Understanding banana bunchy top disease epidemiology in Burundi for an enhanced and integrated management approach

This study on the epidemiology of banana bunchy top disease (BBTD) was carried out in the context of small‐scale farms in Burundi for an integrated management approach. Banana trials were established in farmers’ fields comparing different plot locations, while spatial and seasonal occurrence of aphid vectors was evaluated at three different altitudes. In addition, serological tests were performed on banana leaf samples to confirm the presence and titre of the virus. The results showed that BBTD incidence varied among banana cultivars and locations. Nine months after plot establishment, BBTD incidence ranged from 21·8% to 56·4% in plots within affected fields, while a range of 0–12·3% was reported in plots located between 5 and 30 m away from affected banana fields. Aphid numbers were highest in the dry season. These aphids were able to acquire and transmit the virus irrespective of altitude. A mean incubation period of 21 and 84 days was observed at low (780 m a.s.l.) and high (2090 m a.s.l.) altitude, respectively. Thus, a holistic approach, taking into account banana cultivar, plot location, disease‐free planting material and regular field sanitation, should be promoted for long‐term BBTD management.     

Sources of resistance to Pseudocercospora fijiensis,the cause of black Sigatoka in banana

Black Sigatoka, caused by Pseudocercospora fijiensis, is one of the most devastating diseases of banana. In commercial banana-growing systems, black Sigatoka is primarily managed by fungicides. This mode of disease management is not feasible for resource-limited smallholder farmers. Therefore, bananas resistant to P. fijiensis provide a practical solution for managing the disease, especially under smallholder farming systems. Most banana and plantain hybrids with resistance to P. fijiensis were developed using few sources of resistance, which include Calcutta 4 and Pisang Lilin. To broaden the pool of resistance sources to P. fijiensis, 95 banana accessions were evaluated under field conditions in Sendusu, Uganda. Eleven accessions were resistant to P. fijiensis. Black Sigatoka symptoms did not progress past Stage 2 (narrow brown streaks) in the diploid accessions Pahang (AA), Pisang KRA (AA), Malaccensis 0074 (AA), Long Tavoy (AA), M.A. Truncata (AA), Tani (BB), and Balbisiana (BB), a response similar to the resistant control Calcutta 4. These accessions are potential sources of P. fijiensis resistance and banana breeding programmes can use them to broaden the genetic base for resistance to P. fijiensis.     

Induction of systemic resistance in banana (<Emphasis Type="Italic">Musa</Emphasis> spp.) against <Emphasis Type="Italic">Banana bunchy top virus</Emphasis> (BBTV) by combining chitin with root-colonizing <Emphasis Type="Italic">Pseudomonas fluorescens</Emphasis> strain CHA0

Pseudomonas fluorescens strains CHA0 and Pf1 were investigated for their biocontrol efficacy against Banana bunchy top virus (BBTV) in banana (Musa spp.) alone and in combination with chitin under glasshouse and field conditions. Bioformulation of P. fluorescens strain CHA0 with chitin was effective in reducing the banana bunchy top disease (BBTD) incidence in banana under glasshouse and field conditions. In addition to disease control, the bioformulation increased the economic yield significantly compared to the untreated control. Increased accumulation of oxidative enzymes, peroxidase (PO), polyphenol oxidase (PPO), phenylalanine ammonia lyase (PAL), pathogenesis-related (PR) proteins, chitinase, β-1,3-glucanase and phenolics were observed in CHA0 bioformulation amended with chitin-treated plants challenged with BBTV under glasshouse conditions. Indirect ELISA indicated the reduction in viral antigen concentration in P. fluorescens strain CHA0 with chitin-treated banana plants corresponding to reduced disease ratings. The present study revealed that induction of defence enzymes by P. fluorescens with chitin amendment reduced the BBTD incidence and increased bunch yield in banana.     

大豆花叶病的流行因素及综合防治

1985年设置流行观察圃,以4个品种、4种播期、两种种子传毒率的种子,系统观察各因素对病害流行的作用。另设综合防治田,品种与观察圃相同;采取隔离种植、种传率低、适期播种及早期拔除病苗等措施。1986年用相同的4个品种,设5个隔离试区,各播种不同种传率(0—33％)的种子,测定播种种子传毒率对流行的影响。结果证明:传毒种子是当地最主要的侵染来源,田间杂草不传病,蚜虫传播距离多在100米以内,花期以后被侵染的植株,其种传率显著降低。播种种子传毒率与田间流行程度及收获种子传毒率相关显著。所用防病措施效果明显,可控制田间病情指数为4％、种传率为0％、种子斑驳率低于4％。     

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.     

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.     

Long-lasting systematic roguing for effective management of CABMV in passion flower orchards through maintenance of separated plants

In Brazil, passion flower is grown across almost the entire country. The predominant disease of the passion flower crop is passion fruit woodiness, caused by the potyvirus cowpea aphid-borne mosaic virus (CABMV), and transmitted by aphids in a nonpersistent manner. The disease reduces the useful life of the orchard from 36 months to approximately 18 months. Up to now, there has not been an efficient method for disease management. The aim of this work was to evaluate the efficiency of systematic roguing of diseased plants through weekly inspections, for disease management in the field. The latent and incubation periods of CABMV in passion flower vines were determined in order to optimize roguing efficiency. Passion fruit plants inoculated with CABMV started to act as sources of inoculum from 3 days after inoculation (DAI), and the symptoms were expressed, on average, at 8 DAI. Five field experiments, conducted in the states of São Paulo and Bahia, Brazil, demonstrated that systematic roguing of diseased plants was significantly efficient for managing passion fruit woodiness disease. In order to facilitate identification and subsequent removal of the infected plants, they need to be grown separately. This cultural practice can be recommended for managing passion fruit woodiness disease, provided it is applied on a regional scale by all passion fruit growers. The development of some pilot plantings for the application of roguing in a passion flower-producing region is recommended to validate the use of this technique for managing passion fruit woodiness disease.     

Quantitative epidemiology of Banana Bunchy Top Virus Disease and its control

Models of a banana bunchy top virus disease epidemic were developed to incorporate the two key features of an epidemic in a plantation in the Philippines: an exponential increase in disease incidence over 10 years, and a declining gradient of incidence from the outside edge of the plantation to the centre. A non-spatial model consisted of three difference equations to describe the numbers of latently infected and of infectious plants in the plantation and the size of the inoculum source outside the plantation. In a spatial model the outside portion of the plantation was divided into eight blocks running parallel to the outside edge. The dispersal gradient of the inoculum was assumed to be negative exponential. Analysis of the two models showed that for disease incidence to increase exponentially over time, the rate of disease progress could be dependent either on internal spread and roguing rate (proportion of diseased plants removed and replaced per unit time) or on the rate of increase of external inoculum pressure. The observed incidence gradient from the edge to the centre of the plot could be explained only if external inoculum dominated the parameters in the spatial model. This model was also used to explore a variable roguing rate across blocks. Simulations indicated that this may produce small gains over the adoption of a constant roguing rate over all blocks, but was risky because a shift of roguing emphasis only slightly too far towards the outside blocks can result in a dramatic increase in disease.     

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.     

Epidemiology of dark leaf spot caused by Alternaria brassicicola and A. brassicae in organic seed production of cauliflower

In organic seed production of Brassica vegetables, infections by Alternaria brassicicola and A. brassicae can cause severe losses of yield and seed quality. Four field experiments with or without artificial inoculation with A. brassicicola were conducted in organically managed seed‐production crops of cauliflower cv. Opaal RZ in 2005 and 2006 in the Netherlands. The development of A. brassicicola and A. brassicae on pod tissues and developing seeds was followed and seed quality was assessed. Alternaria brassicicola was externally present on 1·2% of the seeds 14 days after flowering and observed internally within 4 weeks after flowering. In both seasons, seed colonization by the pathogen increased slowly until maturation but sharply increased during maturation. A similar pattern was found for the colonization of pod tissues by A. brassicicola as quantified by TaqMan‐PCR. The incidence of A. brassicicola on mature seeds reached 70–90%. Internal colonization was found for 62–80% of the seeds. External and internal seed colonization by A. brassicae was much lower, with incidences below 3%. The quality of harvested seeds was generally low, with less than 80% of seeds able to germinate. Seed quality was not affected by warm water treatments. It was concluded that A. brassicicola and A. brassicae have the potential to infect pods and seeds soon after flowering. For the production of high quality seeds, producers must prevent such early infections. Therefore, new control measures are needed for use in organic cropping systems.     

Factors Affecting the Spread of Plum pox virus Strain M in Peach Orchards Subjected to Roguing in France

ABSTRACT We evaluated the impact of roguing on the spread and persistence of the aggressive Plum pox virus strain M (PPV-M) in 19 peach orchard blocks in Southern France. During a 7- to 10-year period, orchards were visually inspected for PPV symptoms, and symptomatic trees were removed every year. Disease incidence was low in all orchards at disease discovery and was <1% in 16 of the 19 orchard blocks. The spread of Sharka disease was limited in all 19 blocks, with an annual disease incidence between 2 and 6%. However, new symptomatic trees were continuously detected, even after 7 to 10 years of uninterrupted control measures. An extended Cox model was developed to evaluate to what extent tree location, orchard characteristics, environment, and disease status within the vicinity influenced the risk of infection through time. Eleven variables with potential effect on tree survival (i.e., maintenance of a tree in a disease- free status through time) were selected from survey data and databases created using a geographical information system. Area of the orchard, density of planting, distance of a tree from the edge of the orchard block sharing a boundary with another diseased orchard, and distance to the nearest previously detected symptomatic tree had a significant effect on the risk for a tree to become infected through time. The combined results of this study suggest that new PPV-M infections within orchards subjected to roguing resulted from exogenous sources of inoculum, disease development of latent infected trees, as well as infected trees overlooked within the orchards during visual surveys. A revision of the survey and the roguing procedures used for more effective removal of potential sources of inoculum within the orchards and in the vicinity of the orchards would improve disease control suppression of PPV.     

In search of commercially acceptable Panama wilt-resistant natural variants in Ney Poovan banana (Musa AB)

Ney Poovan banana, the most widely cultivated mixed diploid banana, has been reported to be susceptible to Fusarium wilt like many other varieties in the world. The identification of natural variants possessing disease tolerance or resistance is one strategy to prevent losses. The aim of this study was to identify resistant lines in Ney Poovan banana, through extensive field surveys and screening of putative variants using a detached leaf-based challenging technique. The selected lines were screened under field condition to determine their economic feasibility for commercial-scale use. A total of 26 lines were observed to be resistant to the disease, out of which 24 exhibited a commercially acceptable productivity index and five best lines possessing desirable attributes were obtained.     

Use of fungicides to control chocolate spot (Botrytis fabae) on winter field beans (Vicia faba)

Less chocolate spot (% leaf area affected) developed on winter–sown field beans sprayed with benomyl than on untreated beans or beans sprayed with iprodione, prochloraz or thiabendazole. A single benomyl spray during flowering reduced disease development and increased yield by 20% in 1982 when a severe chocolate spot epidemic developed in untreated plots during flowering. Benomyl sprays at flowering also increased yield in 1980 when there was less disease at flowering than in 1982, but did not increase yield in 1981 when there was little disease at flowering. In 1981 a severe chocolate spot epidemic developed early in the season and less chocolate spot developed on lower leaves of beans grown from seed treated with benomyl plus thiram than on beans grown from untreated seed. Treated plots yielded 70% more than untreated plots, although by early flowering there was no difference in chocolate spot between them. Seed treatment did not increase bean yields in 1982 and 1983, when chocolate spot did not develop early in the season. In the glasshouse, benomyl seed treatment prevented the development of aggressive lesions on lower leaves but thiram did not.     

Cassava brown streak disease and the sustainability of a clean seed system

One method of reducing disease in crops is the dissemination of disease‐free planting material from a multiplication site to growers. This study assesses the validity and sustainability of this method for cassava brown streak disease, a threat to cassava crops across East Africa. Using mathematical modelling, the effects of different environmental and control conditions on pathogen spread were determined in a single‐field multiplication site. High disease pressure, through large vector populations and disease in the surrounding area, combined with poor roguing practice, resulted in unsuccessful disease suppression. However, fields may produce sufficiently clean material for replanting if these factors can be overcome. Assessing the sustainability of a low‐pressure system over multiple harvests, well‐managed fields were found to maintain low disease levels, although producing sufficient cuttings may prove challenging. Replanting fields from the previous harvest does not lead to degeneration of planting material, only cutting numbers, and the importation of new clean material is not necessarily required. It is recommended that multiplication sites are only established in areas of low disease pressure and vector population density, and the importance of training in field management is emphasized. Cultivars displaying strong foliar symptoms are to be encouraged, as these allow for effective roguing, resulting in negative selection against the disease and reducing its spread. Finally, efforts to increase plant multiplication rates, the number of cuttings that can be obtained from each plant, have a significant impact on the sustainability of sites, as this represents the primary limiting factor to success.     

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.     

Tomato brown rugose fruit disease: current distribution,knowledge and future prospects

Tomato is the most economically important fruit/vegetable crop grown worldwide. However, viral diseases remain an important factor limiting its productivity, with estimated quantitative and qualitative yield losses in tomato crops often reaching up to 100%. Many viruses infecting tomato have been reported, while new viral diseases have also emerged. The climatic changes the world is experiencing can be a contributing factor to the successful spread of newly emerging viruses, as well as the establishment of disease in areas that were previously either unfavourable or where the disease was absent. Because antiviral products are not available, strategies to mitigate viral diseases rely on genetic resistance/tolerance to infection, control of vectors, improvement in crop hygiene, roguing of infected plants and seed certification. Tomato brown rugose fruit virus (ToBRFV) is an emerging viral threat to tomato productivity and is currently spreading into new areas, which is of great concern to the growing global production in the absence of mitigation measures. This review presents the current knowledge about ToBRFV and future prospects for an improved understanding of the virus, which will be needed to support effective control and mitigation of the impact it is likely to cause.     

Characterization of early leaf spot suppression by strip tillage in peanut

ABSTRACT Epidemics of early leaf spot of peanut (Arachis hypogaea), caused by Cercospora arachidicola, are less severe in strip-tilled than conventionally tilled fields. Experiments were carried out to characterize the effect of strip tillage on early leaf spot epidemics and identify the primary target of suppression using a comparative epidemiology approach. Leaf spot intensity was assessed weekly as percent incidence or with the Florida 1-to-10 severity scale in peanut plots that were conventionally or strip tilled. The logistic model, fit to disease progress data, was used to estimate initial disease (y(0)) and epidemic rate (r) parameters. Environmental variables, inoculum abundance, and field host resistance were assessed independently. For experiments combined, estimated y(0) was less in strip-tilled than conventionally tilled plots, and r was comparable. The epidemic was delayed in strip-tilled plots by an average of 5.7 and 11.7 days based on incidence and severity, respectively. Tillage did not consistently affect mean canopy temperature, relative humidity, or frequency of environmental records favorable for infection or spore dispersal. Host response to infection was not affected by tillage, but infections were detected earlier and at higher frequencies with noninoculated detached leaves from conventionally tilled plots. These data suggest that strip tillage delays early leaf spot epidemics due to fewer initial infections; most likely a consequence of less inoculum being dispersed to peanut leaves from overwintering stroma in the soil.     

Farm Management Effects on Rhizosphere Colonization by Native Populations of 2,4-Diacetylphloroglucinol-Producing Pseudomonas spp. and Their Contributions to Crop Health

ABSTRACT Analyses of multiple field experiments indicated that the incidence and relative abundance of root-colonizing phlD+ Pseudomonas spp. were influenced by crop rotation, tillage, organic amendments, and chemical seed treatments in subtle but reproducible ways. In no-till corn plots, 2-year rotations with soybean resulted in plants with approximately twofold fewer phlD+ pseudomonads per gram of root, but 3-year rotations with oat and hay led to population increases of the same magnitude. Interestingly, tillage inverted these observed effects of cropping sequence in two consecutive growing seasons, indicating a complex but reproducible interaction between rotation and tillage on the rhizosphere abundance of 2,4-diacetlyphloroglucinol (DAPG) producers. Amending conventionally managed sweet corn plots with dairy manure compost improved plant health and also increased the incidence of root colonization when compared with nonamended plots. Soil pH was negatively correlated to rhizosphere abundance of phlD+ pseudomonads in no-till and nonamended soils, with the exception of the continuous corn treatments. Chemical seed treatments intended to control fungal pathogens and insect pests on corn also led to more abundant populations of phlD in different tilled soils. However, increased root disease severity generally was associated with elevated levels of root colonization by phlD+ pseudomonads in no-till plots. Interestingly, within a cropping sequence treatment, correlations between the relative abundance of phlD and crop stand or yield were generally positive on corn, and the strength of those correlations was greater in plots experiencing more root disease pressure. In contrast, such correlations were generally negative in soybean, a difference that may be partially explained by difference in application of N fertilizers and soil pH. Our findings indicate that farming practices can alter the relative abundance and incidence of phlD+ pseudomonads in the rhizosphere and that practices that reduce root disease severity (i.e., rotation, tillage, and chemical seed treatment) are not universally linked to increased root colonization by DAPG-producers.     

Protection of potatoes against pests in Ukraine1

Potato production in Ukraine is now mainly in the private sector, and the phytosanitary state of potato crops leaves much to be desired. Control of Leptinotarsa decemlineata is generally well managed, but potato late blight (Phytophthora infestons) is out of control. Potato wart disease continues to cause problems in areas where it occurs because of the unavailability or unacceptability of resistant cultivars. Potato viruses and the losses they cause are little studied or taken into account. Other fungal, bacterial and insect pests are causing increasing problems in the field and in store. Integrated pest management systems are proposed by scientific institutions, including adequate forecasting of potato blight and use of certified seed potatoes of cultivars resistant to wart and cyst nematodes, but these systems do not adequately reach private growers. The range of plant protection products authorized for sale to the general public is very limited, and this is a serious obstacle to adequate plant protection in potato.