Spatial relationships between entomopathogenic nematodes and nematophagous fungi in Florida citrus orchards

Stress tolerance in entomopathogenic nematodes: Engineering superior nematodes for precision agriculture

Entomopathogenic nematodes (EPNs) are soil-dwelling parasitic roundworms commonly used as biocontrol agents of insect pests in agriculture. EPN dauer juveniles locate and infect a host in which they will grow and multiply until resource depletion. During their free-living stage, EPNs face a series of internal and environmental stresses. Their ability to overcome these challenges is crucial to determine their infection success and survival. In this review, we provide a comprehensive overview of EPN response to stresses associated with starvation, low/elevated temperatures, desiccation, osmotic stress, hypoxia, and ultra-violet light. We further report EPN defense strategies to cope with biotic stressors such as viruses, bacteria, fungi, and predatory insects. By comparing the genetic and biochemical basis of these strategies to the nematode model Caenorhabditis elegans, we provide new avenues and targets to select and engineer precision nematodes adapted to specific field conditions.

Historical Differentiation and Recent Hybridization in Natural Populations of the Nematode-Trapping Fungus Arthrobotrys oligospora in China

Maintaining the effects of nematode-trapping fungi (NTF) agents in order to control plant-parasitic nematodes (PPNs) in different ecological environments has been a major challenge in biological control applications. To achieve such an objective, it is important to understand how populations of the biocontrol agent NTF are geographically and ecologically structured. A previous study reported evidence for ecological adaptation in the model NTF species Arthrobotrys oligospora. However, their large-scale geographic structure, patterns of gene flow, their potential phenotypic diversification, and host specialization remain largely unknown. In this study, we developed a new panel of 20 polymorphic short tandem repeat (STR) markers and analyzed 239 isolates of A. oligospora from 19 geographic populations in China. In addition, DNA sequences at six nuclear gene loci and strain mating types (MAT) were obtained for these strains. Our analyses suggest historical divergence within the A. oligospora population in China. The genetically differentiated populations also showed phenotypic differences that may be related to their ecological adaptations. Interestingly, our analyses identified evidence for recent dispersion and hybridization among the historically subdivided geographic populations in nature. Together, our results indicate a changing population structure of A. oligospora in China and that care must be taken in selecting the appropriate strains as biocontrol agents that can effectively reproduce in agriculture soil while maintaining their nematode-trapping ability.

Reclaimed desert habitats favor entomopathogenic nematode and microarthropod abundance compared to ancient farmlands in the Nile Basin

Characterizing entomopathogenic nematode (EPN) biogeography with a goal of augmentation and conservation biological control requires fine-scale taxonomic resolution, because closely related EPN species can exhibit divergent phenotypes for key properties such as habitat adaptation and insect host specificity. Consequently, we employed high throughput genome sequencing (HTS) to identify and compare EPNs and natural enemies of EPNs in 58 citrus orchards in 2 ecoregions in Egypt (El Beheira and Al Qalyubia governorates). We designed improved primers targeting the ITS2 rDNA to discriminate EPN species and used pre-reported primers targeting D2-D3 region for soil microarthropods. Five EPN species (Heterorhabditis bacteriophora, H. indica, H. taysearae, Steinernema glaseri, and S. scapterisci) and one steinernematid not represented in Genbank databases were detected. This is the first report of S. scapterisci and possibly the unknown (perhaps undescribed) species in Egypt. Only heterorhabditid species, dominated by H. indica, were detected in the reclaimed, sandy desert soils of El Beheira governorate. In the fine textured, ancient farming lands of the Nile delta all six species were detected, but at lower frequency and abundance. Microarthropod family richness (P = 0.01) and abundance (P = 0.001) was higher in the reclaimed lands than in the Nile Delta. Soil clay content, pH and elevation explained significant variation in the mite community structure. Population density of H. indica, the only EPN found consistently and at high abundance in El-Beheira, was inversely related to abundance of species in the nematophagous mite family Rhodacaridae.

Managing nematodes in Egyptian citrus orchards

Backed by its Mediterranean climate, fertile soils, cheap labor, and geographic location, Egypt ranks high among citrus producer countries. These factors can collectively offer early fruiting and long harvest season, good quality, low production costs, and closeness of export markets. However, citrus trees are attacked by many plant-parasitic nematode species in Egypt and abroad; all but one, the citrus nematodeTylenchulus semipenetrans, of which are of limited distribution.Tylenchulus semipenetransimpacts the size and quality of citrus yields differently under various conditions, but guidelines have been published to help interpret soil sample results. Other species such as migratory endoparasites (lesion nematodes), sedentary endoparasites (root-knot nematodes), and several species of ectoparasitic nematodes (spiral and stunt nematodes) can damage citrus in Egypt. Yet, due to their limited distribution and very low population densities in citrus orchards, their damaging level is uncertain or not believed to cause significant damage of citrus yield. Clearly,T.semipenetransinteracts differently with other plant pathogens often found in citrus rhizosphere. While its infection to citrus seedlings can reduce subsequent infection of roots byPhytophthora nicotianae, it can increase the virulence ofFusarium solani. Therefore, issues closely connected with citrus yield losses are presented herein. Perspectives for management of nematodes associated with citrus in Egypt were discussed in the light of their economic importance, sampling accuracy, phytosanitary measures, ecology, and biology. Cultural practices should tackle other citrus problems before managing these nematodes.

Optimizing for taxonomic coverage: a comparison of methods to recover mesofauna from soil

Manipulating soil properties to modify the dynamics between nematodes and their natural enemies has been proposed to conserve services such as the biological control of insect pests by entomopathogenic nematodes. Many soil microarthropods including acari mites and collembola are natural enemies of nematodes; however, little is known about the naturally occurring assemblages of these two soil dwelling groups and how they might be influenced by soil conditions. A method to efficiently recover both nematodes and microarthropods from environmental samples would be helpful to characterize communities of these two groups in different habitats. Because samples of nematodes extracted from soil by sucrose centrifugation (SC) also contain soil mites, collembola, protozoans, and fungal and bacterial propagules, the efficiency of SC to recover microarthropods was compared to more conventional methods of microarthropod recovery such as heptane flotation (HF), Berlese funnels (BF), and a modified flotation Berlese method (FBF). Microarthropods were identified using an inverted microscope to class in one experiment and to order in a second. Significantly more microarthropods of all taxa were recovered by SC than with either Berlese method (BF or FBF). In total, 40% more microarthropods comprising seven orders were recovered by HF compared to SC, but the difference was not significant. Ecological indices (diversity, richness, and evenness) derived from HF and SC were congruent and significantly higher than those derived from BF. Excessive organic matter in the HF extractions, compared to those of SC, BF, and FBF, made mite detection and identification difficult and time consuming. Moreover, unlike SC, neither HF nor any Berlese method recovered nematodes. Accordingly, we found SC to be the most efficient method for microarthropod extraction, making it an ideal method for studies of communities of nematodes and many of their natural enemies in the soil.

Fungi-Nematode Interactions: Diversity, Ecology, and Biocontrol Prospects in Agriculture

Fungi and nematodes are among the most abundant organisms in soil habitats. They provide essential ecosystem services and play crucial roles for maintaining the stability of food-webs and for facilitating nutrient cycling. As two of the very abundant groups of organisms, fungi and nematodes interact with each other in multiple ways. Here in this review, we provide a broad framework of interactions between fungi and nematodes with an emphasis on those that impact crops and agriculture ecosystems. We describe the diversity and evolution of fungi that closely interact with nematodes, including food fungi for nematodes as well as fungi that feed on nematodes. Among the nematophagous fungi, those that produce specialized nematode-trapping devices are especially interesting, and a great deal is known about their diversity, evolution, and molecular mechanisms of interactions with nematodes. Some of the fungi and nematodes are significant pathogens and pests to crops. We summarize the ecological and molecular mechanisms identified so far that impact, either directly or indirectly, the interactions among phytopathogenic fungi, phytopathogenic nematodes, and crop plants. The potential applications of our understanding to controlling phytophagous nematodes and soilborne fungal pathogens in agricultural fields are discussed.