CHAPTER 3: EFFECTS OF A NEEM-AMENDED PRODUCT ON PLANT-PARASITIC
3.4 DISCUSSION
91
92
combinations of neem extract with 5% cow manure on the growth and yield of two potato cultivars viz. Burren and Riviera. Their findings indicated that combinations of cow manure with neem leaf extract improved the overall potato growth and yield significantly compared to the untreated control and chemical fertiliser applications (Shayaa & Hussein, 2019). The combination of cow manure with neem leaf extracts resulted in healthier crops that can naturally resist or outgrow the potential of PPN to attack their root systems (Dutta et al., 2019). Hajji and Horrigue-Raouani (2012) also observed better potato (cv. Spunta) emergence and development in the treatments containing manure and compost in Tunisia, although RKN densities were not reduced by these treatments.
A previous study investigating the nematicidal potential of neem leaf extracts and poultry manure on sweet potato in Ghana indicated that the addition of neem leaf extracts had also been successful in reducing especially LN populations significantly compared to untreated plots and plots treated with poultry manure. The nematicidal effect of the manure treatment showed variable results between different cultivars used (Osei et al., 2017). This significant reduction in LN population numbers due to the addition of neem leaves, or neem leaf extracts, is supported by several studies on various crops: Radish (Raphanus sativus L. ‘Early Scarlet Globe’), cucumber (Cucumis sativus L. ‘Marketmore 76’) and tomato in Canada (Abbasi et al., 2005), cucumber in Ghana (Agyarko & Asante, 2005), black pepper (Piper nigrum L.) in Vietnam (Hieu et al., 2014) and crossandra (Crossandra undulaefolia L.) in India (Jothi et al., 2004). A study by Khan et al.
(2015) on peach (Prunus persica (L.) Batsch) seedlings in Pakistan found that, although the neem powder reduced LN population numbers, the chemical nematicide standard carbofuran provided superior control. Ali et al. (2021) found that the use of different animal manures (chicken, sheep and cow) each increased plant growth in cucumber in Eqypt significantly while also reducing the population densities of various PPN, including LN. A study conducted by Cole et al. (2020) to control LN in potato in Michigan, United States of America (USA) had similar findings using poultry manure. A recent study by Izuogu and Usman (2019) in Nigeria showed that making a compost tea from either poultry or cow manure significantly reduced LN population densities in maize (Zea mays L.). The findings from this Deerpark study correlates to a lesser degree with the findings from these authors, in that all the treatments, including the neem-amended cow manure treatments, reduced the population densities of LN, even though only Nemacur 400 EC stand alone and the combination of all three products together showed statistically significant differences from the untreated control.
Previous research focussing on the use of neem amendments and animal manure on other nematodes, such as spiral (Hoplolaimidae) and ring (Criconematidae) nematodes showed that the use of neem leaves and especially poultry and cow manure reduced these populations
93
significantly (Agyarko & Asante, 2005; Ali et al., 2021; Khan et al., 2015; Osei et al., 2017), although this was not observed in this study. Although the population densities showed a slight decline over time, this could not be attributed directly to the application of the neem-amended cow manure granules. It must be kept in consideration that the nematode numbers of these nematodes were generally low, compared to what is known and experienced in potato fields in South Africa (Jones et al., 2017), throughout the trial period, making it more difficult to demonstrate significant differences.
The lack of effect by the neem-amended cow manure granule on the number of PPN eggs produced contradicts those by Hayat et al. (2012) in Pakistan who found that potato plants grown in soil amended with ground neem seeds produced significantly less RKN eggs than the untreated control and other soil amendments. Controversy in results from the Deerpark study and the Pakistan study is difficult to explain but severe drought conditions experienced in the trial site are most probably the greatest contributor towards the inferior performance of the tried and tested synthetically derived nematicides. Such nematicides are known to be effective in reducing PPN in local potato fields (Sesweni, 2016), with the same explanation being proposed for the Kalahari 3:1 product performing non-optimal in this study.
Results from this study, however, strongly correlate with the review of different RKN management strategies by Collange et al. (2011) in different vegetable crops indicating that the application of organic amendments, including neem extracts in larger field studies, have shown variable or inconclusive results with substantial variation existing between the studies referred to. Except for severe drought conditions, another possibility is related to the product itself in that the concentration of neem extract in the cow manure granules used was too low and therefore not sufficient to effectively reduce RKN and other PPN numbers. It might thus be interesting to investigate a different formulation with a higher concentration of the neem extract. Another possible explanation why the Kalahari 3:1 treatment was not effective in the present study could be, as in previous research, neem leaves, instead of a leaf extract and raw (ungranulated) manure, provided better and more consistent results (Abbasi et al., 2005; Agyarko & Asante, 2005; Hieu et al., 2014; Jothi et al., 2004; Khan et al., 2015; Osei et al., 2017; Shayaa & Hussein, 2019). It is important to then also again accentuate that the drought might have had a considerable effect on results obtained in this trial since a lack of sufficient soil moisture puts considerable strain on plant growth, plant vigour, nematode survival and population development.
A severe drought was experienced in the area from the summer of 2014/2015 up until early in the 2019/2020 rainy season with rainfall averaging between 50 and 75% of the long-term figures. The average rainfall on the farm where the trial was conducted before the drought was approximately 1 100 mm per season (personal records of Pieter Buys, farm owner, dating back to the 2008/2009
94
season). The rainfall records from 2008 are shown in Figure 3.5 and indicate the rainfall per month, per season, of Nyalani Estates, the farm on which the trial was conducted. Plant-parasitic nematodes are aquatic animals that require sufficient levels of soil water to survive and reproduce (Perry. 1996), meaning their survival and motility was likely limited to the areas in the soil that received drip irrigation, which could not be optimally applied due to the severe drought and low levels of water in the cement storage dam on the farm situated about 900m away from the trial site. Water for agriculture was restricted with household use receiving priority from the start of 2019 until the start of 2021.
In essence, the use of drip irrigation would concentrate the PPN in the immediate crop root zone.
However, with restricted irrigation being allowed for agricultural purposes combined with reduced rainfall, the adverse effect on the efficacy of the products tested was expected, as the performance of the products was not optimal in this trial. It is known that drought impact adversely on the efficacy of nematicides as they must be transported and dissolved in this aqueous medium to ensure optimal efficacy (Jones et al., 2017). Riekert (1996) explicitly mentioned that drought periods impacted adversely on nematicide performance in maize trials. The water supplied by irrigation at the Deerpark trial site during this study was firstly not sufficient to bring the soil water back to capacity and could have played a role in the spread of the products around the crop root zone. This could also have played a role in the ability of the plants to take up the fertiliser component of the Kalahari 3:1 treatments. The reduced amount of rainfall between 2014 - 2020 is clearly shown in Figure 3.5, suggesting that one of the reasons for the poor performance in this trial was due to lack of sufficient moisture. The temperature records for the Tzaneen area, shown in Figure 3.6, only show two abnormal spikes in the average maximum temperatures (approximately March 2014 and again between December 2015 – January 2016). This suggests that temperature did not play as big a role in the results of this trial as the amount of rainfall experienced.
Moreover, except for the proposed negative effect of the drought and potential insufficient content of the neem and manure ingredients in Kalahari 3:1 (to optimally act as a nematicide), the RKN M. enterolobii, occurring at the site in combination with M. javanica, is known to be more virulent than other RKN species (Castagnone-Sereno, 2012), which may have been an additional contributing factor to the poor performance of the products in this trial, while some populations of M. javanica are also known to be quite virulent (Ornat et al., 2007), which could also have influenced the results obtained.
95