First Report of Root-Knot Nematode Meloidogyne enterolobii Infecting Sweetpotato in the State of Rio Grande do Norte, Brazil.

Abstract

The sweetpotato (Ipomoea batatas L., Convolvulaceae family) originated in Latin America and is currently cultivated worldwide. The storage roots, rich in calories, have made this crop one of the main caloric sources for low-income populations, especially in developing countries. Brazil annually produces about 805,000 tons, with the Northeast region responsible for 34% of this production (Albuquerque et al. 2020). In October 2019, sweetpotato plants cv. Campina, from a field in the region of Touros, state of Rio Grande do Norte (RN), Brazil (5°12 31 S 35°34 42 W), presented deformed storage roots, with galls, typical of root-knot nematodes. The roots were sent to the Nematology Laboratory (LabNema) where 14,032 eggs and 3,312 second-stage juveniles (J2s) of Meloidogyne sp., in 10 g of roots, were recovered. The species of adults was identified through morphological, biochemical, and phylogenetic analysis. The perineal region of females (n = 10) presented an oval shape, with a high and semi-trapezoidal dorsal arch and streak-free perivulval region. The labial region of males (n=10) presented high and rounded head cap, labial region slightly set off from the body, without annulations. The morphological characters were compatible with the original description of Meloidogyne enterolobii (Yang and Eisenback 1983). The phenotype of esterase isoenzymes showed two major bands (VS1-S1) also characteristic of M. enterolobii (Esbenshade and Triantaphyllou 1985). Sequences of 18S rDNA (~1200bp) of individual females (Holterman et al. 2006) obtained from sweetpotatoes before (SPme1 and 2) and after inoculation (SPme3 and 6), and from guava, used as M. enterolobii species control, were submitted to Bayesian analysis. The sequences presented genetic diversity among them resulting from seven SNPs (Single Nucleotide Polymorphism) and 99.4 to 99.9% identity with M. enterolobii sequences deposited in the NCBI GenBank (accession numbers MW209034-MW209039). The pathogenicity test was carried out under greenhouse conditions, in which 3,000 eggs and J2s from the original population isolated of M. enterolobii were inoculated in sweetpotato seedlings cv. Campina (n = 6). After three months, the roots presented galls and deformations typical of root-knot nematodes, while non-inoculated plants did not present any symptoms. An average of 15,900 eggs and J2s of M. enterolobii (RF = 5.3) were recovered from the roots, proving that sweetpotatoes were a host of this species. Meloidogyne enterolobii is known to cause great damage to sweetpotato (Ye et al. 2020). In Brazil, Meloidogyne nematode had been reported once, isolated from a sweetpotato field in the Ceara state and the species suggested by the authors according to esterase electrophoresis was M. enterolobii. Nonetheless, the authors did not present taxonomic, isoenzyme phenotypes and molecular species identification integratively, nor included pathogenicity tests (Silva et al. 2016). Therefore, it is the first time that M. enterolobii, with reliable identification by different methods, including sequencing, was detected in commercial sweetpotato fields in the RN state and in Brazil. The local farmers reported that this nematode deforms the storage roots which make them useless for commercialization, resulting in minimal losses of 50% of production in the infested areas. Furthermore, as sweetpotatoes are vegetatively propagated, the spread of this nematode through planting material is favored. Considering the importance of this crop in Brazil, this report is essential for control measures of this pathogen to be taken in order to avoid its spread to other regions.