Tesfamariam Mekete

The importance and management strategies of cereal cyst nematodes, Heterodera spp., in Turkey

AbstractCereal cyst nematodes (CCNs) can cause significant economic yield losses alone or in combination with other biotic and abiotic factors. The damage caused by these nematodes can be enormous when they occur in a disease complex, particularly in areas subject to water stress. Of the 12 valid CCN species, Heterodera avenae, H. filipjevi, and H. latipons are considered the most economically important in different parts of the world. This paper reviews current approaches to managing CCNs via genetic resistance, biological agents, cultural practices, and chemical strategies. Recent research within the soil borne pathogen program of the International Maize and Wheat Improvement Center has focused on germplasm screening, the potential of this germplasm as sources of resistance, and how to incorporate new sources of resistance into breeding programs. Breeding for resistance is particularly complicated and difficult when different species and pathotypes coexist in nature. A lack of expertise and recognition of CCNs as a factor limiting wheat production potential, combined with inappropriate breeding strategies and slow screening processes limit genetic gains for resistance to CCNs.

Plant-Parasitic Nematodes Are Potential Pathogens of Miscanthus × giganteus and Panicum virgatum Used for Biofuels

A survey of Miscanthus × giganteus and switchgrass plots throughout the midwestern and southeastern United States was conducted to determine the occurrence and distribution of plant-parasitic nematodes associated with these biofuel crops. During 2008, rhizosphere soil samples were collected from 24 Miscanthus × giganteus and 38 switchgrass plots in South Dakota, Iowa, and Illinois. Additional samples were collected from 11 Miscanthus × giganteus and 10 switchgrass plots in Illinois, Kentucky, Georgia, and Tennessee the following year. The 11 dominant genera recovered from the samples were Pratylenchus, Helicotylenchus, Xiphinema, Longidorus, Heterodera, Hoplolaimus, Tylenchorhynchus, Criconemella, Paratrichodorus, Hemicriconemoides, and Paratylenchus. Populations of Helicotylenchus, Xiphinema, and Pratylenchus were common and recorded in 90.5, 83.8, and 91.9% of the soil samples from Miscanthus × giganteus, respectively, and in 91.6, 75, and 83.3% of the soil samples from switchgrass, respectively. Prominence value (PV) (PV = population density × √frequency of occurrence/10) was calculated for the nematodes identified. Helicotylenchus had the highest PV (PV = 384) and was followed by Xiphinema (PV = 152) and Pratylenchus (PV = 72). Several of the nematode species associated with the two biofuels crops were plant parasites. Of these, Pratylenchus penetrans, P. scribneri, P. crenatus, Helicotylenchus pseudorobustus, Hoplolaimus galeatus, X. americanum, and X. rivesi are potentially the most damaging pests to Miscanthus × giganteus and switchgrass. Due to a lack of information, the damaging population thresholds of plant-parasitic nematodes to Miscanthus × giganteus and switchgrass are currently unknown. However, damage threshold value ranges have been reported for other monocotyledon hosts. If these damage threshold value ranges are any indication of the population densities required to impact Miscanthus × giganteus and switchgrass, then every state surveyed has potential for yield losses due to plant-parasitic nematodes. Specifically, Helicotylenchus, Xiphinema, Pratylenchus, Hoplolaimus, Tylenchorhynchus, Criconemella, and Longidorus spp. were all found to have population densities within or above the threshold value ranges reported for other monocotyledon hosts.

Association analysis of resistance to cereal cyst nematodes (Heterodera avenae) and root lesion nematodes (Pratylenchus neglectus and P. thornei) in CIMMYT advanced spring wheat lines for semi-arid conditions

To identify loci linked to nematode resistance genes, a total of 126 of CIMMYT advanced spring wheat lines adapted to semi-arid conditions were screened for resistance to Heterodera avenae, Pratylenchus neglectus, and P. thornei, of which 107 lines were genotyped with 1,310 DArT. Association of DArT markers with nematode response was analyzed using the general linear model. Results showed that 11 markers were associated with resistance to H. avenae (pathotype Ha21), 25 markers with resistance to P. neglectus, and 9 significant markers were identified to be linked with resistance to P. thornei. In this work we confirmed that chromosome 4A (~90–105 cM) can be a source of resistance to P. thornei as has been recently reported. Other significant markers were also identified on chromosomal regions where no resistant genes have been reported for both nematodes species. These novel QTL were mapped to chromosomes 5A, 6A, and 7A for H. avenae; on chromosomes 1A, 1B, 3A, 3B, 6B, 7AS, and 7D for P. neglectus; and on chromosomes 1D, 2A, and 5B for P. thornei and represent potentially new loci linked to resistance that may be useful for selecting parents and deploying resistance into elite germplasm adapted to regions where nematodes are causing problem.

Phasmarhabditis bonaquaense n. sp. (Nematoda: Rhabditidae), a new slug-parasitic nematode from the Czech Republic

Phasmarhabditis bonaquaense n. sp. is described and illustrated from the body of Malacolimax tenellus, from the locality of České Švýcary near the village of Dobrá Voda, the Czech Republic. Females are characterized by a body length of 2349 (1878-2626) µm and a cupola shaped tail with a long hyaline hair-like tail tip. Extremely prominent papilla-like phasmids present. Males 1829 (1414-2121) µm long. Peloderan bursa with nine pairs of rays (papillae), 1/1/1/2/1/3. One non-paired apparent papilla-like structure located near the ventral appendage anterior to the cloaca. Prominent papilla-like phasmids located close to the tail tip. Small subunit (18S), ITS, and D2-D3 expansion segments of the large subunit of ribosomal DNA were used to analyze the phylogenetic relationships of sequenced species in the genus Phasmarhabditis and other closely related species. Phasmarhabditis bonaquaense n. sp. varied from other related nematodes both in morphological characterizations and phylogenetic analysis. The life cycle of the newly described species is not well known but it is probably a facultative, mollusc-parasitic nematode able to survive permanently in the saprobic phase on decaying organic matter.

Research on plant-parasitic and entomopathogenic nematodes in Ethiopia: a review of current state and future direction

Food self-sufficiency is Ethiopia’s national priority goal. Given that pest management seriously impacts agriculture, research on crop diseases is of paramount significance to the national goal. Here we provide a comprehensive account of research on plant-parasitic and entomopathogenic nematodes in Ethiopia. We show that the limited information available indicates that plant-parasitic nematodes impact crop production. There exists a serious gap in knowledge with regard to the effects of plant-parasitic nematodes on almost all major crops. This gap includes surveys with appropriate levels of identification, distribution, and yield loss and damage threshold studies on target crops. The current state of knowledge hinders the nation’s ability to design and implement appropriate control strategies for plant-parasitic nematodes. We propose a strategic assessment of plant-parasitic nematodes of all major crops, the need for systematic manpower training and continued search for entomopathogenic nematodes in the major agro-ecological zones of the nation and further research on those entomopathogenic nematodes already identified.

E-typing for nematodes: an assessment of type specimen use by nematode taxonomists with a summary of types deposited in the Smithsonian Nematode Collection

We assessed 301 taxonomic papers published in nine journals between 1999-2011 to determine the use of type specimens and to evaluate the habitat focus and the number of new species described per year. A total of ca 100 new nematode species were described every year, primarily from terrestrial habitats. Two-thirds were terrestrial, 16% were aquatic and the remaining 9% were animal-parasitic nematodes. Only 2.5% of the taxonomic literature reported a comparative study of type material for making a decision on the identity of the target taxon. The overwhelming majority (i.e., 97.5%) relied only on literature comparisons. Our closer scrutiny of the 61 papers revealed a number of shared problems: a third stated that inadequacy of original descriptions, or unavailability/inaccessibility of type specimens had hindered them from unequivocally determining the identity of their species. Fourteen percent reported a discrepancy between the text descriptions and the illustrations, and a tenth revealed the absence of designated types for taxa relevant to their work. A similar number indicated deterioration of types to be a reason for either making wrong conclusions in previous descriptions, or for rendering their descriptions incomplete. We argue for E-typing of nematodes as a solution to enhance the future accessibility of type specimens. We stress the need for a concerted effort between museum curators, nematological journals and nematological societies to address the problem and thereby to forge a brighter future for the science.

Influence of Root Exudates and Soil on Attachment of Pasteuria penetrans to Meloidogyne arenaria

The bacterium Pasteuria penetrans is a parasite of root-knot nematodes (Meloidogyne spp.). Endospores of P. penetrans attach to the cuticle of second-stage juveniles (J2) and subsequently sterilize infected females. When encumbered by large numbers of spores, juveniles are less mobile and their ability to infect roots is reduced. This study looked at different factors that influence spore attachment of P. penetrans to the root-knot nematode Meloidogyne arenaria. Pretreatment of J2 with root exudates of eggplant (Solanum melongena cv. Black beauty) reduced spore attachment compared with pretreatment with phosphate-buffered saline (PBS), suggesting that the nematode surface coat was altered or the spore recognition domains on the nematode surface were blocked. Spore attachment was equally reduced following exposure to root exudates from both host and nonhost plants for M. arenaria, indicating a common signal that affects spore attachment. Although phytohormones have been shown to influence the lipophilicity of the nematode surface coat, auxins and kinetins did not affect spore attachment compared with PBS. Root exudates reduced spore attachment more in sterilized soil than in natural soil. Sterilization may have eliminated microbes that consume root exudates, or altered the chemical components of the soil solution or root exudates. Root exudates caused a greater decrease in spore attachment in loamy sand than in a sandy loam soil. The sandy loam had higher clay content than the loamy sand, which may have resulted in more adsorption of compounds in the root exudates that affect spore attachment. The components of the root exudates could have also been modified by soil type. The results of this study demonstrate that root exudates can decrease the attachment of P. penetrans endospores to root-knot nematodes, indicating that when these nematodes enter the root zone their susceptibility to spore attachment may decrease.

Genotypiing of single spore isolates of a pasteuria penetrans population occurring in florida using snp‐based markers

To generate single spore lines of a population of bacterial parasite of root‐knot nematode (RKN), Pasteuria penetrans, isolated from Florida and examine genotypic variation and virulence characteristics exist within the population.

First Report of Meloidogyne haplanaria Infecting Mi-Resistant Tomato Plants in Florida and Its Molecular Diagnosis Based on Mitochondrial Haplotype

Meloidogyne haplanaria is a species originally found infesting peanut in Texas and, more recently, in Arkansas. In this study, we confirmed the presence of M. haplanaria in Florida based on morphological and molecular characterization. This species was identified from a sample submitted for diagnosis collected from Mi-resistant tomato rootstock grown in Naples, FL. The major diagnostic criteria to distinguish M. haplanaria from other closely related root-knot nematode (RKN) species are based on morphological differences and host range tests, which are time consuming and labor intensive and require living or well-preserved specimens. In our study, we provide an easy diagnostic strategy to distinguish M. haplanaria from other RKN species based on amplification of two mitochondrial DNA regions. These regions span the intergenic spacer and part of the adjacent large subunit ribosomal RNA gene (lrDNA) and sequence polymorphisms in lrDNA revealed by the restriction pattern following digestion with the restriction enzymes HinfI and MnlI. A unique haplotype pattern, which has not been observed in any of the RKN species described thus far, was observed in M. haplanaria. The outcome of molecular analysis of M. haplanaria aligned with morphological measurement and characteristics as well as perineal pattern originally described for M. haplanaria.