mycosphaerella fijiensis) and Fusarium wilt (Fusarium oxysporum f. sp. cubense). However, bacteria cause significant impacts on bananas globally and management practices are not always well known or adopted by farmers. Bacterial diseases in bananas and enset can be divided into three groups: (1) Ralstonia-associated diseases (Moko/Bugtok disease caused by Ralstonia solanacearum and banana blood disease caused by R. syzygii subsp. celebesensis); (2) Xanthomonas wilt of banana and enset, caused by Xanthomonas campestris pv. musacearum and (3) Erwinia-associated diseases (bacterial head rot or tip-over disease Erwinia carotovora ssp. carotovora and E. chrysanthemi), bacterial rhizome and pseudostem wet rot (Dickeya paradisiaca formerly E. chrysanthemi pv. paradisiaca). Other bacterial diseases of less widespread importance include: bacterial wilt of abaca, Javanese vascular wilt and bacterial fingertip rot (probably caused by Ralstonia spp., unconfirmed). This review describes global distribution, symptoms, pathogenic diversity, epidemiology and the state of the art for sustainable disease management of the major bacterial wilts currently affecting banana and enset.

The banana weevil, Cosmopolites sordidus (Germar), is a potential vector of Xanthomonas campestris pv. musacearum in bananas

Abstract This study was carried out to investigate the potential role of banana weevils as vectors of Xanthomonas campestris pv. musacearum (Xcm), causal agent of banana wilt. Weevils captured from Xcm-infected plants were tested for presence of Xcm, and further raised on Xcm-infected corms for later use as vectors to transmit the pathogen to healthy tissue-cultured plantlets. Analysis of weevils captured from diseased fields revealed more weevils contained Xcm originating from ‘Mbwazirume’ compared with ‘Kayinja’ cultivars. Colonies of Xcm were recovered from the weevil external body surface, internal organs (mouth parts and abdomen) and faecal matter. There was significantly higher Xcm presence and cfu mL−1 on the external weevil body surface than within the internal organs. Bacterial populations declined progressively from the external body surface, internal mouth parts, internal abdominal parts and the faecal matter. Following placement of weevils previously fed on Xcm-exuding corms in close proximity to healthy potted plants, infection occurred, with characteristic disease symptoms observed on all cultivars evaluated except ‘Kayinja’ which remained symptomless. Isolation from both symptomatic and asymptomatic plants revealed erratic Xcm incidence and cfu g−1 that did not correlate to the number of weevils released in all cultivars, except for ‘Kayinja’. This study showed that Xcm can survive on and within the banana weevil and potentially spread the pathogen to neighbouring plants.

Musa germplasm diversity status across a wide range of agro-ecological zones in Rwanda, Burundi and Eastern Democratic republic of Congo.

Objectives: This study assessed the on-farm Musa germplasm diversity across different agro-ecologies of Rwanda and the socio-economic utilization options and selection practices that create/maintain this diversity on-farm. Methodology and results: A Musa germplasm diagnostic survey was carried out in 2007 in five Rwandan districts on a transect from Lake Kivu (West) to Kirehe district (East) bordering Tanzania. Across all sites, 118 farms, each having at least 50 mats were sampled for determining Musa diversity. Forty three Musa cultivars were recorded across the five districts. Higher diversity was observed in the east declining westwards to the Lake Kivu region as reflected by the number of cultivars and their relative abundance. Nearly half of the recorded cultivars had a low diversity index (Gini-Simpson 1-D < 0.2) and therefore prone to genetic erosion. Cooking cultivars only dominate in the district of Kirehe, while beer cultivars dominate the banana production landscape in the other districts. Taste/flavor, bunch size and market demand were the most important criteria for banana cultivar selection and thus greatly influenced cultivar conservation and distribution on-farm. Diseases such as Fusarium wilt and Xanthomonas wilt greatly contributed to genetic erosion. Conclusion and application of results: Musa cultivar diversity in Rwanda is under threat. Ex-situ conservation of the menaced cultivars is of crucial importance. Beer cultivars dominated the landscape. Cultivar diversity on-farm was influenced by the prevailing altitude; taste/flavor, bunch size, and market demand of the cultivars; and their susceptibility to diseases especially Fusarium and Xanthomonas wilt. Banana breeding or adaptation strategies therefore should take into account the farmer preferred traits. In addition, strategies for managing these diseases are critical for preventing the genetic erosion of the affected cultivars.

Variation in nectar volume and sugar content in male flowers of Musa cultivars grown in Rwanda and their non-effect on the numbers of visiting key diurnal insect vectors of banana Xanthomonas wilt

Insects are a major mode of banana Xanthomonas wilt (XW) spread. High insect activity has been blamed for the high XW incidence in ‘Kayinja’ (ABB-genome) dominated banana landscapes across east and central Africa. ‘Kayinja’ male bud nectar composition reportedly contributes to high insect activity. The variation in nectar composition with agro-ecological zones and banana cultivars and its influence on the number of visiting insects in Rwanda were assessed. Three male buds were collected per cultivar for nectar extraction and analysis using a high performance liquid chromatography. Nectar volume and sugar concentrations varied (P<0.001) across 27 banana cultivars, annual seasons and agro-ecological zone. The highest nectar volume was recorded among the East African highland cooking cultivars (AAA-genome) in the high altitude site and the short-heavy rainy season. Nectar contained three sugars: glucose, fructose and sucrose, though hexose (glucose and fructose) was dominant. The three sugars varied significantly (P<0.001) within each cultivar. The total nectar-sugar concentration ranged from 2.3–32%, with the highest among dessert cultivars ‘Kamaramasenge’ (AABgenome) and ‘Gisukari’ (AAA-genome). No strong correlation occurred between insect population and total nectar sugar concentration or nectar volume. Insect populations were rather influenced by the weather conditions, the long rainy season characterized by moderate well distributed rainfall recording the highest insect populations as compared to the short rainy season (with heavy rainfall) and the dry seasons.

Seed degeneration of banana planting materials: strategies for improved farmer access to healthy seed

Vegetatively propagated crops suffer from yield loss and reduced stand density and longevity caused by the build‐up of certain pests and pathogens between successive plantings via infected planting material. Here, six seedborne phytosanitary problems of banana are reviewed to evaluate whether a seed degeneration framework is a useful tool to identify approaches to achieve healthier planting materials. Phytoparasitic nematodes and weevils generate gradual declines in yields and in sucker health. Fusarium wilt and banana bunchy top virus cause progressive mat collapse across the field. Symptomless suckers from any mat in infested fields represent a risk of transmitting the disease to a new field. Xanthomonas and ralstonia wilts, due to incomplete systemicity, are intermediate in their threat to yield loss and frequency of transmission in suckers. Losses to banana streak virus are triggered by abiotic stress, although sucker transmission of episomal banana streak virus also contributes. A qualitative equation described here for seed degeneration covers a cycle beginning with the quality and risk factors of the planting material used to plant a new field and ends with the quality and risk factors of the suckers extracted from the field to plant a new field. This review of five planting material multiplication methods commonly used in banana contrasts their differing usefulness to address seed degeneration in the small farm context. It is proposed that initiatives to offset banana seed degeneration should integrate the role of off‐farm actors into decentralized initiatives rather than attempt to duplicate national seed certification frameworks from other true seed or vegetatively propagated crops.

Risks Posed by Intercrops and Weeds as Alternative Hosts to Xanthomonas campestris pv. musacearum in Banana Fields

Alternative host plants are important in the survival and perpetuation of several crop pathogens and have been suspected to play a role in the survival of Xanthomonas campestris pv. musacearum (Xcm) and perpetuation of Xanthomonas wilt (XW) disease of banana and enset. This study determined the potential risk posed by two weeds (Canna spp. and wild sorghum) and common banana intercrops (maize, millet, sorghum, taro, and sugarcane) as alternative hosts to Xcm. The study employed screenhouse experiments, laboratory procedures and diagnosis of banana fields in XW-affected landscapes. Typical XW symptoms were only observed in artificially inoculated Canna sp., with an incidence of 96%. Leaf lesions characteristic of xanthomonads occurred on millet (50%) and sorghum (35%), though the plants recovered. No symptoms occurred in maize, sugarcane, taro or wild sorghum. However, Xcm was recovered from all these plant species, with higher recoveries in Canna sp. (47%), millet (27%), sugarcane (27%), and wild sorghum (25%). Only isolates recovered from Canna sp., millet, sorghum and wild sorghum caused disease in banana plantlets. The presence and incidence of XW on-farm was positively associated with the presence of susceptible ABB Musa genotypes and negatively with number of banana cultivars on farm and household access to training on XW management. Only 0.02% of field sampled Canna spp. plants had Xcm. Risk posed by Canna spp. on-farm could be limited to tool transmission as it has persistent floral bracts that prevent insect-mediated infections. Given the high susceptibility, perennial nature and propagation through rhizomes of Canna sp., it could pose a moderate-high risk, thus warranting some attention in the management of XW disease. Sugarcane could offer a low-moderate risk due to its perennial nature and propagation through rhizomes while risk from maize, millet, and sorghum was deemed zero-low due to their annual nature, wind-mediated mode of pollination and propagation through seed. Understanding the interactions of a crop pathogen with other plants is thus important when diversifying agroecosystems. The study findings also suggest other factors such as cultivar composition and management of the disease at farm and landscape level to be important in the perpetuation of XW disease.

Diversity and morphological characterization of Musa spp. in North Kivu and Ituri provinces, Eastern Democratic Republic of Congo

Eastern Democratic Republic of Congo constitutes an area where the East African Highland bananas (Musa-AAA-EA) and plantains (Musa-AAB) meet. However, Musa diversity in this region has never been characterized nor represented in national or international collections, yet increasing human activities and build-up of pests and diseases, especially Xanthomonas wilt of banana and banana bunchy top disease, could negatively affect this diversity. This study assessed, collected and morphologically characterized on-farm Musa diversity in the North Kivu and Ituri provinces. Musa accessions collected were added to the UCG-Butembo in situ field collection. A total of 90 and 150 farms were assessed, respectively, in the North Kivu and Ituri provinces. High Musa variability was observed in both provinces. Forty-one cultivars (six presumed new) were identified in Ituri Province compared to 38 cultivars (13 presumed new) in North Kivu Province. Nineteen cultivars occurred in both provinces. Beni and Lubero territories in North Kivu and Mambasa territory in Ituri Province had greater Musa diversity, varying between 27 and 38 cultivars, approximately double of the number of cultivars observed in the other territories. Twenty-three of the 34 Musa descriptors contributed more to the discrimination of the cultivars identified across the provinces and were therefore used for grouping the cultivars using principal component analysis (PCA). The Morisita index of similarity between Ituri Province and North Kivu Province territories was less than 0.5, suggesting dissimilarity in diversity between these sites. Six cultivars: ‘Kirisirya’, ‘Pakuma’, ‘Nziravahima’, ‘Vuhindi’, ‘Tundu’ and ‘Kisubi musa’ were reportedly at risk of genetic erosion. Key words: Banana, conservation, cultivars, ex-situ, genetic erosion, genotypes, plantain.

An alternative to complete banana mat uprooting: assessing the effectiveness of continuous cutting at soil level of all shoots in a mat on speed for corm decay

The complete uprooting of diseased mats/fields (CMU) is one of the recommended control options for Xanthomonas wilt of banana. CMU is labour intensive, time consuming and disturbs the soil structure, exposing fields to erosion. CMU often involves exportation of whole plant biomass, affecting soil fertility. The potential of continuous cutting at soil level of all shoots in a mat until complete corm decay in situ as an alternative to CMU was assessed. The first experiment was established using 224 banana mats in their third cropping cycle. All the plants were cut down at soil level, meristems were removed, and sweet potato and bush bean planted. In a repeat experiment with 180 banana mats, a wide range of treatments were applied on top by cutting and removing the apical meristems. These included the: injection of 2,4-D herbicide into the centre of each corm; removal of a cone shaped section from the center of each corm; and creation of a 20 cm deep incision in the center of each corm; in combination with the application of soil or farmyard manure substrate on cut surface. In the first experiment, resprouting stopped at 8 months while corms fully decayed after 25 months. Annual intercrops did not influence re-sprouting and corm decay rate. Similar re-sprouting trends occured in the repeat experiment. However, 2,4-D application significantly (P<0.05) lowered decay time, with 12-47% of corms decomposed at 8 months compared with 0-20% in other treatments without 2,4-D. In the 2,4-D treatments, 100% of corms had decomposed compared with 36-80% in other treatments by the 20 th