Mg is the most prevalent PPN on rice and considered a major threat to rice as yield losses can reach up to 70% [12,94,112]. Mg densities of 120, 250, and 600 eggs/plant in seedlings 10, 30, and 60 days after planting were reported by Rao et al. [110], causing 10% losses. In a later study, Cuc and Prot [65] stated that a density of 100 J2/g root could be considered as high infestation. Most recently, Win et al. [73] found that population densities could exceed 1000 J2/g root with 12–16 galls/plant, contributing to a 65% yield reduction. It has also been found that there is a decline in yield when more than 75% of the roots are affected by nematodes [32]. Additionally, the water regime is an important environmental factor that influences the development and population dynamics of Mg, and the damage and yield loss that it can cause to rice. Soriano et al. [91] showed that rice cultivar tolerance to Mg varies with the water regime and that yield losses may be prevented or minimized when the rice crop is flooded early and maintain inundated until harvesting. For example, losses in lowland rainfed rice in Bangladesh can range between 16 and 20%, while in India, losses range between 16 and 32% under irrigated conditions and between 11 and 73% under flooded conditions [102,113]. In China, the highest incidence of the disease is in the Hunan provinces, exceeding 85% in infested paddy fields [19]. Furthermore, reports of Mg infestations in rice–wheat agroecosystem of India, Nepal, and Pakistan suggest that the damage caused by the rice RKN may be responsible for the poor productivity in this cropping system [10,11,35,114].

Changes in agricultural policy and adoption of new rice production technologies in South East Asian countries have influenced the status of the rice RKN problem [64]. For instance, in the Philippines, Mg became a major constrain due to the intensification of rice cropping and shortage of water supply. This situation forced the farmers to grow direct wet seeding, and intermittent irrigation, providing favorable conditions for Mg infestation and increasing the economic losses [9,64]. In India, the system of rice cultivation shifted to the so-called “system of rice intensification practice”, where a new ecological condition is being developed through modification of rice cultivation practices that includes planting younger and tender seedlings, the creation of greater aeration in soil, and regulation in irrigation. All these conditions provide a suitable environment to increase the infestation levels of the rice RKN [112,114,115].

Spatio-temporal studies have also demonstrated that densities of Mg J2 in the soil fluctuate throughout the year [116]. Moreover, Mg’s ability to survive and reproduce in off-seasons on weeds and forage crops contributes to increase the population levels in the soil, and rice infection in the next season [35]. Besides alternative hosts and irrigation, the soil type influenced the tolerance of plants to Mg and showed differences in the multiplication of the nematode [91]. Studies have also revealed that infestation levels depend on the rice cultivar [117,118], and the aggressiveness differs between populations, suggesting intraspecific variability [35,119]. It was also found that Mg consists of more than one race. In fact, populations from Florida have shown less aggressiveness and difference on the host infection and reproduction patterns than the Asian populations, and populations from Vietnam are not able to reproduce on tomato (Solanum lycopersicum), soy (Glycyne max), or green beans (Phaseolus vulgaris), despite these species being reported as a host of Mg [16,119,120].

4. Host Plants

The best strategy for management of Mg is to prevent the movement of plant and soil that in some cases may adhere to machinery or tools. In a recent pest risk analysis for Mg in Italy, it was concluded that the main ways of dispersion of this nematode are likely to be through the movement of infected plants and infested soil, non-host plants that may have grown near areas infested with Mg, and floating roots or plant material in the water [121]. Migrant waterbirds, machinery, and travelers were considered a secondary source of entrance. On the other hand, changes in the water regime (intermittent irrigation or water shortages) in many parts of the world are also contributing to the spread and infectivity of the nematode.

To minimize the losses resulting from Mg, management strategies are of extreme importance, and studies have shown that a combination of methods is the best approach to control this nematode in rice fields. The methods that have been applied to control Mg include the use of synthetic nematicides, known as the most efficient strategy, cultural methods, biological agents, and natural nematicides.

Some synthetic nematicides were, recently, strictly regulated or banned from the market, due to the adverse impacts on the environment and human health, reducing the alternatives for RKN control. Cultural methods (fallowing, soil solarization, crop diversification and rotation, etc.) also appeared to have some efficacy. For instance, crop rotation studies with non-host crops, like sweet potato (Ipomoea batatas), cowpea (Vigna unguiculata), sesame (Sesamum indicum), castor (Ricinus communis), sunflower (Helianthus annuus), soybean (Glycine max), turnip (Brassica rapa subsp. rapa), and cauliflower (Brassica oleracea var. botrytis), showed to prevent Mg development [110,132,173]. Nonetheless, none of these practices have gained importance among farmers, because of the high cost and unsatisfactory results. Furthermore, as many weeds found in rice fields are hosts for Mg, serving as nematode reservoirs for the next crops, a weed management programmme must be implemented to maintain a low nematode population in infested fields.

Alternative strategies, such as the “rice field flooding technique”, used by the Italian National Plant Protection Organization (Ministerial Decree of 6 July 2017) to control Mg, had some effect on the nematode population densities. Mg can still propagate under flooding conditions, but the damage induced by this nematode is lower than in shallow intermittently flooded fields [80,174]. Nevertheless, this method of control also has some limitations, as there are areas where this practice is not applicable due to the soil structure, characterized by a low water retention capacity, or restriction in water use. Another approach explored by Sacchi et al. [174] was the use of rice plants as trap crops. Preliminary results indicate that trap cropping for the management of the rice RKN is efficient in most rice-growing areas, especially those with water shortages. However, additional studies are required to establish the most effective number of trap crop cycles that are necessary to reduce Mg population density. Additionally, this technique, in our opinion, could be highly influenced by climate in northern latitudes in order to sow rice in advance and the cost of machinery and water.

The use of biological control agents, such as the fungi Paecilomyces lilacinus, Trichoderma harzianum, T. viride, and other Trichoderma spp.; the bacteria Bacillus subtilis; and the rhizobacterium Pseudomonas fluorescence, have shown promising results against Mg [175,176,177,178]. Studies by Amarasinghe and Hemachandra [178], in Sri Lanka, revealed that T. viride reduces gall formation and production of egg masses, which represents a potential strategy to be included in integrated pest management programs.

Similarly, the use of essential oils (EOs) has been explored to control RKN, as an alternative to the synthetic nematicides. The nematicidal effects of EO from spices and medicinal plants on RKN have been widely reported. The high effect of Cymbopogon spp. EO (C. martini motia, C. flexuosusand, and C. winterianus) on J2 mortality has been described [179,180,181]. Chavan et al. [182] stated that basil (Ocimum basilicum), peppermint (Mentha×piperita), and lemongrass (Cymbopogon citratus) EOs have nematicidal properties against Mg. In order to confirm the efficacy of these EOs, the in vitro tests must be complemented by in vivo soil-based experiments.

Host plant resistance is an environmentally friendly and cost-effective strategy to mitigate damage caused by Mg. A promising alternative for the control of Mg is the screening of germplasm for genotypes that are resistant/tolerant and the development of resistant/tolerant cultivars [80,108,183]. Resistance sources against Mg have been identified in African wild accessions of rice (O. glaberrima and O. longistaminata and O. rufipogon) [184], and variability to a certain extent has been perceived [162]. Wild accessions that are partially or fully resistant to Mg can therefore act as resistant donors for interspecific crosses with Asian cultivars of rice [184,185]. Introgression of O. glaberrima into O. sativa has led, for example, to the new rice for Africa, NERICA cultivars [186], but the introgression has not been very successful [187]. Therefore, natural resistance in O. sativa cultivars is potentially very important. In Asian rice, using the Bala and Azucena mapping population, chromosomes 1, 2, 6, 7, 9, and 11 have been reported as having quantitative trait loci (QTL) for partial resistance to Mg [111]. Mapping of Mg resistance on chromosome 10 in Asian rice (cv. Abhishek), using bulk segregant analysis, was reported by Mhatre et al. [188]. A hypersensitivity-like reaction to Mg infection found in the Asian rice cv. Zhonghua 11 suggests that resistance to Mg was qualitative rather than quantitative, involving (a) major gene(s) [189]. Galeng-Lawilao et al. [190] reported the main effect QTL for field resistance in Asian rice on chromosomes 4, 7, and 9 plus two epistatic interactions (between loci on chromosome 3 and 11, and between 4 and 8).

Few studies have used genome-wide association studies (GWASs) as a viable strategy to identify novel QTLs for PPN resistance or susceptibility in different plants [191,192]. For example, Dimkpa et al. [191] confirmed the robustness of GWAS to screen for rice–nematode interactions and identified two resistant accessions (Khao Pahk Maw and LD 24). Studies carried out, in India, by Hada et al. [193] allowed the identification of 40 highly resistant accessions. Alternatively, the profiling of the defense response of 36 rice cultivars to Mg infection revealed a variation in the expression of plant defense genes [194]. Among all the selected plant defense genes, the expression of mitogen-activated protein kinases (MAPK20), isochorismate synthase genes (ICS1), nonexpressor of pathogenicity expression genes1 (NPR1), phytoalexin-deficient 4 (PAD4), allene oxidase synthase (AOS2), jasmonic acid-inducible rice myb gene (JAMYB), and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO7) was upregulated, possibly providing resistance against Mg. This observation matches the insignificant expression in the susceptible genotypes. These outcomes are significant and can be exploited for breeding purposes.