CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW
1.4 MANAGEMENT STRATEGIES
1.4.2 Alternative strategies for management of plant-parasitic nematodes
Biological control of agro-economically important nematodes refers to the use of biological agents, such as bacteria or fungi (natural parasites/predators) that share the rhizosphere with PPN and are constantly interacting, to reduce the negative effects that nematodes have on the cultivated crops (Strom et al., 2020; Varandas et al., 2020). This type of control has recently enjoyed a lot of attention. Some of the strategies that have been studied and show potential for use include:
1. Predatory nematodes (Dropkin, 1980b).
2. Soil arthropods that feed on nematodes – e.g. mites (Azevedo et al., 2020).
3. Nematophagous fungi and some bacteria species – e.g. Pochonia chlamydosporia (Goddard) Zare & W. Gams, 2001, Trichoderma harzianum Rifai, 1969, Purpureocillium lilacinus (Thom) Samson, 1974, Pasteuria Metchnikoff, 1888, Clostridium Prazmowski, 1880, Serratia Bizio, 1823 (Mazzuchelli et al., 2020; Peiris et al., 2020; Varandas et al., 2020).
Bionematicides
A bionematicide can be defined as a nematicide of biological origin (Glare et al., 2012). Naturally occurring compounds with nematicidal activity can be found in plant extracts (alkaloids, glucosinolates, etc.) and residues or by-products derived from roots, fruits skin, bark, leaves, stems, seeds and flowers (Chelinho et al., 2017; Pino-Otin et al., 2019;). Examples of plant extracts used as bionematicides include Cucurbitacin A and B which can be extracted from e.g.
African wild cucumber (Cucumis africanis L.) (Dube et al., 2018). Hydrolate extracts from wormwood (Artemisia absinthium L. var. Candial) (Pino-Otin et al., 2019) and leaf extracts from goat weed (Ageratum conyzoides L.) (Pavaraj et al., 2010) are also used in such formulations.
Research into bionematicides has investigated various bacteria and their metabolites for PPN control, such as various rhizobacteria, Bacillus Cohn, 1872, Pasteuria and Pseudomonas Migula, 1894 (Aballay et al., 2017; Berini et al., 2018, Horak et al., 2019). The use of Bacillus spp. for use in agriculture is of particular interest, due to its ability to control PPN, especially RKN (Abbasi et al., 2014; Perez-Garcia et al., 2011; Zhao et al., 2018). Some of the locally available registered bionematicides include Spartacus L7125 (Beauveria bassiana (Bals.-Criv.) Vuill., 1912) combined with Romulus L7124 (T. harzianum isolate DB104) on carrot (Daucus carota L.) and Mytech WP L9077 (P. lilacinus) for use on various flowers and vegetables (www.biologicalcrophealth.co.za).
Studies conducted using Trichoderma and Bacillus isolates to control PPN on soybean (Glycine max (L.) Merr.) and tomato in South Africa showed potential as bionematicides (Chinheya et al., 2017; Engelbrecht et al., 2018). Bayer (Pty) Ltd also produced a bionematicide, namely BioAct,
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containing Purpureocillium lilacinum (Paecilomyces lilacinus) 251, but it is currently not available in South Africa.
1.4.2.2 Soil amendments, cover and green manure crops /organic amendments
One of the earliest methods suggested to assist in nematode control, was to ensure that the organic contents in the soils were high. This was usually achieved using green manure (working plant residues back into the soil after harvest), improved organic manures and soil amendments obtained from seed oils and oilseed cakes (Dutta et al., 2019; Mojumder, 2000). Soil amendments most likely only improve the soil structure and quality, reducing stress on the plants, although some do exhibit antinematocidal properties (Abdelnabby, 2012; Khalil, 2013). This ultimately leads to healthier crops that naturally resist or outgrow the potential of PPN to attack their root systems while soil amendments do not function as a host, as some cover crops would (Dutta et al., 2019).
Some organic amendments with antinematodal properties include components found in the neem plant, Azadirachta indica A. Juss., whether used as green manure, seed oil cake or neem oil extract (Campos et al., 2016; Kosma et al., 2011; Lokanadhan et al., 2012). Little information on the nematicidal effect of neem in terms of nematode control is however available for potato cropping systems. Two studies investigating the use of animal manure and/or neem leaves to control PPN and improve potato yields have shown these strategies to be very promising (Rossi et al., 2021; Shayaa & Hussein, 2019). A study by Hajji and Horrigue-Raouani (2012) also showed improved potato growth and yields, although the treatments did not affect the RKN densities. In potato, the use of manures and neem amendments have yielded inconsistent and variable results (Kimpinski et al., 2003). Successful suppression of nematode populations with a variety of neem amended products and formulations were however observed in various controlled studies on different vegetable crops (Kumar & Khanna, 2006; Singh & Hali, 2017). Field trials mostly show variable and inconclusive results, although two studies have shown that poultry and cattle manure amended with neem extract show significant suppression of RKN populations with an increase in crop production of rice and okra (Abelmoschus esculentus (L.) Moench) (Auwal et al., 2015;
Collange et al., 2011; Galadima et al., 2015). Neem extract contains various triterpenoids of which azadirachtin is the main compound and is extracted from the plant’s seed kernels (Khalil, 2013).
Studies have shown that azadirachtin influences insect feeding behaviour and development and is effective against many different insects. Neem products have also been shown to be effective against PPN by reducing the ability of juveniles to hatch from eggs and the mobility of juveniles (Khalil, 2013). When applied as a seed treatment, drench or root dip, tomato plants have shown significantly reduced numbers of RKN J2 and reduced development of galls compared to treatments without neem-based products (Kumar & Khanna, 2006; Singh & Hali, 2017).
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The neem-oil amended cow manure granular product, Kalahari 3:1, that was used in the field trial conducted in this study, consists of a combination of 80% cow manure, 10% lime and 10% carbon and is a mixture of Neem oil @ 125 L/ton, Cosmoroot @ 5 g/ton (200N, 87P, 166K g/kg – root stimulant), H85 @ 5 kg/ton (850g/g Humus (carbon)), Soilex @ 5 L/ton (64 g/L organic extracts, 63 g/L carboxylic acids – sodium neutraliser), Mainstay Ca @ 5 L/ton (200 g/L – 42%CaO).
Cosmoroot, Mainstay, H85 and Soilex are specialised fertiliser products produced and sold locally by Cosmocel® (https://cosmocel.co.za). The mixture and formulation of the Kalahari 3:1 granule product, which is, in essence, an organic fertiliser with a nematicidal component, is facilitated by Organic Fertilizer Manufacturers Botswana (OFMB) (https://www.organicfmb.com). This product has thus far been tested in pineapple (Ananas comosus (L.) Merr.) in KwaZulu-Natal and tomato in Botswana without conclusive results (Personal communication, Mike Hallam, Managing Director, OFMB, 2018). The composition of this product does have the potential to be a more environmentally friendly option for producers and was therefore investigated in this study for its effect on the existing PPN population at the site near Tzaneen where potato was grown.
Another strategy used to reduce PPN population densities, namely crop rotation with resistant varieties, non-host plants or incorporating fallow periods also aid in the management of other pests and diseases such as potato scab (Personal communication, Jordaan J., 2017; Pederson, 2015). Emmond and Ledingham (1972) stated that the success of crop rotation depends on using non-host plants and a short survival time of the disease-causing agent in the absence of the host crop. Resistant cultivars allow PPN to feed but will inhibit reproduction and in turn, lead to lower PPN population densities. This strategy should not be used continuously and needs to be combined with other control strategies to reduce the risk of selection for other PPN to become dominant (Dutta et al., 2019; Pederson, 2015). This is true in nematode management strategies as well, but as previously stated, PPN encountered in potato cropping systems can infest a wide range of cover and rotational crops which makes effective crop rotation more challenging (Jones et al., 2013; Onkendi et al., 2014).
In South African potato growing areas, producers tend to use different graze crops, mostly grasses, also acting as cover crops between potato plantings (Personal communication, Du Raan, C. (PSA), 2021). Many crop rotation or cover cropping strategies focussed on rotating with crops, e.g. Brassicaceae, that have biofumigant properties (Dutta et al., 2019). However, this was not the focus of this study and is therefore not discussed further. In the study forming part of this project, the field was allowed to recover naturally and the predominant vegetation that grew during the fallow period was White Buffalo grass (Urochloa mosambicensis (Hack.) Dandy. (Family:
Poaceae)) (Malan, 2018; Mashau, 2010), which is different from what most producers plant in South Africa (Personal communication, Du Raan, C. (PSA), 2021). This is a perennial grass, with
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a tufted inflorescence consisting of numerous spike-like racemes arranged alternately on a central axis (Fig. 1.13). It is stoloniferous, which is sometimes rooting and branching from its lower nodes.
It is indigenous to Africa, occurring in Kwa-Zulu Natal, spreading northwards up into East Africa.
White Buffalo grass is commonly found in disturbed areas, fallow lands and trampled ground. It is considered a palatable grass with average leaf production and is favoured by cattle for grazing.
This plant is considered a good indicator of disturbed areas, drought-resistant, preferring sandy or loam soils and summer rainfall (Malan, 2018; Mashau, 2010). A benefit of using White Buffalo grass as a cover crop is that it is reported to have shown a slight degree of galling (<10% of the root system) due to infection by M. javanica when grown in combination with Desmodium ovalifolum (Guill. & Perr.) in Brazil (Lenné, 1981). No information could however be found regarding the host status of this grass to other Meloidogyne spp.
Figure 1.13: White Buffalo grass (Urochloa mosambicensis (Hack.) Dandy (http://pza.sanbi.org/urochloa-mosambicensis).
Practices followed by producers can vary greatly depending on their location, resources, disease management needs, whether they are planting table or seed potato and more (Steyn et al., 2016;
Van der Waals et al., 2016). A standard 4-year rotation is followed by most producers in South Africa, where the potato crop will be rotated with maize or soybean (Glycine max L.) and then planted with a grass crop for a year (resting year for the soil) before the next potato planting. This strategy is implemented frequently in the Mpumalanga province. Variations do occur depending on the needs of the producer (Personal communication, Riley, R., Agronomist, 2020). In the irrigation areas of the North-West and Free State provinces, producers follow a similar rotation, but would use rotation crops such as sunflower (Helianthus annum L.), onion (Allium cepa L.) and sometimes alfalfa (Medicago sativa L.) (Personal communication, Wethmar, J., Agronomist,
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2020). Producers in the Sandveld area rotate with cereal crops or onion and most producers follow longer rotation cycles, with one producer planting potato for 4 years followed by onion and cereals (oat: Avena sativa L.) for the next 4 years before planting potato again (Personal communication, Baloyi, A., Agronomist, 2020). These crop rotation cycles are generally not only implemented for PPN management, but also to reduce pressure from other diseases such as potato common scab (Streptomyces scabies, Lambert & Loria) and to rest the soil between plantings (N’Dayegamiye et al., 2017; Wright et al., 2016). In the Limpopo province, the crop rotation cycles differ greatly between the different climatic zones. In the Dendron-Polokwane area, producers follow a 3-5-year rotation. Most producers plant cover crops used for cattle grazing until the next potato planting, while others include an onion planting after the potato crop and a fallow year before planting potato again. Producers located in the warmer Bushveld of Limpopo have different strategies, although table/process potato farmers follow a 5-year rotation and seed potato producers follow a 7-year rotation. Large scale producers tend to plant onion and feed or fallow until the next planting, while smaller producers rotate potato with onion or a bean (Fabaceae) variety and then plant a sequence of vegetable crops before leaving the soil naturally fallow before the next planting. This is done to supplement cash flow in most cases (Personal communications: Van Zyl, A., Area Manager, 2020 & Jordaan, J., Producer, 2017).
1.4.2.3 Other strategies
Other non-chemical strategies that can be implemented by local potato producers include soil solarization and trap crops.
• Soil solarization:
- Clear polyethylene sheets are placed over moist soil (summer) to increase topsoil temperature (>50 °C) (Dutta et al., 2019).
- This practice is not practical or cost-effective for large-scale/commercial producers. In addition, it is expensive and weather conditions could counteract sterilisation effects, while pathogenic and beneficial microbes will be killed (Dutta et al., 2019).
• Trap crops:
- Suitable for small-scale crop production
- A trap crop is destroyed after J2 infection, before females can lay eggs (Sasanelli et al., 2021).
• Sanitation practices:
- Prevent and reduce the spread of PPN to other fields.
- Clean equipment and shoes are prerequisites for this practice to be effective, while soil must be removed from vehicles and implements before moving from one field to the next.
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- Plant certified clean planting material (Dufour et al, 2003; Sasanelli et al., 2021).
• Plastic mulching:
- Red plastic mulch promotes growth above ground, limiting food production for PPN in the soil.
- Similar effect to that of soil solarization expected.
- Can be expensive and labour intensive to install, especially on a large scale (Dufour et al., 2003).
It is essential to properly integrate the existing and new knowledge obtained from research (from identification of nematodes to new agricultural tools and strategies) in the application of management strategies. These strategies ensure the sustainable and effective reduction of PPN population densities. This was attempted in the study, firstly by accurately identifying the RKN species dominant in the field (Chapter 2) and secondly by evaluating the effect of Kalahari 3:1 against PPN populations (Chapter 3) as well as the effect of this product and grass fallow on the beneficial soil nematodes present in the field (Chapter 4).