STUDIES ON THE OCCURRENCE AND POPULATION DISTRIBUTION OF NEMATODES WITH YAM ( DIOSCOREA ROTUNDATA POIR ) TUBERS IN BENUE STATE , NIGERIA

Yam (Dioscorea spp) nematodes have been variously reported to reduce tuber yield and quality worldwide. In the present study, a survey was done to estimate types, frequency and population density of nematodes associated with yam tubers at each of the Local Government headquarters and two villages in Benue state of Nigeria. A total of 250 yam tubers were sampled randomly in the survey markets between April and May, 2017. Each yam sample was processed by peeling the periderm (oneinch-thick) out of which a 10 g sub-sample tuber was separately put in a blender with some quantity of water and macerated at low speed for 15 seconds. Plant-parasitic nematodes were recovered from the yam tubers using a modified extraction tray method. Five nematode species were found in association with yam tubers as follows: Scutellonema bradys, Meloidogyne spp, Pratylenchus spp, Rotylenchulus spp and Tylenchus spp. Scutellonema bradys was the most ubiquitous, occuring at a 100% frequency rating in all the samples while Meloidogyne spp, Pratylenchus spp, Rotylenchulus spp and Tylenchus spp. were 29.60%, 16.80%, 8.80% and 4.80% respectively. Though the study revealed that Rotylenchulus spp and Tylenchus spp. were poor parasites of yam tubers, yet there was a widespread distribution of Scutellonema bradys, Meloidogyne spp and Pratylenchus spp on yam tubers in Benue State of Nigeria, thus control of plant-parasitic nematodes is encouraged for improved tuber quality.


INTRODUCTION
Yam (Dioscorea spp) is a staple food crop in Nigeria.There are four main varieties widely cultivated in the country [7].Large production of the yam crop is in West Africa and out of that, Nigeria is the largest producer and Ghana and Cote d'Ivoire follows Nigeria [19].Among the world's root crops, yam is the second most used one [26].The rise in yam consumption in the world today is due to its high nutritional value.Yam is rich in many nutrients and also have medicinal properties [18].The crop is also used for cultural events and the harvest is always celebrated annually by some tribes in Nigeria [24].The set standard for exportable crops including yam is necessary to be followed by stakeholders in order to raise the quality of yam export to internationally acceptable level.For example, as far back as 2000, Jamaica's yam export industry was valued at 15 million dollars and serves as source of revenue for the 15,000 small farmers [18].
Despite above listed advantages of yam to human health, economic, cultural and social benefits, the damage by agents like plant-parasitic nematodes is a major threat to this crop in all yam producing countries throughout the world [8,10,30,2,14,23,21,7].In West Africa, [16] and in Central America [1] the extent of damage is high.Meloidogyne sp.type causes losses after harvest [12] there are different factors enhance the spread of nematodes in Nigeria, like a suitable a host [3].
In a bid for Nigeria Agricultural Quarantine Service (NAQS) to carry out one of her functional objectives which is to facilitate international trade in agricultural products by enforcing compliance with World Trade Organization (WTO), Sanitary and Phytosanitary (SPS) standards, a survey was carried out for yam's associated nematodes in the State with a view to controlling them for safe export and preventing postharvest losses.sampled from an open market.This made a total of two hundred and fifty (250 tubers) samples in the entire state.Samples (yam tubers) from each market were pooled and sealed in plastic bags and protected from the sun [26].The samples were properly labelled and taken to Nematology Research Laboratory of the Nigeria Agricultural Quarantine Service (NAQS), Moor Plantation, Ibadan-Nigeria for analysis and identification of plant parasitic nematodes.The coverage of the state surveyed is shown in fig. 1 while The Geographic Information System (GIS) was used to indicate coordinate of each market where nematodes on the sampled yams were intercepted (fig.2).

MATERIALS AND METHODS
In the laboratory, every yam tuber collected was freed of soil, washed under a gentle stream of cool tap water and mopped dry.Each yam was processed by peeling the periderm (oneinch-thick) with a knife and cut transversely with scissors into about 1-2 cm pieces, mixed carefully and 10 g sub-sample was weighed out of the bulk sample per tuber and assayed for nematode using the Whitehead and Hemming [31] and identified [15,17,22] as described previously [4].

Nematode species isolated from yam tubers
Five species of parasitic nematodes were identified and recorded in association with yam tubers from Benue state of Nigeria.They include; Scutellonema bradys, Meloidogyne spp, Pratylenchus spp, Rotylenchulus spp and Tylenchus spp.(Plate 1(a-e)).The occurrence and widespread distribution of these nematode species on yam tubers in Benue State surveyed is described in table 2 while the occurrence and widespread distribution of these nematode species on yam tubers recovered from each market surveyed in Benue State of Nigeria is described in Tables 3 and 4.

Population density of nematode species associated with yam tubers
The population densities are described in table 2 and 3. Scutellonema bradys (71.81 %) and Rotylenchulus spp and Tylenchus spp were recorded in relatively low from the tuber samples.

DISCUSSION
The survey revealed a large diversity and population of Plant-Parasitic Nematodes (PPN) found in association with the yam tubers in Benue state of Nigeria.The result agree with earlier reports by [24] in which seven species of plant-parasitic nematodes associated with yam tubers from various countries were identified.The nematode species that were reported as important pest of yam tubers include; Scutellonema, Meloidogyne, Pratylenchus, Paratrichodorous, Rotylenchulus, Radopholus and Aphelenchoides.
Nematodes like Scutellonema bradys, Meloidogyne spp and Pratylenchus spp attack yam in many countries [24].Estimates of the damaging potentials for Pratylenchus coffeae indicate that where the number of individuals in soil exceeds 600 per plant of Dioscorea rotundata [13].Yields of yams severely infested with M. arenaria can be reduced by 24-80 % in Chlna [20] while population of M. incognita as low as 100 juveniles per plant are said to reduce tuber yields of D. rotundata in India [25].In this study, survey was carried out in April when yam tubers must have been harvested and stored for a long period of time which might increase nematode species [30] as temperatures of 24-31 °C can increase yam nematode populations.Other factors that could have contributed to the widespread distribution of these nematodes on yam tubers include planting of nematode infested seed tubers and other environmental factors such as soil type as well as the presence of a suitable alternate host [4].
In Nigeria, yams usually propagate from tubers [10] and thus the risk of dissemination by this means is greater and can act as inoculum [24] for the spread.The type of soil also a determining factor for nematodes [9,28,29].The cumulative effect of the damage caused by the feeding habit of these nematodes could have a negative impact on the plant vigour thereby constituting serious impediments to the quality and yield of yams which is the ultimate to farmers [5].Due to the insidious nature of damage caused by Plant-Parasitic Nematodes (PPN), farmers and marketers are in most cases not aware of them and often times do not control nematodes on the field and store respectively thus quality decline becomes inevitable.
Awareness programmes should therefore be created to sensitize yam marketers on damage potential and the need to embark on effective management strategies on the yams in store in order to maximise market price.It is also imperative that farmers plant resistant varieties and avoid spread of PPN.Efficient regulatory measures and plant quarantine services could also help prevent the spread of such plant-parasitic nematodes from infested to uninfested farmlands.
There is a widespread distribution of plant-parasitic nematodes associated with yam tubers in Benue State of Nigeria, thus nematode control is recommended in order to improve crop quality and yield.

Fig. 1 :Fig. 2 :
Fig. 1: Map of Nigeria showing the surveyed state for yam nematodes

1 :
Micrographs of parasitic nematodes found on yam tubers in Benue state of Nigeria during a market survey in 2017 (X60) The survey was carried out in Benue State, North Central geopolitical zone of Nigeria between April and May 2017.

Table 4 : Frequency of occurrence of nematode species associated with yam tubers in the survey market of Benue State in Nigeria Nematode species Survey market Scutellonema bradys
Meloid = Meloidogyne Prat = Pratylenchus Roty = Rotylenchulus Freq.= Frequency