The efficacy of abamectin in reducing plant-parasitic nematodes in cotton
De Beer, Ernst Friedrich Ludwig
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Cotton production is worldwide hampered by infection of various pests and diseases, including plant-parasitic nematodes (PPN). Root-knot nematodes (RKN), in particular Meloidogyne incognita race 4 is the predominant nematode species and race that adversely affects the production of cotton in South Africa and thus result in substantial yield losses. Management strategies that are frequently used to minimize yield losses in cotton locally are limited to a few registered nematicides and to a lesser extent, crop rotation. In addition, no resistant cotton cultivars are available that are adapted to local climatic conditions. The main objective of this study was to evaluate the efficacy of abamectin against PPN, particularly M. incognita race 4, in greenhouse and field trials. The host suitability of four cultivars Delta OPAL®, Nu OPAL®, Nu OPAL RR® and Delta OPAL RR® that were used in the greenhouse trial were concurrently also evaluated against M. incognita race 4. To conduct this study, mature RKN females that were present in roots of tomato (cv. Rodade) and produced egg masses from which eggs and J2 (used as inoculum source for the greenhouse trial) were identified using Deoxyribonucleic Acid (DNA)-based techniques. The same procedure was followed for females that were present in roots of cotton cultivars that were planted in field trials. With regard to the greenhouse trial, two treatments namely abamectin at a dosage rate of 0.15mg a.i./seed as well as a non-abamectin (untreated control) treatment were used. Approximately 2 500 M. incognita race 4 eggs and J2 were inoculated per cotton seed at planting for four cultivars (Delta OPAL ®, Nu OPAL®, Nu OPAL RR® and Delta OPAL RR®) that were used. Nematode parameters viz. numbers of eggs and J2, egg masses and galls per root system as well as egg-laying female (ELF) indices and reproduction (Rf) values were obtained during five sampling intervals. These intervals represented the major growth stages of cotton plants, namely first true leaf, square, flower and boll development as well as when 50% of the bolls were opening. In addition, root mass (g) and biomass (g) data per cotton plant were also obtained. The trial layout was a randomized complete split-plot design including the two treatments, five sampling intervals and the four cultivars, which were replicated six times. Nematode and plant growth stage data were subjected to a factorial analysis of variance (ANOVA) with treatments as factor 1, sampling intervals as factor 2 and cultivars as factor 3. Means were separated by the Tukey Test and degrees of freedom (error) > 18 were always pursued. Nematode data for nematode parameters (dependent variables) were non-linearly regressed on the various sampling intervals (independent variable) using polynomial models (Genstat for Windows), while plant data (dependent variable) were linearly regressed using the sampling intervals (the independent variable). The RKN species and race that was used as inoculum for the greenhouse trial proved to be M. incognita race 4 using the specific sequenced characterized amplified region (SCAR)-polymerase chain reaction (PCR) method. Although the 0.15mg abamectin a.s./seed treatment resulted in a significant (P ≤ 0.05) reduction in M. incognita race 4 population levels in roots of the four cultivars, these levels were still relatively high. Significant differences (P ≤ 0.05) were also evident among the sampling intervals for both the abamectin and non-abamectin treatments with regard to all nematode and plant growth parameters. Further, the four cultivars were identified as susceptible hosts to this RKN species and race and generally had similar non-linear regression lines for the non-abamectin treatment in terms of M. incognita race 4-population development during the duration of the trial. These cultivars did, however, differ significantly (P ≤ 0.05) from each other in terms of particularly the eggs and J2/root system only for the abamectin treatment when data were pooled for the five sampling intervals. Cultivar Nu OPAL® maintained significant (P ≤ 0.05) higher egg and J2 numbers/root system than those maintained by the other three cultivars. This cultivar was, however, still classified as being highly susceptible (like the other three cultivars) to M. incognita race 4 using Rf values. For the latter as well as other nematode parameters, namely egg mass and gall numbers/root system as well as ELF indices significant (P ≤ 0.05) differences were only evident between the two treatments and the five sampling intervals, but not for the cultivars. With regard to interaction data those that were significant (P ≤ 0.05) between the two treatments and five sampling intervals for all the nematode and plant parameters, were regarded as the most important. This indicated that the treatments reacted differently during these intervals for all parameters measured. Since this trial was conducted in a greenhouse under controlled conditions, nematode and plant growth data obtained should be verified in field trials throughout the cotton-producing areas of South Africa under natural occurring environmental conditions. Only then can final conclusions be made in this regard. For evaluation and verification of the efficacy of abamectin as a seed treatment in reducing PPN populations particularly M. incognita race 4, field trials were conducted at five sites where cotton was commercially grown during the 2005/2006 and 2006/2007 growing seasons. Four trials were conducted at three sites that are located in commercially-grown cotton fields in the Marble Hall area (Limpopo Province), while the other trial was done in the Vaalharts area near Jan Kempdorp (Northern Cape Province). For abamectin, two dosage treatments, namely 0.15mg a.s./seed and 0.30mg a.s./seed were used in all field trials. Standard treatments included were the classical nematicides aldicarb and fenamiphos that are registered on cotton in South Africa. An untreated control as well as a thiamethoxam 0.3mg a.s./seed treatment were also included for the 2005/2006 trials. In addition to these treatments, a seventh treatment containing abamectin 0.15mg a.s./seed + thiamethoxam 0.3mg a.s./seed was included during the 2006/2007 season. Cotton seed used to plant trials during the 2005/2006 season were those for cultivar Nu OPAL®, while Nu OPAL RR® was used during the 2006/2007 season. Trial layouts for all trials constituted a randomized complete block design with nine and six replicates during the 2005/2006 and 2006/2007 growing seasons, respectively. Both root and soil samples were taken for nematode extraction, counts and identification purposes from the outer two rows of each plot at 42 as well as 84 days after planting (DAP), except when excessive rainfall occurred. Nematode and yield data for all trials were subjected to analyses of variance (ANOVA). For yield estimation, cotton lint was also harvested for all trials, weighed and subjected to ANOVAS. Meloidogyne incognita race 4 has been identified as the predominant PPN species and race being present at all trial sites, while low population levels of individuals from the Hoplolaimidae, Criconema spp., Pratylenchus spp. and Paratrichodorus spp. were also present. The standard nematicide treatments aldicarb and fenamiphos generally resulted in the lowest number of M. incognita race 4 eggs and J2/root system in all trials and differed significantly from those for the untreated control treatments for three trials. The 0.15mg abamectin dosage treatment in particular did generally not differ significantly (P ≤ 0.05) from the untreated control treatments nor from the standard nematicide and the thiamethoxam 0.3mg treatment as well as for the abamectin 0.15mg a.s./seed + thiamethoxam 0.3mg treatment during sampling interval one for two of the trials and during sampling intervals one and two for the other. Yield for the abamectin 0.15mg a.s./seed treatment was significantly (P ≤ 0.05) higher than that of the untreated control only for Trial A. In terms of the cost-effectiveness, the estimated cost of the 0.15mg abamectin a.s./seed treatment was calculated to be substantially lower than those for the two standard nematicide treatments for the latter trial. This scenario poses a potential benefit for producers when this abamectin dosage will be used. Although the 0.15mg abamectin dosage treatment showed potential to reduce population levels of M. incognita race 4 during this study, data varied between trials and seasons for the field trials. It must, however, be emphasised that since M. incognita race 4 populations in roots of abamectin-treated cotton plants were comparable to those for the standard nematicides as well as those of the untreated controls, additional management strategies should be used in combination with the abamectin treatment. It further accentuates that abamectin should preferably be used only where population levels of M. incognita race 4 are not particularly high.