Impact of low-cost management techniques on population dynamics of plant-parasitic nematodes in sweet potato

Highlights

• Forty-seven nematode genera were identified.

• Goat manure had the most pronounced effect on PPN.

• There were differences in some metabolic footprints, ecological and functional indices during LR and SR.

Abstract

Sweet potato is an important food security crop but its production is limited by various biotic constraints including plant-parasitic nematodes (PPN). In Kenya, current PPN management practices in sweet potato have several limitations hence the need for alternative low-cost management strategies. This study evaluated the impact of intercropping maize and sweet potato (MS) and application of Tithonia diversifolia (MG), cow (CM) and goat manure (GM) on population dynamics of PPN and the effect on metabolic footprints, ecological and functional indices. Field experiments were established in a randomized complete block design involving the four treatments and unamended controls. Soil samples were collected during long (LR) and short (SR) rains seasons. Forty-seven nematode genera were identified in both seasons. Principle response curves analysis revealed that goat manure had the most pronounced effect on PPN. There were differences in metabolic footprints, ecological and functional indices during LR and SR. In CM plots, predator footprint was high during the long rains season. Functional metabolic footprints categorized all plots as degraded in both seasons except MS which was structured in SR. However, CM bordered a structured ecosystem in both seasons while GM bordered a structured ecosystem in LR. Goat manure may have enhanced the natural ability of soil to regulate PPN affecting sweet potato and it may provide an alternative sustainable method of PPN control for smallholders.

Introduction

Sweet potato (Ipomoea batatas) is a preferred food security crop in sub-Saharan Africa because it is drought tolerant, performs well under unfavorable conditions and requires little or no management inputs compared to other crops (Woolfe, 1992; Kaguongo et al., 2012; Stathers et al., 2013). In Kenya, sweet potato is mainly cultivated by smallholder farmers in Central, Rift Valley, Nyanza, Western and Coast Provinces with intensive production in Nyanza and Western regions (Kaguongo et al., 2012). The entire sweet potato plant has varied uses such as human food, feed for livestock and as industrial raw material (Githunguri and Migwa, 2007; Claessens et al., 2008; Duvernay et al., 2013). Sweet potato yields in Kenya have steadily declined from 125,050 hg ha^-1 in 2014 to 94,220 hg ha^-1 in 2017 (FAOSTAT, 2019). Production is limited by various biotic constraints including weevils, aphids, and nematodes (Kivuva et al., 2014; Echodu et al., 2019).

Nematodes are distributed in all environments as both parasites and free-living organisms, and they influence crop production in different ways. Free-living nematodes play crucial roles such as decomposition, mineralization of organic materials and regulation of parasitic nematodes (Xiao et al., 2010; Neher et al., 2012; Ferris et al., 2012a). In addition, they are useful bioindicators of soil health status in agro-ecosystems (Bongers, 1990; Neher, 2001). Free-living nematodes occupy various trophic levels and are classified as bacterivores, fungivores, omnivores and predators based on their feeding habits (Yeates et al., 1993). Plant-parasitic nematodes (PPN) on the other hand negatively affect crop productivity. Worldwide, PPN cause an annual estimated yield loss of up to $118 billion in many crops (Atkinson et al., 2011).

Sweet potato is characterized by three distinct growth phases with variations in the time for each phase based on the cultivar (Woolfe, 1992). Different parasitic nematodes affect sweet potato at particular growth stages. For instance, the reniform nematode, Rotylenchulus reniformis attacks sweet potato during the first two stages and reduces the root system through a “pruning” effect (Clark and Wright, 1983) while Meloidogyne incognita affects the second and third growth stages (Lawrence et al., 1986; Agu, 2004). Concomitant infection by multiple PPN species cause varied responses in sweet potato (Thomas and Clark, 1983; Agu, 2006). Root galls, necrosis and root cracking are key symptoms of sweet potato infected by parasitic nematodes (Coyne et al., 2014). Root galls limit absorption and transport of nutrients and water while cracking and necrosis reduce quality and marketability of storage root (Lawrence et al., 1986). The infection sites create entry points for secondary pathogens such as fungi and bacteria which further lowers yields (Cervantes-Flores et al., 2008; Nicol et al., 2011). Economically important PPN associated with sweet potato include Meloidogyne, Pratylenchus, Rotylenchulus and Ditylenchus species (Niere and Karuri, 2018). Most PPN are controlled using nematicides and soil fumigants. Despite these chemicals being fast-acting and effective (Adegbite and Agbaje, 2007; Dubey and Trivedi, 2011; Abbas et al., 2015), their use is limited due to their negative effects on the environment, non-target organisms, human and animal health (Udo and Ugwuoke, 2010; Renco and Kovacik, 2012). Other nematode management strategies include use of resistant cultivars, crop rotation, use of cover crops, soil solarization and use of microorganisms which also have their own limitations (Mcsorley, 2011; Hajji and Horrigue-Raouni, 2012; Suzuki et al., 2012; Gregory et al., 2017). Resistant cultivars are not available for most crops and those that exist are nematode species-specific and hence not suitable for PPN control in agro-ecosystems with multiple species (Hockland et al., 2012; Briar et al., 2016). Crop rotation is complex in its design and implementation especially where many nematode species are present. Soil solarization on the other hand is expensive and hence inapplicable in large scale farming (Briar et al., 2016).

Use of organic amendments presents a promising strategy which not only increases overall yields and improves plant growth and soil parameters, but also control soil borne pests and diseases including parasitic nematodes (Akhtar and Malik, 2000; Oka, 2010; Renco et al., 2011). Commonly used organic amendments with suppressive effects on PPN include animal manure, green manure and composted materials (Kimenju et al., 2008; Rivera and Aballay, 2008; Hu and Qi, 2010; Renco et al., 2011; Renco, 2013; Amulu and Adekunle, 2015; Osunlola and Fawole, 2015). A decrease in PPN which results from application of different organic amendments is associated with production of nematicidal compounds such as ammonia, different secondary metabolites and organic acids (Thoden et al., 2011). However, nematicidal activity depends on physico-chemical parameters of soil such as pH and temperature (Oka, 2010). In addition, to the toxic effect of these materials on PPN, the natural ability of soil to regulate PPN is increased by addition of organic matter due to the increase in predatory and omnivorous free-living nematodes and the general improvement of the soil food web (Sánchez-Moreno and Ferris, 2018). Effectiveness of organic amendments on PPN infecting sweet potato in Kenya has not been evaluated. We sought to determine the effect of intercropping sweet potato and maize, application of Tithonia diversifolia, cow and goat manure on PPN abundance, nematode metabolic footprints, ecological and functional indices.