Occurrence of Meloidogyne javanica as a Pathogen of Cockscomb (Celosia cristata) in Brazil

Abstract

Cockscomb (Celosia cristata L., family Amaranthaceae) is an ornamental plant species cultivated in pots or used as cut flowers in floral arrangements. In April 2018, cockscomb ‘Comum’ exhibiting many galls on the roots was detected and sampled from several areas (27°21′34″S, 53°23′37″W) of Frederico Westphalen county, Rio Grande do Sul State, Brazil. Identification of Meloidogyne sp. (Nematoda: Meloidogynidae) was carried out by observations of perineal patterns (n = 30) and esterase phenotypes (n = 30), morphological measurements of second-stage juveniles (J2) (n = 50), and amplification and sequencing of the ITS1-5.8S-ITS2 rRNA region (primer set: forward 5′-TTGATTACGTCCCTGCCCTTT-3′ and reverse 5′-TCCTCCGCTAAATGATATG-3′) and the D2 to D3 fragment of the 28S rRNA gene (primer set: forward 5′-ACAAGTACCGTGAGGGAAAGTTG-3′ and reverse 5′-TCGGAAGGAACCAGCTACTA-3′). In addition, cockscomb roots were processed to determine the number of J2 and eggs of Meloidogyne sp. Nematode population density observed in the sample was 2,100 J2 per gram of cockscomb roots. The J2 had the following morphometric characteristics: length (L) = 405.5 ± 61.0 (368.5 to 415.6) µm, a = 29.6 ± 2.5 (26.8 to 33.4), c = 8.00 ± 0.5 (6.0 to 9.6), stylet = 13.5 ± 0.5 (11.3 to 15.9) µm, dorsal esophageal gland orifice (DGO) = 3.20 ± 0.4 (2.9 to 3.9) µm, tail length = 55.5.5 ± 4.0 (54.1 to 59.5) µm, and hyaline tail terminus = 10.3 ± 0.8 (9.0 to 11.5) µm. Female perineal patterns included a low trapezoidal dorsal arch and smooth striae interrupted by a pair of incisions on both sides. These lateral ridges that divide the dorsal and ventral striae are unique to Meloidogyne javanica (Treub, 1885) Chitwood, 1949 (Rammah and Hirschmann 1990). The polymorphisms in the esterase bands observed during electrophoresis revealed the typical phenotype of J3 (Rm = 1.00,1.25, 1.40) M. javanica populations (Carneiro et al. 2000). Sequence homologies were determined by comparing sequences of polymerase chain reaction (PCR) amplicons of D2 to D3 of 28S and ITS1-5.8S-ITS2 rRNA region that were noted to be 516 bp (GenBank MK072827) and 554 bp (MK072828) in length, respectively. The sequences of both markers exhibited 99 and 100% identity, respectively, with sequences being more similar to those of M. javanica isolates from Brazil, Vietnam, and China. The confirmation of nematode species identity was performed by PCR species-specific sequence characterized amplified region (SCAR) using a primer set composed of Fjav (5′-GGTGCGCGATTGAACTGAGC-3′) and Rjav (5′-CAGGCCCTTCAGTGGAACTATAC-3′). The PCR amplification using SCAR produced a specific fragment of expected size (∼670 bp) for M. javanica (Donkers-Venne et al. 2000). In greenhouse tests, cockscomb Comum plants being maintained in pots containing sterilized soil were inoculated with 5,000 eggs and J2 of the original M. javanica population; a noninoculated control was included, and treatments were replicated 10 times. After 60 days, all inoculated plants showed reduced growth compared with the control. Galling symptoms on the roots were similar to those observed in field-grown plants, and the nematode reproduction factor (final population/initial population) was 20.5. The noninoculated plants did not present gall formation in the roots, and their development was not affected either. The morphological and molecular characteristics of reisolated root-knot nematodes were identical to those of M. javanica. These results confirmed the nematode’s pathogenicity to cockscomb. To our knowledge, this is the first report on M. javanica parasitizing cockscomb in Brazil; therefore, our findings are of great importance to Brazilian flower growers to help them establish management measures M. javanica.