Report, pathogenicity and parasitism habit of Helicotylenchus pseudorobustus on onion

Spiral nematodes (Helicotylenchus spp.) are globally distributed and are currently considered secondary pests in onion. However, a population of Helicotylenchus was recently detected causing reduced development in an onion-producing area in Rio Grande do Sul state, extreme south of Brazil. In this context, the objective of this study was to conduct a morphological, morphometric, and molecular characterization of this population associated with onion, and to evaluate its reproduction, pathogenicity, and parasitism on onion under greenhouse and laboratory conditions. The experiment was conducted with soil naturally infested with 730 nematodes with individual ‘Bola Precoce’ onion plants. Plants were evaluated for shoot, bulb, and root fresh weights, bulb diameter, leaf and root lengths, number of leaves, and nematode final population and reproduction factor (RF). Additionally, the feeding behavior was determined in the roots of infected plants. The studied population was identified as Helicotylenchus pseudorobustus, constituting the first report of this species in onion crops in Brazil and worldwide. This nematode significantly impacted plant development, reducing bulb fresh weight, shoot fresh weight, number of leaves, leaf length and total chlorophyll content by 22.2 to 61.5%. The cultivar Bola Precoce was susceptible, yielding in RF nematode value equal to 4.34. Furthermore, H. pseudorobustus behaved as endoparasitic and semi-endoparasitic in the infected onion roots.