Incomplete systemic movement of Xanthomonas campestris pv. musacearum and the occurrence of latent infections in xanthomonas wilt‐infected banana mats

Management of banana xanthomonas wilt (XW) (caused by Xanthomonas campestris pv. musacearum, Xcm) has been impeded by poor adoption of control options that are complex, cumbersome and costly. To improve XW management, this study investigated Xcm survival and latent infections in subsequent generations, survival of latently infected planting materials (suckers), incidence of latent infections in symptomless plants in mats having diseased plants, and XW status across farms and markets in districts previously devastated but currently endemic. On‐station experiments were protected from new infections. Latent bacteria at low levels were detected in up to 20% of the third generation suckers, with a significant (P < 0·05) reduction (43–20%) in subsequent generations. Only 3–6% of latently infected suckers succumbed to XW. Incidence of Xcm in symptomless suckers from farmers' fields (with up to 70% incidence) was low (3%) while it increased (8–25%) with disease severity in mats in controlled experiments. In the surveyed districts, incidence had significantly declined with yields observed to have recovered relative to earlier reports, although latent infections remained high. This study provides evidence that if new infections are prevented, fields with high XW incidence can be rejuvenated. It showed incomplete systemic movement of Xcm in mats coupled to a gradual decline of bacterial load in subsequent generations to levels that cannot initiate disease. These studies explain the current successes in farms practising single diseased plant removal instead of whole mat rouging, and gives hope to farmers lacking access to clean planting material.     

Comparing effectiveness,cost- and time-efficiency of control options for Xanthomonas wilt of banana under Rwandan agro-ecological conditions

European Journal of Plant Pathology - Xanthomonas wilt of banana (XW) is a major disease affecting banana throughout East and Central Africa (ECA). Initial control was through complete diseased mat...     

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.     

Systemicity of Xanthomonas campestris pv. musacearum and time to disease expression after inflorescence infection in East African highland and Pisang Awak bananas in Uganda

Banana xanthomonas wilt (XW) caused by Xanthomonas campestris pv. musacearum (Xcm) attacks all banana cultivars. Xcm in inflorescence‐infected Pisang Awak plants with wilting male bud bracts is restricted to the upper parts of the true stem; therefore, cutting these plants at the pseudostem base has been recommended to prevent further Xcm spread. In order to fine‐tune existing control strategies, this study examined the movement of Xcm into plants and mats, in relation to disease incubation period. Mature Pisang Awak and East African highland (AAA‐EA) plants were inoculated with Xcm through abscission wounds of female bracts, male bud bracts, male flowers, a combination of male bud bracts and flowers, and by cutting male buds with a contaminated machete. Thirty plants per genotype and treatment were monitored for 24 months for disease symptoms. An additional 68 AAA‐EA and 33 Pisang Awak plants were sampled weekly to assess the rate of Xcm spread within the plants. All floral entry points resulted in disease, with the highest incidence in combined male bract and male flower abscission wound inoculations. The study confirmed the systemicity of Xcm, with the pathogen able to live within the mat for long periods (5–16 months) without causing disease. Reliance on disease symptom expression to manage XW is therefore not sufficient. The long incubation period in lateral shoots may explain the current resurgence of the disease in locations where the disease was thought to have been successfully eradicated.     

A control package revolving around the removal of single diseased banana stems is effective for the restoration of Xanthomonas wilt infected fields

Xanthomonas campestris pv. musacearum, the causal agent of Xanthomonas wilt of banana (XW), does not infect or cause symptom development in all physically attached shoots in an infected mat. Incomplete/partial systemicity and latent infections often occur. The single diseased stem removal (SDSR, the removal of only symptomatic plants) technique depends on these observations. The SDSR technique, as an alternative or complementary practice to complete mat uprooting (CMU) for XW control, was evaluated at eight XW pilot sites in eastern Democratic Republic of Congo as a novel control option. This technique is low-cost, simple and easily applicable. Within one month, XW plant incidence at the experimental sites declined to below 10%, while within three and 10 months declined to below 2% and 1%, respectively. Restoration of banana plots was observed even in plots that initially had over 80% plant disease incidence. CMU removes a larger portion of the inoculum in a field but is very tedious, time consuming and costly in terms of labour and lost production, due to the premature cutting of symptomless plants that potentially could bear a bunch. CMU can potentially prevent further spread when XW appears for the first time on a farm or location. The choice of CMU relative to SDSR also depends largely on farming objectives. CMU can be carried out in intensive and market-oriented production systems, whose ultimate target is eradication, for example, in South-Western Uganda. In contrast, SDSR is more appealing to subsistence-oriented production, such as in eastern DR Congo, Burundi or central Uganda, whose target is more oriented towards management/control. SDSR can be suggested where access to clean planting material is difficult, thus could be recommended to a very large percentage of small-scale farmers in the currently affected banana-based production systems in east and central Africa.     

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.     

Sorghum (Sorghum bicolor) as a bean intercrop or rotation crop contributes to the survival of bean root rot pathogens and perpetuation of bean root rots

Root rots (RR) are the main cause of declining bean (Phaseolus vulgaris) production in southwestern Uganda. Here, beans are mainly intercropped/rotated with maize (Zea mays), sorghum (Sorghum bicolor), sweet potato (Ipomoea batatas), potatoes (Solanum tuberosum) and garden peas (Pisum sativum). These crops also suffer from RR and bean RR pathogens have been isolated from some of these crops. This study aimed to determine the extent of RR on maize, sorghum and peas, and their potential to contribute to the survival of bean RR pathogens. Therefore, experiments were carried out in bean RR‐infested farmers’ fields as well as soils inoculated with bean RR pathogens (Pythium spp. and Fusarium spp.) under screen house conditions and a susceptible bean cultivar served as a control. High RR incidence/severity scores were recorded in beans and sorghum in both farmers’ fields and screen house experiments. The high field RR incidence/severity in sorghum correlated with the screen house scores. This study shows that RR is also a problem to other crops, especially sorghum, warranting attention. The findings also imply that sorghum plays a potential role as an alternate host to bean RR pathogens, increasing inoculum density of bean RR pathogens and potentially negatively impacting the bean RR problem. Intercropping or rotating beans with sorghum in this region is not recommended. However, maize was RR‐resistant and therefore appropriate as an intercrop/rotational crop to beans in the system. A holistic rather than commodity approach is recommended for managing RR in this cropping system.