Plant-parasitic nematodes associated with sweet potato rhizosphere soil in the Semi- Deciduous Forest and Coastal Savannah Zones of Ghana

A survey was conducted in nine major sweet potato producing districts across the semideciduous forest and coastal savannah zones of Ghana to determine the prevalence of plantparasitic nematodes parasitizing the crop. Soil samples were collected at 90-days after planting from the rhizosphere of sweet potato crop and analysed using Modified Baermann tray method from 100 farms across the study area. Seven plant-parasitic nematode genera were extracted from soil samples collected and morphologically identified under a microscope with four of them, namely Meloidogyne, Pratylenchus, Rotylenchulus, and Helicotylenchus being the most prevalent. Scutellonema sp. occurred in 89% while Tylenchus sp. occurred in 33% of the districts sampled. The ring nematode, Criconemella sp. was found in only two of the nine districts covered; Ketu North and Akatsi South which incidentally recorded 100% of the seven nematodes encountered in the survey. The abundant nematode was Meloidogyne sp. which represented 39% and Criconemella, the least (0.1%) of the total nematodes recovered in the survey. This study has shown that high diversity, incidence and density of economically important plant-parasitic nematodes are associated with sweet potato crop. Development of appropriate management strategies to mitigate the negative effects of plant-parasitic nematodes on sweet potato is recommended.


Introduction
Sweet potato (Ipomoea batatas L. (Lam.) is a major staple crop recognized for its contribution to food security, particularly in developing countries where hunger and malnutrition are high (Makini et al., 2017). It is a preferred crop cultivated throughout the tropical and sub-tropical regions due to its wide adaptability to a wide range of environmental conditions and low cost of production (Woolfe, 1992). Due to its high yielding, nutritional and food security potential, sweet potato production has increased in Ghana from 89,600 to 145,886 tons between 199689,600 to 145,886 tons between and 201789,600 to 145,886 tons between (FAOSTAT, 2017. The crop is grown largely for its starchy, and sweet-tasting tuberous roots which serve as food, feed or industrial crop (Odebode, 2008) and is a rich source of carbohydrates, vitamins A and C, thiamine, niacin, and riboflavin containing significant amounts of calcium and iron (Oversweet, 2009). Varieties such as the orange and yellow flesh varieties have high beta-carotene content thereby used in addressing the issue of vitamin A -deficiencies among children and women (Makini et al., 2017;Odebode, 2008). Sweet potato yield and quality are however reduced by the activities of plant-parasitic nematodes such as rootknot nematodes (Meloidogyne spp.), lesion nematodes (Pratylenchus spp.) and reniform nematodes (Rotylenchulus spp.), being a major production constraint (Olabiyi, 2007;Coyne et al., 2003;Cervantes-Flores, 2000). Apart from the direct damage caused by the feeding activities of plant-parasitic nematodes, they also provide avenues for penetration and establishment of several secondary pathogenic organisms in the roots of the crop which constrains sustainable production. Despite the importance of plant-feeding nematodes in sweet potato production, little is known about the genera composition, distribution, frequency and abundance of the pest associated with the crop in Ghana. Knowledge of these will aid in the development of sustainable management strategies against phytonemtodes parasitizing the sweet potato crop. The study was therefore aimed at determining the genera composition, frequency and abundance of plant-parasitic nematodes in sweet potato rhizosphere soil across the study area.

Sampling
Soil samples from sweet potato rhizosphere were collected from 100 farms in nine districts across different agro-ecological zones of Ghana (Table 1). All farms sampled were monoculture systems and no nematode management practices were employed by farmers. Soil samples were collected at 90-days after planting in an N-shaped pattern across each of the selected farms averaging 0.5 acres using a 15-cm soil auger. Three soil cores at a depth of 20 cm from each farm were collected and pooled into one composite sample. Each sample was then carefully placed into a cooler with Styrofoam and disposable ice packs, transported to the laboratory, stored at 4 o C for 24 hours before processing. Total 100 Plant-parasitic nematodes associated with sweet potato...

Nematode extraction and identification
Nematodes were extracted from 200 cm 3 soil subsamples using the Modified Baermann tray method (Coyne et al., 2007) for 48 hours after carefully breaking up soil lumps and sieving samples through a 1.27 cm mesh. Second stage juveniles of nematodes extracted from the different soil samples were preserved in 4% formalin for identification. Nematodes were counted from 1-ml aliquot of each sample under stereo microscope at magnification 40x. Plant-parasitic nematodes were identified to the genera level using gross body morphological features (Mai & Mullin 1996). Plant-parasitic nematodes were counted and expressed as nematodes per 200 cm 3 of soil. For each genus, percent frequency was calculated as the total number of samples in which a particular nematode genus was detected, divided by the total number of samples collected and multiplied by 100 (Osei et al., 2005) to express as a percentage. The relative abundance (RA) of the nematode genus was determined as the sum of nematodes for all samples containing that genus divided by the number of positive samples (Osei et al., 2005). The values so obtained were expressed as a decimal logarithm (log x + 1). A nematode was regarded as frequent in the soil when it was observed in at least 30% of the soil samples.

Results
Nematode diversity expressed in terms of the number of genera differed between the districts. Seven plant-parasitic nematode genera, namely Meloidogyne, Pratylenchus, Rotylenchulus, Helicotylenchus, Scutellonema, Tylenchus and Criconemella were encountered in the survey. All of them belong to the order Tylenchida and of five different families viz Meloidogynidae, Pratylenchidae, Hoplolaimidae, Tylenchidae and Criconematidae respectively. Meloidogyne, Pratylenchus, Rotylenchulus, and Helicotylenchus nematodes were the most prevalent (100 %) across the districts (Table 2) (Table 3). Meloidogyne population density (7278.8 per 200 cm 3 ) was highest (Table 3). Of all the districts, Komenda Edina Eguafo Abirem recorded the highest number of Meloidogyne individuals (Table 3). Contrary, larger populations of Rotylenchulus were recovered from both Abura Asebu Kwamankese and Fanteakwa North districts in coastal savannah and semideciduous forest zones respectively (Table  3). Criconemella spp, did not only record the lowest incidence ( Fig.1) but also was the least abundant (Fig. 2). Generally, Meloidogyne spp. were the most abundant across the districts except in Fanteakwa North and Abura Asebu Kwamankese where Rotylenchulus nematodes were the most abundant recording 47 and 33 % respectively (Fig. 2). In three districts; Upper Manya Krobo, Akatsi South and Ketu South, Meloidogyne spp. relative abundance was more than 50 % (Fig. 2).

Figures in brackets are (log x + 1) transformed data
Discussion Seven plant-parasitic nematodes genera of the order Tylenchida and belonging to five different families were identified in some major sweet potato growing districts of the semideciduous forest and coastal savannah zones of Ghana. Nematodes of the order Tylenchida are considered the most important nematode pests of cultivated plants. The nematodes identified this study are economically important pathogens reported to be associated with sweet potato production (Karuri et al., 2016;Olabiyi et al., 2016;Haougui et al., 2011) in several producing regions globally. The production constraint and losses caused by the activities of plant-parasitic nematodes on sweet potato production may exacerbate due to the limited information, ineffective nematodes management strategies employed by farmers and the synergistic interaction between plant nematodes and other microorganisms. Meloidogyne, Rotylenchulus, Pratylenchus and Helicotylenchus spp. were the most prevalent nematode pests with high incidence, population densities and abundance in all surveyed farms in the districts. Meloidogyne spp. are a globally distributed pest consisting of more than 126 species (Moens et al., 2009). Of these, M. incog nita, M. indica, M. javanica, M. hapla, M. gramini cola, M. arenaria, and M. triiticoryzae are considered the most destructive and devastating threat to crop production (Mangala & Mauria, 2006;Jones et al. 2013;Postnikova et al., 2015). Root-knot nematodes are serious pests of sweet potato and cause high economic damage through the penetration of roots by the infective juveniles. Their penetrative activities lead to gall formation on roots of the plant leading to a reduced uptake of water and nutrients leading to stunted growth, leaf chlorosis, abnormal flower production and low yields. The extent of damage on crops has been found to depend on factors such as the initial population density of the pest and susceptibility of the cultivar. High population density of Meloidogyne spp., across the districts therefore pose serious threat to sweet potato production as continuous cultivation of susceptible cultivars may cause higher yield loss to farmers.
The association and parasitism of the reniform nematode, Rotylenchulus spp. on sweet potato have been reported under both field and screen house studies (Montasser et al., 2019;Karuri et al., 2016). This nematode species unlike Meloidogyne, reduces yield in sweet potato through pruning of roots leading to reduced absorption of nutrients and water. Again, their activities reduce host quality by causing deeper root cracks leading to low storage quality of sweet potato (Henn, 2006). Understanding the interaction between nematode species is key for effective management. It was observed that at both Abura Asebu Kwamankese and Fanteakwa North districts, high reniform nematodes population led to reduction in population of Meloidogyne spp. Osman et al. (2012) reported a reduction in population parameters of root-knot nematodes in the presence of R. reniformis. The spiral nematodes, Helicotylenchus spp, are common plant-feeding nematodes found in association with several crops including sweet potato (Karuri et al., 2016). Their feeding activities lead to the formation of necrotic lesions in the root which under severe conditions coalesce leading to root necrosis and die-back. Lesion nematodes, Pratylenchus spp., are economically important pests to several crops. Its recovery from sweet potato rhizosphere soil in the current study is in line with previous work which identified P. coffeae, P. brachyurus and P. goodeyi (Niere & Karuri, 2018;Coyne et al., 2003) as common and important pest of sweet potato production. In the presence of Meloidogyne, these nematodes, according to Agu (2004), retard leaf growth in sweet potato and cause lesions on both feeder and storage roots. Other nematodes genera such as Scutellonema, Tylenchus and Criconemella spp. recovered from the sweet potato rhizosphere soil in the current study have all been found and reported to be associated with the crop (Kurari et al., 2016Coyne et al., 2003. The physical environment should not be compromised in attempts to devise management strategies to curb the menace of nematodes in sweet potato production in Ghana.

Conclusion
Based on the results of this study, sweet potato farms across the districts presented high diversity, incidence and abundance of economically important plant-parasitic nematodes. The nematode abundance reported, however, varied due to factors such as edaphic and climatic conditions. Notwithstanding these, the high frequency and abundance of plant-feeding nematodes in the districts deserve the attention of crop protectionists and plant breeders to develop sustainable management strategies and practices against the identified pests to improve yield and enhance livelihood.