First Report of Meloidogyne enterolobii Causing Tuber Galling Damage on White Yam (Dioscorea rotundata) in Nigeria

The white yam (Dioscorea rotundata) is the most widely grown species for food purposes in West Africa. Plant-parasitic nematode attack is among the major constraints to yam production (Bridge et al. 2005). Several species of nematodes are known to infect yams, including root-knot nematodes (RKN; Meloidogyne spp.), which cause tuber and root galling (Hunt and Handoo 2009). To date, M. incognita and M. javanica are reported in yam in Nigeria and elsewhere in Africa (Onkendi et al. 2014). During a taxonomic nematode survey, galled yam tubers were collected from markets and farmers’ fields between November 2012 and February 2013 in Nigeria. Steam-sterilized soil planted with susceptible tomato (Solanum lycopersicum cv. Marmande) was infested with nematodes isolated from galled tubers and used for RKN population increase and species identification after maturation of nematodes. Ten young egg-laying females taken from these populations were used for species identification based on esterase and malate dehydrogenase isozymes patterns (Carneiro et al. 2000). The M2 and N1a isozyme electrophoresis phenotype identical to the M. enterolobii pattern described in Carneiro et al. (2000) was observed in 11% of 36 galled tubers examined. M. enterolobii was recovered from galled white yam in the Derived Savanna (Anambra State) and in the Southern Guinea Savanna (FCT Abuja and Niger States). Individual egg masses were removed and single-egg mass cultures were established on susceptible tomato to generate pure cultures of RKN. DNA isolated from a single juvenile of each of the 10 populations established was used for PCR amplification of mitochondrial COX2 using primer pair, COX2F (5′-TTGAATTTAAGTGTTGTTTATTAC-3′) and COX2R (5′-GATTAATACCACAAATCTCTGAAC-3′). The 432-bp PCR DNA product was sequenced on both strands (GenBank accession no. KU881046) and found to be 100% identical to M. enterolobii (KU881056, KP202351) (Janssen et al. 2016). To confirm M. enterolobii as a causal agent of the galling damage on white yam, egg masses removed from pure cultures were inoculated at a rate of 1 nematode/ml soil onto white yam cv. Makakusa planted in pots containing 6 liters of steam-sterilized top soil/white sand mixture (2:1 v/v) under screen house conditions. At harvest, 3 months after inoculation, roots and tubers exhibited severe galling symptoms typical of RKN. The nematode reproduction factor was on average 29 times the initial nematode density. The RKN, M. enterolobii, is therefore established as a new RKN species causing galling damage on yam. To our knowledge, this is the first record of M. enterolobii infecting yam and causing galling damage. This species is known to be present in West Africa and other continents affecting a range of crops, but not yam (Hunt and Handoo 2009). This extends the known host range for this damaging and highly aggressive nematode, and warrants consideration during resistance screening activities and other management strategies for yam.