Soil chemical properties and their relationship with phytonematode populations inside and outside patches of soybean fields
Introduction
Nematodes are among the most economically damaging pathogens of cultivated plants. Plant-parasitic nematode infestations cause an estimated annual loss of US$100 to 150 billion worldwide (Singh et al., 2015). These vermiform organisms inhabit the soil and, most frequently, feed on plant roots. Plant-parasitic nematodes are obligate parasites and can be classified according to their feeding habits as endoparasites (remain inside the host roots for most of the life cycle), semiendoparasites (part of the body remains inside the roots and the other part, on the root surface, where eggs are deposited), and ectoparasites (remain in the soil and feed on host roots introducing only the stylet) (Lambert and Bekal, 2002). Nematodes can migrate when vermiform, moving freely within the root system or the soil in search of food, as adults, some females become sedentary, acquiring an aberrant form (Castillo and Vovlas, 2007; Dias et al., 2010; Favoreto et al., 2019). Because of these differences, some nematodes are found more frequently in roots and others in soil.
The occurrence of parasitic nematodes in the field is indicated by plants with stunted growth, yellow leaves, early defoliation, and reduced yield. Direct symptoms, that is, those observed in the roots, include gall formation, root deformation, necrosis, and reduced size. Such symptoms vary with nematode species (Dias et al., 2010; Favoreto et al., 2019).
In the tropical climate regions of South America, particularly in Brazil and Paraguay, where soybean is the most cultivated crop, the most common nematodes are Meloidogyne spp. (root-knot nematodes), Pratylenchus brachyurus (lesion nematode), Rotylenchulus reniformis (reniform nematode), and Heterodera glycines (cyst nematode). Other species have also gained importance in recent years, such as Scutellonema brachyurus (Pedrozo, 2008; Favoreto et al., 2019).
Although Meloidogyne spp. are the most common in tropical regions, the increase in soybean cultivation on weathered sandy soils with low fertility and organic matter content has increased the problems with P. brachyurus (Pedrozo, 2008; Dias et al., 2010). Soil properties related to management practices also influence nematode occurrence. For instance, soil compaction can limit root growth to surface layers, where plant-parasitic nematodes are found at high. Thus, there seems to be an important relationship between soil physical (texture) and chemical (pH, organic matter, nutrient content) properties and phytonematode populations (Chávez and Araya, 2009; Franchini et al., 2011; Costa et al., 2014; Arévalo et al., 2016).
One aspect that has been little studied is the behavior of nematode populations inside and outside patches with symptoms of nematode damage in soybean fields. The location of symptomatic patches may change between crop seasons, and with this, there is the hypothesis that high nematode populations can also be found outside patches and they may have a correlation with soil properties. Then, this study aimed to describe the relationship between nematode populations and soil physicochemical properties inside and outside patches with symptoms of nematode damage in soybean.
Section snippets
Material and methods
The study was conducted in soybean-producing regions in Nueva Esperanza, Curuguaty, and Katueté, Department of Canindeyú, eastern Paraguay (MAG, 2014). The agroecological zone is characterized by gently undulating to undulating plains suitable for mechanized agriculture and soils of predominantly basaltic origin with signs of laterization, classified as reddish-brown or dark-red Latosol (transition phases between basalt-derived soils and sandy soils) (Galeano, 2014). The climate is
Results
Most samples (80%) had mixed populations of nematodes. Two, three, and four different genera were identified in 16, 61, and 5% of root samples, respectively, and in 57, 23, and 2% of soil samples, respectively. Pratylenchus spp. were identified in 95% of root samples, nematode eggs in 86%, Helicotylenchus spp. in 75%, and H. glycines in 61%. Helicotylenchus spp. were the most prevalent in soil samples (80%), followed by nematode cysts (64%), H. glycines (57%), and Pratylenchus spp. (36%). In
Discussion
The positive correlation between Pratylenchus spp., Meloidogyne spp., and eggs is directly related to the fact that both nematodes are endoparasites and that, most likely, the eggs were derived from both genera. Pratylenchus oviposition occurs within the roots of host plants (Castillo and Vovlas, 2007), and Meloidogyne individuals produce egg masses on the surface of the root system or inside the roots within the cortical parenchyma, depending on the females’ location (Vovlas et al., 2005). The
Conclusions
Pratylenchus spp. was the most frequently detected nematode, particularly in root samples collected from inside patches with symptoms of nematode damage in soybean fields. There was positively correlated between Pratylenchus and Meloidogyne spp. The most prevalent nematodes in soil samples were Helicotylenchus spp., H. glycines, cysts, R. reniformis, and S. brachyurus. Pratylenchus spp. was positively correlated with sand content and negatively with sum of bases, micronutrients, and K.
Declaration of competing interest
The authors of the manuscript entitled “Soil chemical properties and their relationship with phytonematode populations inside and outside patches of soybean fields” declare that they have no conflict of interest or financial related to this research.
Acknowledgements
We thank the Brazilian National Council for Scientific and Technological Development (CNPq) for granting a research productivity fellowship to Claudia R. Dias-Arieira.
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