Print version ISSN 1982-5676
Trop. plant pathol. vol.33 no.2 Brasília Mar./Apr. 2008
RESEARCH ARTICLE ARTIGO
Cover crops for reniform nematode suppression in cotton: greenhouse and field evaluations
Culturas de cobertura para o manejo do nematóide reniforme em algodoeiro: avaliações em casa de vegetação e campo
Guilherme L. AsmusI; Mário M. InomotoII; Roseane A. CargninIII
IEmbrapa Agropecuária Oeste, Cx. Postal 661, 79804-970, Dourados, MS, Brazil
IIDepartamento de Entomologia, Fitopatologia e Zoologia Agrícola, ESALQ, Universidade de São Paulo, Piracicaba, SP, Brazil
IIIDepartamento de Agronomia, Universidade Federal de Mato Grosso do Sul, 79804-970, Dourados, MS, Brazil
Two greenhouse and one field experiment were carried out to evaluate the reaction of cover crops to reniform nematode, Rotylenchulus reniformis, and their effect on nematode populations in a naturally infested soil (2,359 nematodes/200cm3) and on cotton yield. Oil radish (Raphanus sativus), Mulato grass (Brachiaria ruziziensis x B. brizantha), forage sorghum (Sorghum bicolor), tef (Eragrostis tef), foxtail millet (Setaria italica), Algerian (Avena byzantina) and black (A. strigosa) oats, pearl millet (Pennisetum glaucum), and finger millet (Eleusine coracana) were determined to be poor hosts for R. reniformis in greenhouse experiments. Grain amaranth (Amaranthus cruentus) and quinoa (Chenopodium quinoa) were good hosts to R. reniformis. In the field, lower nematode densities were observed after Mulato grass, oil radish and forage sorghum. Higher cotton fiber yields were obtained from plots cultivated with Mulato grass or sorghum during the winter compared to clean fallow. Cotton yield was inversely correlated with both reproduction factor (p < 0.05) of the nematode on the winter cover crops and population of R. reniformis at cotton planting (p < 0.01).
Keywords: Gossypium hirsutum, nematode management, no-till cropping system, Rotylenchulus reniformis.
Foram realizados dois experimentos em casa de vegetação e um experimento em campo, com o objetivo de avaliar a reação de culturas de cobertura ao nematóide reniforme, Rotylenchulus reniformis, e seu efeito sobre a população do nematóide em uma área naturalmente infestada (2.359 nematóides/200cm3) e sobre a produção de algodão. As culturas de nabo forrageiro (Raphanus sativus), capim-mulato (Brachiaria ruziziensis x B. brizantha), sorgo forrageiro (Sorghum bicolor), tef (Eragrostis tef), capim-moa (Setaria italica), aveias amarela (Avena byzantina) e preta (A. strigosa), milheto (Pennisetum glaucum) e capim-pé-de-galinha (Eleusine coracana) comportaram-se como maus hospedeiros de R. reniformis nos experimentos em casa de vegetação. Amaranto (Amaranthus cruentus) e quinoa (Chenopodium quinoa) foram bons hospedeiros de R. reniformis. No campo, as menores densidades dos nematóides foram observadas nas parcelas onde foram cultivados capim-mulato, nabo forrageiro e sorgo forrageiro. As maiores produtividades de fibra de algodão foram obtidas nas parcelas cultivadas durante o outono e inverno com capim-mulato ou sorgo forrageiro, quando comparadas com aquelas que permaneceram em alqueive. A produtividade de algodão correlacionou-se negativamente com o fator de reprodução do nematóide nas culturas de cobertura (P < 0,05) e com a população de R. reniformis no plantio de algodão (P < 0,01).
Palavras-chave: Gossypium hirsutum, manejo de nematóides, sistema plantio direto, Rotylenchulus reniformis.
The reniform nematode (Rotylenchulus reniformis Linford & Oliveira, 1940) is a major plant-parasitic nematode of cotton worldwide (Starr, 1998), especially in the USA, where it is widely distributed. Reniform nematode is also becoming a very important pathogen in Brazil, being found in almost all important cotton growing areas in the central region of the country (Asmus, 2004). Due to the limited knowledge about non-host plants for crop rotation and resistant cotton cultivars (Robinson et al., 1997; Robinson, 2002), its management has been restricted to the use of nematicides, whose detrimental effects on the environment are well known. The use of cotton cultivars more tolerant to R. reniformis, and spring or autumn crop rotations with non-hosts to this nematode have minimum impact on the environment, and should be encouraged.
In the absence of host plants and/or under adverse climatic conditions, nematode populations in soil tend to decrease (McSorley, 1998). Therefore, overwinter fallow with weed control (clean fallow) should contribute to nematode management. However, in a no-tillage cropping system, the soil is kept covered by one or more crops during autumn and winter seasons, a condition that is particularly important in tropical areas with high temperature and low rainfall in winter, such as in the Brazilian Central Region (Salton et al., 2001). Depending on the susceptibility of the cover crops used locally, R. reniformis populations can increase and reach damaging levels to crops planted the following season (Gallaher et al., 1988; Jones & McLean, 2004). Approximately 13.5 million hectares have been cultivated under the no-tillage system in Brazil (Oliveira & Veiga Filho, 2002), and pearl millet represents the main cover crop used (Zancanaro & Tessaro, 2006). Other plant species including amaranth (Amaranthus cruentus), quinoa (Chenopodium quinoa), forage sorghum (Sorghum bicolor and S. bicolor x S. sudanense) and several annual or perennial grasses have also been tested as cover crops (Spehar & Santos, 2002; Spehar et al., 2003; Lamas, 2007). However, their susceptibility to R. reniformis was not known. The objectives of this work were: 1) to evaluate the ability of selected plant species potentially suitable for cover crops for suppression of reniform nematode under greenhouse conditions and, 2) to evaluate the effect of these cover crops on soil populations of R. reniformis and on cotton yield under field conditions.
MATERIAL AND METHODS
Two greenhouse and one field experiment were carried out from September 2003 to April 2005. The greenhouse experiments were located at Embrapa Agropecuária Oeste, in Dourados, Mato Grosso do Sul State, Brazil. The field experiment was conducted in a cotton field naturally infested with R. reniformis (2,359 nematodes/200 cm3 soil), in Aral Moreira, Mato Grosso do Sul State, Brazil.
Host status for R. reniformis of 12 cover crop species was evaluated in Experiment 1. The tested plants were grain amaranth (Amaranthus cruentus BRS Alegria), two cultivars of black oat (Avena strigosa Embrapa 140 and Common), finger millet [Eleusine coracana Agronorte], two cultivars of forage sorghum [Sorghum bicolor Santa Elisa 38 and IPA 7301011], foxtail millet (Setaria italica), oil radish (Raphanus sativus var. oleiferus Siletina), pearl millet (Pennisetum glaucum BRS 1501), two cultivars of quinoa (Chenopodium quinoa BRS Piabiru and Common), and tef (Eragrostis tef ). French marigold (Tagetes patula) was included as a resistant control, according to Caswell et al. (1991). Three seeds of each plant species were sown in 500-cm3 pots containing 400 cm3 of sterilized substrate (58.5% sand, 7% silt and 34.5% clay) and the seedlings were thinned to one per pot prior to nematode inoculation.
R. reniformis inoculum used in experiment 1 was originally collected from soybean roots (Glycine max) and multiplied for 90 days on castor roots (Ricinus communis) in the greenhouse. Eggs and larvae of R. reniformis were extracted from castor roots using the blender-sieving method followed by sucrose centrifugation (Coolen & DHerde, 1972) one day before nematode inoculation. Inoculum of 1,216 eggs and larvae was distributed into two holes about 3.0 cm deep and 1.0 cm from the plant stem. Final population (Pf) of R. reniformis was estimated 60 days after inoculation. Nematodes were extracted from the roots (Coolen & DHerde, 1972) and from the substrate (Jenkins, 1964), and the reproductive factor (RF = Pf/Pi) calculated.
In a second experiment (Experiment 2), the tested plants were black oat (A. strigosa Embrapa 29), Mulato grass (Brachiaria ruziziensis x B. brizantha), Algerian oat (A. byzantina São Carlos), oil radish Siletina, pear millet BRS 1501 and forage sorghum Santa Elisa 38. Soybean BR 96-25619 and French marigold were included as standard good and poor hosts, respectively. Approximately 1,000 eggs + larvae of R. reniformis were inoculated per pot. Sixty days after inoculation, nematodes were extracted from roots and soil using the same procedures as in Experiment 1.
Both experiments were arranged in a completely randomized design with eight (experiment 1) or six (experiment 2) replications. Each experimental unit consisted of a plastic pot with one plant. Data on nematode reproductive factor were log transformed [log (x+1)] prior to analysis of variance, and treatments were compared using LSD test.
As forage sorghum Santa Elisa 38, Mulato grass, pearl millet BRS 1501, and oil radish Siletina responded as poor hosts of R. reniformis in greenhouse experiments and are species very well adapted for the Central Region of Brazil, they were also included in a subsequent field experiment designed to determine their influence on R. reniformis population density and on cotton production. Clean fallow (with manual weed control) was included as R. reniformis negative control treatment.
The experiment was carried out between April 2004 and April 2005 at a commercial farm located in Aral Moreira, Mato Grosso do Sul State, Brazil (S22º5454 W055º3139, annual average temperature of 25ºC and 1,687 mm annual average rainfall). The experimental design was a completely randomized block with five replications. Each experimental plot was 50 m2 in size. Cover crops were established immediately after mechanical destruction of crop residues following cotton harvest. On 29 April 2004, cover crops were sown manually in rows 0.40 m apart. The amount of seeds planted was 10 kg.ha-1 (forage sorghum and Mulato grass), 15 kg.ha-1 (pearl millet), and 3 kg.ha-1 (oil radish). Cover crops were grown for six months, and precipitation was recorded. Weeds were managed manually. On 22 October 2004, cover crops were sprayed with glyphosate (3.0 kg a.i.ha-1), and on 9 November 2004 cotton Delta Opal was sown directly over the crop residues in eight rows, 0.80 m apart, in each plot. Cultural practices followed local recommendations for no-till practice. On 4 April 2005, all bolls of the two 5.0 m-long central rows were harvested. Lint percentage and boll weight were estimated based on 20 bolls from each plot.
Soil population of R. reniformis was determined from a pooled sample consisting of eight soil cores (20-cm depth) taken at random from each plot. Soil samples were collected on 29 April 2004 (just prior to cover crops sowing; initial population = P1), and on 9 November 2004 (at cotton sowing; population after cover crops = P2). A 200cm3 subsample from each pooled sample was processed for nematode extraction using sieving and centrifugal flotation (Jenkins, 1964). Roots were sampled from four cotton plants in each plot at flowering (11 January 2005; P3) and the nematodes extracted (Coolen & D'Herde, 1972). After counting, the number of nematodes (eggs and vermiform stages) per gram of root was calculated. Nematodes extracted were fixed by heating the suspension to 55ºC for five minutes and were then kept in 2% formalin. Numbers of nematodes were estimated in Peters counting slide under microscope. Changes in R. reniformis population after cover crops (RFC) were estimated by RFC = P2/P1. Nematode counts were log-transformed (log (x+1)) prior to analysis of variance (ANOVA). Means of treatments were compared using LSD test. RFC, P2, and the number of nematodes/ g of roots were correlated with seed cotton yield using Pearsons correlation analysis.
RESULTS AND DISCUSSION
All cover crops tested were poor hosts for R. reniformis, except grain amaranth and quinoa, which resulted in RF > 1 (Table 1). The highest RF was observed on grain amaranth BRS Alegria, followed by the two cultivars of quinoa. Thus, these plant species should not be used as cover crops in areas infested with the reniform nematode. Host status of these plant species for R. reniformis had not been evaluated before. Previously, Lal et al. (1976) evaluated the host status of species of Amaranthus and Chenopodium and rated Chenopodium murale as good host, Amaranthus spinosus as poor host, and A. viridis and C. album as non-hosts to R. reniformis based on the number of egg-masses and young females per plant. Therefore, host status of Amaranthus and Chenopodium for R. reniform is species-dependent.
Robinson et al. (1997) listed host status of R. reniformis on 364 plant species tested, and 314 of them showed contradictory host status, such as that shown by Sorghum bicolor. Our results showed that both forage sorghum cultivars tested are poor hosts for the nematode. Thus, the host status of sorghum cultivars should be characterized prior to their use as cover crops in R. reniformis-infested fields. Our results confirm previous findings that oats and oil radish are poor hosts of R. reniformis (Birchfield & Brister 1962; Hutchinson et al., 2003; Jones & McLean, 2004). Interestingly, all tested monocots rated as poor host species in our study. This is in agreement with the findings of Ferraz (1985) and the previously mentioned paper. Monocots, especially gramineous species, should be the main crops to be used as rotational or cover crops in R. reniformis-infested areas.
Precipitation during the six months that cover crops were grown was 863 mm. All cover crop species established very well with sufficient plant residues for no-till practice. The emergence of cotton was markedly reduced in plots where oil radish had been grown during the winter. For this reason we did not use data from those plots for yield assessment. Allelopathic effects of several Brassicaceae on both cotton germination and vigor were reported by Haramoto & Gallandt (2005). As oil radish proved to have beneficial effects in reducing R. reniformis soil populations during winter, further studies to investigate the time needed between burning down oil radish and sowing cotton are urgently needed. In general, there were significant (P < 0.05) differences among treatments after cover crops (P2) and in the middle season of cotton roots (P3) (Table 2). Since there were significant differences in initial populations (P1), the effect of treatments on R. reniformis populations was better characterized by the RFC.
Due to the absence of hosts, clean fallow led to a natural decline of R. reniformis during winter (Table 2), as was also observed by Sharma et al. (1996). However, in plots where Mulato grass, oil radish and forage sorghum were grown, a decrease in nematode populations during winter was greater (P < 0.05) than under clean fallow. Caswell et al. (1991) observe that the use of Crotalaria juncea, Chloris gayana and Tagetes patula as inter-cycle cover crops in pineapple fields caused a reduction in the population of R. reniformis that could eventually be compared to a fallow situation. Adverse effects on nematode populations were also observed with oat (Avena sativa) cv. Hazek, sorghum (S. bicolor) cv. CSH 6, rye (Secale cereale) cv. Wrens Abruzzi and hairy vetch (Vicia sativa) cv. Chaba White (Ko & Schmitt, 1996; Sharma et al., 1996; Gazaway et al., 2000). Our results suggest that all tested plants can be considered as good winter cover crops for R. reniformis suppression.
The cultivation of cotton after Mulato grass and forage sorghum yielded higher amounts of seed cotton and fiber than cotton cultivation after clean fallow (Table 3). The high fiber percentage and boll weight of cotton after Mulato grass and forage sorghum gave higher cotton fiber production after these cover crops. The number of nematodes at cotton sowing (P2) and in cotton roots [P3 (Rr/ g roots)], and the reproduction factor after cover crops (RFC) were negatively correlated with seed cotton yield (Table 4). From this it can be suggested that cover crops can have a significant effect on cotton yield in infested areas by decreasing R. reniformis population.
Although the main benefit of the cover crops tested was reduction of soil population of R. reniformis, maintenance of soil moisture in no-till cover cropping practice may be another positive effect of cover crops on cotton yield. It is not unusual to observe that, even without significant suppression on R. reniformis populations, some cover crops may still lead to high cotton production of subsequent crops, as was observed by Gazaway et al. (2000) and Jones & McLean (2004), using hairy vetch (Vicia villosa) and Crimson clover (Trifolium incarnatum), respectively. In these studies, nitrogen left after leguminous crops could have been the reason for high cotton yield.
Although the greenhouse experiments have demonstrated that various cover crops can reduce soil population of R. reniformis, field experiments are more important to evaluate the effect of cover crops on cotton yield. Mulato grass and forage sorghum in particular, used as autumn or winter cover crops in fields infested by R. reniformis, proved to reduce the nematode population and thereafter increase the yield of seed and cotton fiber. Different cover crops should be considered if fields have been infested by another important cotton nematode, the root-knot nematode, Meloidogyne incognita. In this case, Dias-Arieira et al. (2003) suggested that Brachiaria species should be grown as cover crop. We concluded that the use of cover crops that are non-hosts or poor hosts is an effective strategy against R. reniformis. This is especially adoptable in cotton fields that practice no-tillage.
This research was partially supported by Fundação Agrisus Agricultura Sustentável (Proc. 061/03).
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Received 3 December 2007
Accepted 18 March 2008
Corresponding author: Guilherme L. Asmus, e-mail: email@example.com
Associate Editor: Regina M.D.G. Carneiro