Conventional Orchards Research Articles

Aulacaspis tubercularis1Newstead in Mango Orchards of Nayarit, Mexico, and Relationship with Environmental and Agronomic Factors

White mango scale, Aulacaspis tubercularis Newstead, was sampled to better understand its biology and fluctuation in abundance in relationship to environmental variables and agronomic management in organic and conventional orchards of mango (Mangifera indica L.) in ‘El Llano’ and ‘Aticama’, Nayarit, Mexico. Sampling was done from January to June 2010 and 2011. White mango scale males group in colonies in the lower canopy of trees (P = 0.0007), while females are isolated and more homogeneously distributed (P = 0.2970) in the whole plant, with a tendency to migrate to the lower canopy when the temperature warms. Females are most abundant between 18 and 22°C and 73 and 78% relative humidity, while males reach maximum abundance at temperatures between 25 and 28°C and 66 and 71% relative humidity. As peak abundance was reached, the proportion of male colonies was more than 20 per individual female. This was more evident in 2011, a year of drought, unlike 2010 that was wet and maintained an average of 0.98 male colony per female. Management practices in organic and conventional plantations of mango influenced the abundance of the insect pest. More abundant females (3.25 per leaf) coincided with the use of humus in organically managed plantations. Tree pruning significantly decreased females from 1.8813 to 0.6456 per leaf (P = 0.0253), but an increase of 0.1750 to 0.4177 male colonies per leaf was observed, although the difference was not statistically significant (P = 0.0681).

Codling moth parasitism is affected by semi-natural habitats and agricultural practices at orchard and landscape levels

Pest control that results from the activity of naturally occurring parasitoids is an important service that could help reduce pesticide use. We analyzed parasitism in codling moth diapausing larvae from a total of 122 apple orchards in southeastern France during five consecutive years (2006–2010) in relation to the agronomic and land cover characteristics at both the local and landscape levels. Three species of hymenoptera parasitoids were observed, including two primary (Braconidae and Ichneumonidae) and one hyperparasitoid (Perilampidae) wasps. Parasitoid community compositions differed according to the year (in term of the relative abundance of the primary parasitoid species) and the presence of windbreaks and spontaneous hedgerows around the orchards (in terms of the relative abundance of primary vs. hyperparasitoid species). The parasitism rates were globally low in all orchards each year (<4.5% in average), but they were significantly higher in organic orchards than in conventional orchards as well as in orchards surrounded by a low proportion of conventional orchards in a 250m vicinity. These results are discussed here in terms of biocontrol enhancement and conservation.

Impact of pest control strategies on the arthropodofauna living in bird nests built in nestboxes in pear and apple orchards

Pesticide applications have a strong impact on biodiversity in agroecosystems. The present study aimed to assess the impact of pest control strategies on the arthropodofauna of Parus major nests built within nestboxes installed in orchards. Unlike many studied groups, these arthropod communities are not in direct contact with pesticide sprays (on account of their being sheltered by nestboxes) and are also unable to move away from the treated area. In this pilot study, we estimated the prevalence and the taxonomic and ecological diversities of arthropodofauna sampled in the nests and assessed the extent to which the whole and nest-specific arthropodofauna were affected by pest control strategies. Sixteen different insect and arachnid Primary Taxonomic Groups (PTGs, order level or below) were found in nests. The best represented PTGs (≥10% occurrence in years 2007 and 2008) were Psocoptera (Insecta, detritivorous/saprophagous), detritivorous/saprophagous Astigmata (Acari) and hematophagous Mesostigmata (Acari). Pest control strategies had a large impact on the prevalence of arthropods in nests, with higher proportions of nests hosting arthropods in organic orchards than in conventional orchards and with intermediate proportions in nests in Integrated Pest Management orchards. In contrast, pest control strategies had no significant effect on the composition of the arthropod communities when only nests hosting nidicolous arthropods were considered.

Comparative tree growth, phenology and fruit yield of several Japanese plum cultivars in two newly established orchards, organic and conventionally managed

The growth, phenology and fruit yield of 14 Japanese plum cultivars (Prunus salicina Lindl) were studied in two newly established experimental orchards under organic and conventional management. The experiment was conducted during 2005-2011 in the province of Seville (SW Spain), an important region of Japanese plum culture. Trunk cross-section areas (TCSA), flowering, yield and tree defoliation before winter dormancy were analysed over several years. After one year, TCSA were larger in the organically managed orchard (OMO) for most of the cultivars, in the next two years they were equal, and from the fourth year, several cultivars showed significantly larger TCSA in the conventionally managed orchard (CMO). Flowering in the conventional orchard started from 2 to 6 days before and lasted for 3 to 5 days more than in the OMO. Several cultivars produced significantly more fruit in the CMO, being the average fruit yield in the organic orchard about 72% of the conventionally managed orchard. Autumn defoliation was significantly advanced in the organic orchard, especially in cultivars highly susceptible to rust (Tranzschelia pruni spinosae), a disease not adequately controlled in the organic orchard.

Dinâmica populacional de Triozoida limbata, Costalimaita ferruginea e inimigos naturais em pomar orgânico e convencional de goiaba

Foi verificada a dinâmica populacional de Triozoida limbata (Hemiptera: Triozidae) e de Costalimaita ferruginea (Coleoptera: Chrysomelidae) e suas correlações com a população de seus inimigos naturais em pomar orgânico e pomar convencional de goiaba. Os experimentos foram realizados em dois pomares distintos de goiaba, na safra agrícola de 2010/2011. Para o monitoramento das pragas e dos inimigos naturais, foram instaladas cinco armadilhas adesivas amarelas em cada pomar. Para obter a correlação entre as densidades populacionais das pragas com os inimigos naturais, foram utilizadas análises de correlação linear de Pearson (SAS). A densidade populacional de T. limbata manteve-se baixa, com pico no mês de outubro, em pomar orgânico de goiaba. A principal praga no pomar convencional de goiaba foi T. limbata, com diversos picos populacionais. Os danos ocasionados por T. limbata, em folhas novas da goiabeira, foram mais pronunciados no pomar convencional. Houve baixa densidade populacional de C. ferruginea em ambos os pomares, porém com um pico populacional no mês de outubro, no pomar orgânico. A espécie C. ferruginea ocasionou danos de maiores proporções em folhas novas de goiabeira, no pomar orgânico. A maior densidade populacional de inimigos naturais foi constatada em pomar orgânico de goiaba, que apresentou correlação positiva entre T. limbata e o coccinelídeo predador Scymnus spp.

Parasitoid wasp diversity in apple orchards along a pest-management gradient

This study examined the impact of pest-management practices on Hymenopteran parasitoid diversity and assemblage composition in six apple orchards in southeastern Michigan, USA. The orchards comprised a gradient of pest-management intensity from organic to IPM to conventional practices. We used a pesticide toxicity index to quantify monthly and seasonal pest-management intensity in each orchard and conducted monthly vacuum-sampling of wasps during summer of 2009.Monthly toxicity scores predicted wasp abundance, setting an upper bound on the number of wasps in the orchard, but did not predict species richness. Total species richness was significantly higher in the organic orchard than in all others, but in August a conventional orchard had the highest wasp abundance and species richness. These results suggest that a toxicity index could guide pesticide choices or application times so as to increase parasitoid wasp populations, which have untapped potential for pest management in apple orchards.

Wild pollination services to California almond rely on semi‐natural habitat

Summary1. Global declines in honeybees have led to concerns about negative impacts on food production because of low levels of pollination. This is exemplified in California where the demand for honeybees Apis mellifera to pollinate almond Prunus dulcis is increasing, but problems with honeybee health suggest it may not be sustainable to rely solely on the pollination service of a single species.2. We investigated the effect of the quantity of surrounding natural habitat, organic management and strips of semi‐natural vegetation on flower visitation frequency of wild and managed pollinators and fruit set in 23 California almond orchards (15 conventional, 8 organic). Five conventional and four organic orchards were surrounded by a low percentage (&lt;5%) of natural or semi‐natural habitat in a 1‐km radius and another five conventional and four organic orchards were surrounded by a high percentage of these habitats (&gt;30%). A further five conventional orchards with a low percentage of surrounding natural habitat had an adjacent strip of semi‐natural vegetation and were included in the study to represent a realistic option for orchard management in intensive agricultural landscapes.3. Wild bee species visited almond flowers but only in orchards with adjacent semi‐natural habitat or vegetation strips. Organic management increased the flower visitation frequencies of hover flies and wild bees. The presence of a strip of semi‐natural vegetation in orchards with a low percentage of surrounding natural habitat increased the number of species and the flower visitation frequency by wild pollinators but only at orchard edges and not to the degree seen when natural habitat was abundant.4. Wild bee species richness and flower visitation frequency, but not honeybee frequency, were related to fruit set. Fruit set increased with increasing percentage of natural habitat surrounding the orchards. Organic farming or the presence of a vegetation strip did not increase fruit set.5. Synthesis and applications. The restoration of high quality habitat strips along the edges of crop fields in highly intensified agricultural landscapes should be encouraged and monitored to conserve pollinators and to determine whether benefits for agriculture can be realized. Although honeybees are the main and most important pollinating insects for many plants, wild pollinators may be necessary to ensure high fruit set. Organic farming alone will not sustain wild pollination services for almond in California.

Shikimate leaf disc assay for early detection of glyphosate resistance in Conyza canadensis and relative transcript levels of EPSPS and ABC transporter genes

Nol N, Tsikou D, Eid M, Livieratos IC &amp; Giannopolitis CN (2012). Shikimate leaf disc assay for early detection of glyphosate resistance in Conyza canadensis and relative transcript levels of EPSPS and ABC transporter genes. Weed Research52, 233–241.SummaryTwenty‐two biotypes of Conyza canadensis (Canadian fleabane, horseweed) from a conventional orchard in Crete displayed varying degrees of reduced glyphosate susceptibility in standard whole plant assays. A refined shikimate leaf disc assay was developed to precisely determine the resistance levels, permitting early detection of resistance evolution and integrated management of the weed. The 5‐enolpyruvoylshikimate‐3‐phosphate synthase (EPSPS) homologue genes (1 and 2) were sequenced for three different biotypes (one of reduced susceptibility from Crete, one resistant from mainland Greece and one resistant from the USA), and no amino acid substitution of Pro106 was found. Real‐time qRT‐PCR was used to study the expression profiles for EPSPS and the M10 and M11 ABC transporter genes, following glyphosate application. The expression levels of the EPSPS genes were not significantly altered following glyphosate application in any biotype, but both M10 and M11 were found to be highly upregulated in glyphosate‐treated reduced susceptibility or resistant biotypes and not in a susceptible biotype. These results are in accordance with data recently reported by other researchers, supporting a role of the M10 and M11 ABC transporter genes in glyphosate resistance in Conyza canadensis, because of reduced translocation.

Qualidade de laranjas 'Valência' produzidas sob sistemas de cultivo orgânico e convencional

O objetivo deste trabalho foi avaliar características qualitativas de laranjas 'Valência' (Citrus sinensis (L.) Osb.) produzidas em dois pomares experimentais: um cultivado sob sistema orgânico e outro sob sistema convencional. Os pomares foram instalados no município de Montenegro - RS, em julho de 2001, em espaçamento de 2,5 m x 5,0 m, cada um possuindo uma área de 0,25 ha. A qualidade dos frutos foi determinada de 2005 a 2010 a partir de cinco amostras por tratamento em cada ano. Amostras de frutos colhidos aleatoriamente de cinco plantas foram analisadas para massa média dos frutos (MMF), o teor de suco (Ts), o teor de sólidos solúveis totais (SST), a acidez total titulável (ATT) e a relação SST/ATT. No ano de 2010, também foram determinados o teor de ácido ascórbico (vitamina C) no suco e a coloração da casca. No sistema de produção convencional, o manejo é realizado através da utilização de adubos minerais, calcário dolomítico, herbicidas, fungicidas e inseticidas. Já no sistema orgânico de produção, foi aplicado composto orgânico, biofertilizante líquido, caldas bordalesa e sulfocálcica, e a cobertura do solo foi mantida com vegetação espontânea, que recebeu, no inverno, aveia e ervilhaca e, no verão, feijão-miúdo. Não foram identificadas diferenças nas características qualitativas entre os frutos de ambos os pomares, exceto para o teor de SST, que foi maior, em média, nos frutos do pomar convencional. Os frutos do sistema convencional também apresentaram maior teor de vitamina C e coloração da casca mais intensa na comparação dos frutos provenientes do sistema orgânico.

Organic amendments and land management affect bacterial community composition, diversity and biomass in avocado crop soils

Background and aims The avocado-producing area of southern Spain includes conventional orchards and organic orchards that use different organic amendments. To gain insight into the effects of these amendments, physicochemical properties and microbial communities of the soil were analysed in a representative set of commercial and experimental orchards.

Predation of codling moth eggs is affected by pest management practices at orchard and landscape levels

The low sustainability of pesticides calls for alternative pest control practices. Among these, the conservation of pest natural enemies is governed both by the local farming practices and by the presence of resources in the surrounding landscape. We investigated the effect of landscape composition on predation of sentinel eggs of the codling moth (Cydia pomonella), a key pest in apple orchards. Thirty egg cards of approximately 15 eggs were exposed twice in each of 12 orchards. Orchards were characterized by their area, their crop protection practices, the abundance of their codling moth population and the abundance of predatory earwigs. The landscape was characterized by the area and management of orchards host to the pest and by the characteristics of the hedgerow network in 50m and 100m wide buffers. On average, 12.4% and 48.3% of the egg cards were attacked by predators in June and August, respectively. Predation depended mostly on the toxicity of the crop protection programs. Predation rates were also lower in orchards surrounded by large areas of conventional orchards. No other landscape variable was significant. This indicates that the spatial distribution of pesticides around orchards impacted the within orchards natural enemies.

THRIPS IN OLEAE EUROPAEA L.: ORGANIC VERSUS CONVENTIONAL PRODUCTION

ISHS XXVIII International Horticultural Congress on Science and Horticulture for People (IHC2010): Olive Trends Symposium - From the Olive Tree to Olive Oil: New Trends and Future Challenges THRIPS IN OLEAE EUROPAEA L.: ORGANIC VERSUS CONVENTIONAL PRODUCTION

Atributos físicos e químicos de um argissolo vermelho, em pomar orgânico de citros com manejo da vegetação nas entrelinhas

O manejo em pomares orgânicos de citros é diferenciado em relação aos pomares convencionais. Deste modo, objetivou-se avaliar alguns atributos físicos e químicos de um Argissolo espessarênico e produtividade do pomar de tangerineiras, cv. montenegrina, sob sistema orgânico de produção, com diferentes manejos da vegetação nas entrelinhas. Os tratamentos estudados foram: gradagem, roçada, acamamento com rolo-faca e com arraste de tronco. A avaliação dos atributos físicos ocorreu sob a projeção da copa e na área de tráfego de máquinas, nas camadas de 0,0-0,1 e 0,1-0,2 m. A fertilidade química do solo foi determinada nas profundidades de 0,0-0,1; 0,1-0,2 e 0,2-0,4 m, enquanto a produtividade de frutos foi estimada a partir das plantas centrais de cada parcela. O tráfego de máquinas influenciou negativamente nos atributos físicos do solo abaixo da interface pneu/solo, embora não tenha sido restritivo à produtividade de frutos. Em todos os tratamentos, houve incremento de matéria orgânica na camada superficial do solo e dos teores de P, K+, Ca2+ e Mg2+ nas três camadas de solo em relação à área adjacente com vegetação nativa. O manejo com gradagem apresentou produtividade significativamente maior em relação ao manejo com roçada.

Resistance of Spanish codling moth (Cydia pomonella) populations to insecticides and activity of detoxifying enzymatic systems

Resistance of codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae) to insecticides has become a major problem in many apple and pear production areas. Our aim was to determine the level of insecticide resistance in Spanish field populations. Seven field populations collected from apple, Malus domestica Borkhausen (Rosaceae), orchards and three laboratory susceptible strains of codling moth were studied. Damage at harvest in all the conventional orchards from which codling moth populations were collected was higher than the economic threshold. The efficacy of eight insecticides, with five modes of action, was evaluated by topical application of the diagnostic concentrations on post-diapausing larvae. The enzymatic activity of mixed-function oxidases (MFOs), glutathione transferases (GSTs), and esterases (ESTs) was evaluated for each population. The susceptibility to insecticides and the biochemical activity of the three laboratory strains and one organic orchard population were not significantly different. Field populations were less susceptible to the tested insecticides than the susceptible strains, especially for azinphos-methyl, diflubenzuron, fenoxycarb, and phosalone. The efficacy of all insecticides was significantly dependent on the activity of MFOs. Only the toxicity of the three insecticides most used in Spain when the populations were collected (azinphos-methyl, fenoxycarb, and phosalone) was also dependent on the activity of ESTs and GSTs activity. According to our results, the control failures were due to the existence of populations resistant to the main insecticides used.

Comparative effects of organic and conventional apple orchard management on soil chemical properties and plant mineral content under Mediterranean climate conditions

The effects of conventional and organic management systems on soil chemical properties and leaf nutrients under Mediterranean conditions were studied over a 2-year period on adjacent commercial apple orchards in Southern Greece. The soil in both orchards was characterised as a clay loam–clay and was uniform in morphological and physical properties. The results indicated no significant differences in soil chemical properties between the different management systems, including soil organic matter (SOM), pH, cation exchange capacity (CEC) and C/N ratio. However, soil samples from the conventional orchard exhibited significantly higher values (p< 0.01) of electrical conductivity (EC) and higher concentrations of K, Ca, Na, Cu and Zn, which were likely the result of chemical fertiliser application. Despite the fact that organic systems promote the accumulation of soil organic matter and fertility over time through the use of organic sources, in our study, the SOM values declined, suggesting that the type and the rate of organic matter input in the organic orchard were insufficient. The leaf nutrients, with the exception of P and Ca, were within the sufficiency range in both management systems. The present findings did not provide evidence of major differences in the leaf macronutrient content between conventionally and organically grown apple trees. Nevertheless, our leaf analysis revealed higher concentrations of Zn in the conventionally grown trees and opposite results for Cu, probably due to the extensive use of copper-containing fungicides in organic orchards in Greece.

Apple orchard pest control strategies affect bird communities in southeastern France

Birds are regarded as appropriate biological indicators of how changes in agricultural practices affect the environment. They are also involved in the biocontrol of pests. In the present study, we provide an assessment of the impact of pest control strategies on bird communities in apple orchards in southeastern France. We compared the structure (abundance, species richness, and diversity) of breeding bird communities in 15 orchards under conventional or organic pest control over a three-year period (2003-2005). Pest control strategies and their evolution over time were characterized by analyzing farmers' treatment schedules. The landscape surrounding the orchards was characterized using a Geographic Information System. We observed 30 bird species overall. Bird abundance, species richness, and diversity were all affected by pest control strategies, and were highest in organic orchards and lowest in conventional orchards during the three study years. The pest control strategy affected insectivores more than granivores. We further observed a tendency for bird communities in integrated pest management orchards to change over time and become increasingly different from communities in organic orchards, which also corresponded to changes in treatment schedules. These findings indicate that within-orchard bird communities may respond quickly to changes in pesticide use and may, in turn, influence biocontrol of pests by birds.

Analysis of the factors affecting apple farming: The case of Isparta province, Turkey

The commercial apple cultivation worldwide is in the midst of a major change in apple production management systems. The effects of these changes have considerably been felt in Turkey compared with the world increasing the production and consumption amounts. Therefore, the conventional orchards have been considerably transformed into modern orchards based on dwarf and semi-dwarf rootstocks in the producer provinces of Turkey, especially Isparta province, in recent years. The aim of the study is to analyze effects of the factors which consist of agricultural structure and infrastructure, the growers’ socioeconomic and demographic characteristics and attributes affecting apple farming in Egirdir district of Isparta, Turkey. To this end, data collected through questionnaires carried out with 125 apple growers in Egirdir district was first used for factor and then multiple regression analyses in SPSS statistical program. The results of the study showed that if the planting distances, by renewing the apple rootstocks and willingness of the farm managers to work out of their own farms, were decreased and the other factors were gradually increased up to the optimum input levels, the growers’ attribute and attitudes could be effectively used. As a result, the total apple production could be considerably increased in terms of technical effectiveness. If the scarce sources used for apple farming could be effectively used, a major contribution to the regional and national economy along with the annual incomes of the farmers could be realized. Key words: Apple farming, factor and regression analysis, rootstocks, apple cultivar, modern apple orchard layout.

Cross‐resistance between azinphos‐methyl and acetamiprid in populations of codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), from Washington State

Codling moth, Cydia pomonella (L.), has been intensely managed with the organophosphate insecticide azinphos-methyl for 50 years, and populations have developed resistance. New management programs have been developed and implemented that rely more heavily on other classes of insecticides. A prerequisite for developing effective resistance management strategies for these compounds is to establish their current levels of effectiveness. Adult and neonate larval assays were conducted to assess the response of field-collected codling moth populations from apple in Washington State. Male codling moth populations exhibited a range of responses to a discriminating concentration of azinphos-methyl in a survey of 20 populations. Populations from certified organic orchards were more susceptible than those from conventional orchards. Mean fecundity was inversely related to azinphos-methyl tolerance. Male responses to azinphos-methyl and acetamiprid varied significantly among populations and were correlated. The residual effectiveness of field applications of both insecticides varied significantly against neonate larvae. Neonate bioassays with insecticide-dipped fruit found significant differences among populations with azinphos-methyl, acetamiprid, methoxyfenozide and spinosad, but not with esfenvalerate. These results support a concern that alternation of insecticides with different modes of action may not be a sufficient strategy to avoid the evolution of broad-spectrum insecticide resistance by codling moth.

The impact of soil carbon management on soil macropore structure: a comparison of two apple orchard systems in New Zealand

SummaryWe analysed the long‐term effect of the addition of organic carbon (C) on the macropore structure of topsoils. For this purpose we compared the top 50 mm in the tree rows of an organic apple orchard with those in an adjacent conventional orchard with the same soil type, texture and previous land‐use history in New Zealand. After 12 years the topsoils of the organic orchard had 32% more soil organic carbon (SOC) sequestered than those of the conventional, integrated orchard because of regular compost applications and grass coverage. We quantified the macropore structure (macropores = pores &gt; 0.3 mm) of nine undisturbed soil columns (43 mm long, 20 × 17 mm in the plane) within each orchard using 3D X‐ray computed tomography. The macroporosity (7.5 ± 2.1%) of the organic orchard soil was significantly greater than that of the integrated orchard (2.4 ± 0.5%). The mean macropore radius was similar in the organic and integrated systems, with 0.41 ± 0.02 mm and 0.39 ± 0.01 mm, respectively. The connectivity of macropores tended to be greater in the organic than in the integrated system, but this was not statistically significant. The pronounced soil C management in the organic orchard increased both the formation of macropores by roots and a larger fresh weight of anecic earthworms, and the stabilization of the macropore structure was increased by a larger aggregate stability and microbial biomass compared with those of the integrated orchard. We simulated the diffusion through the measured pore structures of segments of the soil columns. The segments had the length of the mean aggregate size of the soils. The relative diffusion coefficients at this aggregate scale were significantly greater in the organic (0.024 ± 0.0009) than in the integrated (0.0056 ± 0.008) orchard. In a regression analysis with both the porosity and connectivity of macropores as significant variables, 76% of the variability of the relative diffusion coefficients was explained in the integrated, and, with the porosity as the only significant factor, 71% of the variability in the organic orchard. We hypothesize that a greater relative diffusion coefficient at the aggregate scale would reduce nitrous oxide (N2O) production and emission in a wet soil and suggest that soil C management combats climate change directly by sequestering C and indirectly in the form of a reduction of N2O emissions, by creating more macropores.

Pulse Jet Orchard Heater System Development: Part I. Design, Construction, and Optimization

The objective of this six-year project was to develop a more efficient and effective supplemental orchard field heater technology. In addition to being low-cost and mobile, these devices should be useable with or without wind machines, reduce labor requirements, and greatly reduce air pollutants compared to conventional orchard heating systems. The new generation orchard heaters that were developed in this process are portable, auto-regulating, horizontal pulse jet engines (combustors) with no moving parts. These devices utilize air ejector technology to reduce the temperature of the high-velocity combustion stream by thoroughly mixing it with the surrounding ambient (cold) air, producing a net effective temperature increase of only 2°C to 5°C above ambient. Buoyant forces of warm air are therefore small, and fewer heaters would be needed. The design achieved pressure oscillations that were free of combustion instabilities, and amplitudes were maximized for high exhaust jet velocities for deep hot jet penetration and mixing into the surrounding cold air. The overall size and weight of the heater system were minimized while maintaining optimum operation. Finally, anti-phase operation of paired heaters for noise level reduction from 130 dB to near 75 dB was achieved by implementing optimized silencing plenums at the inlet and exhaust of the paired heaters.