**Preface to "Plant Parasitic Nematodes"**

Plant-parasitic nematodes (PPN), which are small plant/soil-borne pathogens, are economically important pests for agriculture and forestry crops, representing a significant limitation for global food security and forestry health. Damage caused by these nematodes is probably underestimated, because the symptoms that they cause are unspecific, and most growers are often unaware of their presence. However, it has caused estimated losses of 80 billion USD/year. Root knot nematodes (RKN, *Meloidogyne* spp.), potato cyst nematodes (*Globodera* spp.), root lesion nematodes (RLN, *Pratylenchus* spp.), the burrowing nematode *Radopholus similis*, the stem nematode *Ditylenchus dipsaci*, and the pinewood nematode *Bursaphelenchus xylophilus* are some examples of the most economically and scientifically important PPN.

Current approaches to control these PPN include the use of nematicides, but the serious concerns posed by these chemicals for human health and the environment have stimulated the search for eco-friendly strategies of control (https://doi.org/10.3390/plants9060671; https: //doi.org/10.3390/plants9091146; https://doi.org/10.3390/plants9111588). Nevertheless, to cope with this threat, accurate diagnostic methods for nematode detection (https://doi.org/10.3390/ plants9111475; https://doi.org/10.3390/plants10071454; https://doi.org/10.3390/plants9091085; https://doi.org/10.3390/plants10010099; https://doi.org/10.3390/plants10030603; https://doi. org/10.3390/plants10061068) and increased knowledge of nematode intricate relationships with host plants and the environment (https://doi.org/10.3390/plants9060802; https://doi.org/10. 3390/plants10020292) are also crucial for the development of effective integrated nematode management programs.

The first six papers are devoted to the characterization and identification of PPN (*Helicotylenchus* sp., *Meloidogyne* spp., *Paratylenchus* spp., and *Pratylenchus* spp.), which are found to be associated with economically important plants, through integrative taxonomy, essential to adopt appropriate management strategies. In the first paper, https://doi.org/10.3390/plants9091085, Santos et al. identify, for the first time, *M. javanica* parasitizing a bonsai plant of *Ficus macrocarpa*, based on electrophoretic analysis of female esterases, PCR-RFLP of the mtDNA region between COII and 16S rRNA genes and SCAR-PCR, and *H. dihystera*, using morphological characters and sequencing of the D2D3 expansion region of the 28S rDNA gene. Although these PPN species have a worldwide distribution, the findings emphasize the importance of the use and transport of clean, healthy, nematode-free material, to avoid nematode dissemination, and of routine inspections to find out whether imported material is PPN-free. In addition, a first report of the polyphagous RKN *M. luci*, included in the European Plant Protection Organization alert list, in 2017, is reported by Rusinque et al. (https://doi.org/10.3390/plants10010099), in Azores Islands (Portugal) associated with *Solanum tuberosum* (potato), which confirms the need to implement measures to prevent nematode dispersion.

The next three articles focus on *Pratylenchus* spp. characterization and identification. RLN are the most frequent associated with decaying raspberries (*Rubus* sp.) in North Italy (Troccoli et al., https://doi.org/10.3390/plants10061068); are widely distributed in Israel, parasitizing vegetables and crops affecting quality and yield (Bucki et al., https://doi.org/10.3390/plants9111475); and are associated with potato in Portugal (Gil et al., https://doi.org/10.3390/plants10030603). The reduced number of diagnostic characters available and the intraspecific variability makes *Pratylenchus* species identification difficult. The study by Gil et al. (https://doi.org/10.3390/plants10030603) reveals a remarkable amount of variability within and between *P. penetrans* isolates, highlighting that identification based on morphology alone can be inconclusive and should be complemented with molecular markers. Phylogenetic analyses show that the ITS region and cytochrome c oxidase subunit I are useful to differentiate *P. penetrans* from other related species.

Several other *Pratylenchus* spp. were identified by the integration of morphological studies and molecular markers (18S, 28S rRNA gene, ITS region, and/or hsp90 gene), allowing the distinction between *P. thornei* and *P. mediterraneus* and the occurrence of cryptic biodiversity within the genus (Bucki et al., https://doi.org/10.3390/plants9111475; Troccoli et al., https://doi.org/10.3390/ plants10061068). A new species, *P. vovlasi*, is described by Troccoli et al. (https://doi.org/10.3390/ plants10061068), which parasitize raspberries in the Piedmont area (North of Italy); and the efficiency of the most common strategies of control taken to reduce crop losses associated with *Pratylenchus* is discussed by Bucki et al. (https://doi.org/10.3390/plants9111475).

Finally, based on integrative taxonomical approaches, 18 *Paratylenchus* species were identified in Spain; a new pin nematode species, *P. parastraeleni* sp. nov., was described; and the cryptic diversity of the *P. straeleni*-species complex was analyzed, confirming the huge biodiversity of this group (Clavero-Camacho et al., https://doi.org/10.3390/plants10071454).

In the first paper of a second group of five papers, current insights into the biology, parasitism mechanism, and management strategies of important migratory endoparasitic PPN (*Pratylenchus*, *Radopholus*, *Ditylenchus*, and *Bursaphelenchus*) are provided (Mathew and Opperman, https://doi. org/10.3390/plants9060671).

A wide range of management strategies has been employed for PPN control: nematicides, crop resistance, and cultural practices. Despite the adverse impacts on the environment and human health of nematicides, they continue to be an alternative for PPN control. Nevertheless, the research on natural nematicides has been increasing. Fifty commercial plant essential oils (EOs) were tested against the free-living microbivorous nematode *Panagrolaimus* sp. Results indicated that *Cinnamomum cassia* and *C. burmannii* exhibited the best nematicidal activity and the impacts of EOs differ between *Panagrolaimus* sp. and PPN. *Panagrolaimus* sp. was less sensitive; therefore, selected EOs can be potentially used to control PPN without affecting the non-target nematode community (Oro et al., https://doi.org/10.3390/plants9111588).

Crop rotation and the growth of resistant cultivars are ecologically healthy, effective, and widely used strategies for nematode control, but they require knowledge on the host status of a large number of plants. The impact of PPN infection on six biofortified cassava cultivars was assessed, resulting in quality and quantity losses (Akinsanya et al., https://doi.org/10.3390/plants9060802).

*Meloidogyne* spp. circumvent plant defence mechanisms to establish a functional feeding site, develop, and reproduce. Understanding nematode–plant interaction might help to design nematode-resistant/tolerant crops. *M. incognita* strongly affect the β-sitosterol/stigmasterol ratio in *Cucumis sativus*, *Glycine max*, *Solanum lycopersicum*, and *Zea mays*; thus, designing crops with an altered sterol profile can be an option to RKN control (Cabianca et al., https://doi.org/10.3390/ plants10020292).

Lastly, expanding the knowledge of nematodes and their related bacterial microbiota can be an option to develop bionematicidal agents (Oro et al., https://doi.org/10.3390/plants9091146). Species of the family Bacillaceae, reported associated with cysts of *Globodera rostochiensis* (death or in decline), can result in the development of commercial bionematicidal agents (Oro et al., https://doi.org/10. 3390/plants9091146).

Morphological methods require a high level of nematology expertise due to the observation of intra-specific variability; consequently, methods based on molecular biology are an attractive solution since they are potentially discriminatory. Nevertheless, with the increasing number of species described and the complexity within genera, an integrative taxonomical approach is increasingly used.

Although the use of synthetic nematicides is the most frequent and an efficient strategy, there is a growing concern of the risks posed by chemicals. This has stimulated the search for bionematicides derived from plants and the expansion of the knowledge on plant–nematode interactions and on the identification of bacterial microbiota associated with nematodes to develop novel and environmentally friendly strategies for PPN control that is safer to humans and the environment than conventional pesticides.

#### **Carla Maleita, Isabel Abrantes, and Ivˆania Esteves**

*Editors*

*Article*
