Molecular Epidemiology of Ralstonia pseudosolanacearum Phylotype I Strains in the Southwest Indian Ocean Region and Their Relatedness to African Strains.

The increasing requirement for developing tools enabling fine strain traceability responsible for epidemics is tightly linked with the need to understand factors shaping pathogen populations and their environmental interactions. Bacterial wilt caused by the Ralstonia solanacearum species complex (RSSC) is one of the most important plant diseases in tropical and subtropical regions. Sadly, little, outdated, or no information on its epidemiology is reported in the literature, although alarming outbreaks are regularly reported as disasters. A large set of phylotype I isolates (n = 2,608) was retrieved from diseased plants in fields across the Southwest Indian Ocean (SWIO) and Africa. This collection enabled further assessment of the epidemiological discriminating power of the previously published RS1-MLVA14 scheme. Thirteen markers were validated and characterized as not equally informative. Most had little infra-sequevar polymorphism, and their performance depended on the sequevar. Strong correlation was found with a previous multilocus sequence typing scheme. However, 2 to 3% of sequevars were not correctly assigned through endoglucanase gene sequence. Discriminant analysis of principal components (DAPC) revealed four groups with strong phylogenetic relatedness to sequevars 31, 33, and 18. Phylotype I-31 isolates were highly prevalent in the SWIO and Africa, but their dissemination pathways remain unclear. Tanzania and Mauritius showed the greatest diversity of RSSC strains, as the four DAPC groups were retrieved. Mauritius was the sole territory harboring a vast phylogenetic diversity and all DAPC groups. More research is still needed to understand the high prevalence of phylotype I-31 at such a large geographic scale.

First Report of the Root-Knot Nematode Meloidogyne enterolobii Parasitizing Plantain (Musa spp., AAB) in Nigeria.

Plantain (Musa spp., AAB), an important staple food in Africa with West Africa accounting for 32% of global production, is prone to numerous pests and diseases of which plant-parasitic nematodes are a key concern. This includes root-knot nematodes (RKN; Meloidogyne spp.), which infect the roots, causing them to become galled, deformed and swollen. The nematode Meloidogyne enterolobii is considered a global threat to production of many important agricultural crops due to its extremely virulent and aggressive nature (Philbrick et al. 2020). In 2019, during a survey to identify the diversity of nematodes associated with Musa spp. in Nigeria, RKN females (n = 13) were isolated from a heavily galled root (50-75% galling) from a single plantain cv. Agbagba (Musa spp., AAB) plant in Onne, Rivers State, Nigeria (4°43'08.8"N 7°10'37.5"E). Genomic DNA was extracted from three females and processed individually using worm lysis buffer and proteinase K (Bert et al. 2008). The females were identified as M. enterolobii based on Nad5 mtDNA (Janssen et al. 2016), with GenBank accession no. ON010028, ON010027, ON010026, and were 100% homologous to the M. enterolobii sequences MW965454, KU372358 and KU372359 (Supplementary Figure S1). The sampled plant did not show any specific above-ground symptoms but swellings were apparent on the secondary and tertiary roots, which were associated with RKN females that were embedded in the root tissue. All the life stages were found clustered together in the root cortex, where they created necrotic brown-black lesions. A mean value of 2,604 ± 820 (mean ± standard deviation) males, eggs and second-stage juveniles (J2) were extracted from 5 g of root sub-samples (n = 6) using the Hussey and Barker (1973) NaOCl method. On average 39 females were hand-picked (n = 6) from 5 g fresh root. Pure cultures were established from single egg masses and maintained on RKN-susceptible tomato plants (Solanum lycopersicum cv. Marmande). To conduct Koch's postulates, two-month old plantlets of plantain cv. Agbagba (n = 5) were inoculated with 8000 J2s and eggs (initial population) of M. enterolobii pure cultures in 8 L pots in a screenhouse in Nigeria. Non-inoculated plantlets were included as negative controls. The nematode reproduction factor (RF = final density / initial population) and root damage symptoms were assessed 90 days post-inoculation. All the inoculated plantlets had similar galling symptoms and extensive necrosis as was observed in the field (Supplementary Figure S2), with an average RF = 25.9. No symptoms were observed on control plants. Adult females (n = 2) removed from the roots were identified as M. enterolobii based on Nad5 mtDNA (ON532789, ON532790) confirming that plantain cv. Agbagba is a host of M. enterolobii. In Nigeria, M. enterolobii has been reported to be associated with four plant species belonging to four plant families: Euphorbiaceae (Oyetunde et al. 2022), Cucurbitaceae (Bello et al. 2020), Dioscoreaceae (Kolombia et al. 2016), and Solanaceae (dos Santos et al. 2019). To our knowledge, this is the first report of M. enterolobii on a member of the Musaceae in Nigeria and globally the first report on plantain (Musa spp., AAB). The impact of M. enterolobii on plantain productivity has yet to be determined but given the RF value obtained in the pathogenicity test, plantain is a suitable host. This calls for a comprehensive RKN diversity study to evaluate the geographic spread of M. enterolobii on this important staple food crop in West Africa.

Cyst nematode bio-communication with plants: implications for novel management approaches.

Bio-communication occurs when living organisms interact with each other, facilitated by the exchange of signals including visual, auditory, tactile and chemical. The most common form of bio-communication between organisms is mediated by chemical signals, commonly referred to as 'semiochemicals', and it involves an emitter releasing the chemical signal that is detected by a receiver leading to a phenotypic response in the latter organism. The quality and quantity of the chemical signal released may be influenced by abiotic and biotic factors. Bio-communication has been reported to occur in both above- and below-ground interactions and it can be exploited for the management of pests, such as cyst nematodes, which are pervasive soil-borne pests that cause significant crop production losses worldwide. Cyst nematode hatching and successful infection of hosts are biological processes that are largely influenced by semiochemicals including hatching stimulators, hatching inhibitors, attractants and repellents. These semiochemicals can be used to disrupt interactions between host plants and cyst nematodes. Advances in RNAi techniques such as host-induced gene silencing to interfere with cyst nematode hatching and host location can also be exploited for development of synthetic resistant host cultivars. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Potato Cyst Nematodes: A New Threat to Potato Production in East Africa.

Potato cyst nematodes (PCN), such as Globodera rostochiensis and Globodera pallida, are quarantine restricted pests of potato causing major yield and financial losses to farmers. G. rostochiensis was first reported from Kenya's key potato growing area in 2015. We sought to determine the diversity, prevalence and distribution of PCN species across the country by conducting a country-wide survey between 2016 and 2018, which included a more focused, follow-up assessment in three key potato growing counties. A total of 1,348 soil samples were collected from 20 potato growing counties. Information regarding local potato farming practices, potato cultivar use, their diversity and availability was also recorded. PCN cysts were obtained from 968 samples (71.8%) in all the counties surveyed, with Nyandarua County recording the highest PCN field-incidence at 47.6%. The majority of PCN populations, 99.9%, were identified as G. rostochiensis, while G. pallida was recovered from just one field, in a mixed population with G. rostochiensis. Inconsistencies in PCR amplification efficiency was observed for G. rostochiensis using the recommended EPPO primers, compared with ITS primers AB28/TW81, indicating that this protocol cannot be entirely relied upon to effectively detect PCN. Egg density in Nyandarua County varied between 30.6 and 158.5 viable eggs/g soil, with an average egg viability of 78.9 ± 2.8% (min = 11.6%, max = 99.9%). The PCN-susceptible potato cultivar named Shangi was the most preferred and used by 65% of farmers due to its shorter dormancy and cooking time, while imported cultivars (Destiny, Jelly, Manitou, and Markies) with resistance to G. rostochiensis were used by 7.5% of farmers due to unavailability and/or limited access to seeds. Thus, most farmers preferred using their own farm-saved seeds as opposed to purchasing certified seeds. Establishing the distribution and prevalence of PCN and elucidating the local farming practices that could promote the spread of PCN is a necessary precursor to the implementation of any containment or management strategy in the country and ultimately across the region.

Mediation of Potato-Potato Cyst Nematode, G. rostochiensis Interaction by Specific Root Exudate Compounds.

Potato (Solanum tuberosum) is a widely consumed staple food crop worldwide whose production is threatened by potato cyst nematodes (PCN). To infect a host, PCN eggs first need to be stimulated to hatch by chemical components in the host root exudates, yet it remains unknown how most root exudate components influence PCN behavior. Here, we evaluated the influence of eight compounds identified by LC-QqQ-MS in the root exudate of potato on the hatching response of the PCN, Globodera rostochiensis at varying doses. The eight compounds included the amino acids tyrosine, tryptophan and phenylalanine; phytohormones zeatin and methyl dihydrojasmonate; steroidal glycoalkaloids α-solanine and α-chaconine and the steroidal alkaloid solanidine. We additionally tested two other Solanaceae steroidal alkaloids, solasodine and tomatidine, previously identified in the root exudates of tomato, an alternative host for PCN. In dose-response assays with the individual compounds, the known PCN hatching factors α-chaconine and α-solanine stimulated the highest number of eggs to hatch, ∼47 and ∼42%, respectively, whereas the steroidal alkaloids (aglycones), solanidine and solasodine and potato root exudate (PRE) were intermediate, 28% each and 21%, respectively, with tomatidine eliciting the lowest hatching response 13%. However, ∼60% of the hatched juveniles failed to emerge from the cyst, which was compound- and concentration-dependent. The amino acids, phytohormones and the negative control (1% DMSO in water), however, were generally non-stimulatory. The use of steroidal glycoalkaloids and their aglycones in the suicidal hatching of PCN offers promise as an environmentally sustainable approach to manage this pest.

Transcriptional signatures of invasiveness in Meloidogyne incognita populations from sub-Saharan Africa.

Meloidogyne incognita is an economically important plant parasitic nematode. Here we demonstrate substantial variation in the invasiveness of four M. incognita populations relative to tomato. Infective (J2) stage transcriptomes reveal significant variation in the expression of protein-coding and non-coding RNAs between populations. We identify 33 gene expression markers that correlate with invasiveness, and which map to genes with predicted roles in host finding and invasion, including neuropeptides, ion channels, G Protein-Coupled Receptors, cell wall-degrading enzymes and microRNAs. These data demonstrate a surprising diversity in microRNA complements between populations, and identify gene expression markers for invasiveness of M. incognita, to our knowledge for the first time.

Integration of nematology as a training and research discipline in sub-Saharan Africa: progress and prospects.

Within sub-Saharan Africa (SSA), although nematodes are viewed among the most important threats to crop production and food security, the presence of trained nematologists working within this discipline has traditionally been viewed as scarce. The few research studies concerning this subject address this topic from a country or sub-regional perspective and generally portray nematology as 'insufficient'. Over the past two decades, a few initiatives have been instrumental in building greater nematology expertise. For the first time a structured survey was undertaken, involving interviews with individuals from SSA that were (or currently are) involved in nematology training programmes, research, national extension services or in African universities. This paper provides evidence of the positive impact of various initiatives and shows an increase in the number of available nematology positions, together with high rates of graduates that return home to occupy qualified positions. Our findings will help researchers, policy makers and donors to identify areas requiring support to increase the promotion of nematology in SSA and to make an impact for end-users.

Management of Cyst and Root Knot Nematodes: A Chemical Ecology Perspective.

Plant parasitic nematode infection of crops can be highly detrimental to agricultural production. Since the discovery that plant roots release chemicals that attract the infective stage of plant parasitic nematodes some 80 years ago, significant progress in identifying the signaling molecules has occurred only relatively recently. Here, we review the literature on chemical ecological studies of two major plant parasitic nematode groups: root knot nematodes in the genus Meloidogyne and cyst nematodes in the genus Globodera because of the negative impact their parasitism has on farming systems in Africa. We then highlight perspectives for future directions for their management.

Plant-Parasitic Nematodes and Food Security in Sub-Saharan Africa.

Sub-Saharan Africa (SSA) is a region beset with challenges, not least its ability to feed itself. Low agricultural productivity, exploding populations, and escalating urbanization have led to declining per capita food availability. In order to reverse this trend, crop production systems must intensify, which brings with it an elevated threat from pests and diseases, including plant-parasitic nematodes. A holistic systems approach to pest management recognizes disciplinary integration. However, a critical under-representation of nematology expertise is a pivotal shortcoming, especially given the magnitude of the threat nematodes pose under more intensified systems. With more volatile climates, efficient use of water by healthy root systems is especially crucial. Within SSA, smallholder farming systems dominate the agricultural landscape, where a limited understanding of nematode problems prevails. This review provides a synopsis of current nematode challenges facing SSA and presents the opportunities to overcome current shortcomings, including a means to increase nematology capacity.

Meloidogyne virulence locus molecular marker for characterization of selected mi-virulent populations of Meloidogyne spp. is correlated with several genera of betaproteobacteria.

Resistance to root-knot nematodes in tomato is conferred by the Mi resistance gene to the three most important species of Meloidogyne: M. arenaria, M. incognita, and M. javanica. Nevertheless, the Mi gene is unable to inhibit the reproduction of selected and naturally Mi-virulent populations of root-knot nematodes. As pathogenicity assays are time consuming, molecular markers were developed for the easy identification of Mi-virulent populations of Meloidogyne. The sequence characterized amplified region-Meloidogyne virulence locus (MVC) molecular marker is reported to differentiate Mi-avirulent and naturally Mi-virulent from selected Mi-virulent populations. This marker was used to compare acquired virulence in populations of M. javanica from Spain. The original populations used to develop the MVC marker were included as control for reference. Results showed that this marker did not amplify genomic DNA extracted from single juveniles or females of any of the populations tested either from Spain or Japan. In silico analyses performed with the recently published complete genome of M. incognita, indicated that the MVC marker is not correlated to a MVC or to any eukaryotic organism but to several betaproteobacteria genus from the family Comamonadaceae.