Spatial distribution of plant-parasitic nematodes in sugarcane fields

188 Dinardo-Miranda & Fracasso SPATIAL DISTRIBUTION OF PLANT-PARASITIC NEMATODES IN SUGARCANE FIELDS Leila Luci Dinardo-Miranda1*; Juliano Vilela Fracasso2 1 Instituto Agronômico - Centro de Cana-de-açúcar, C.P. 206 - 14001-970 - Ribeirão Preto, SP - Brasil. Instituto Agronômico/Fundag - Centro de Cana-de-açúcar. *Corresponding author <> 2 ABSTRACT: Nematodes are important parasites of sugarcane in Brazil. Because of this, several studies have been conducted in recent years to evaluate the effectiveness of control methods. However, no studies have been reported on the spatial distribution of nematodes in sugarcane fields, and such studies are indispensable for the development of sampling plans, aimed at their application in integrated management programs. The spatial distribution of Meloidogyne javanica, Pratylenchus zeae, and mixed populations of P. zeae and P. brachyurus was studied in six commercial fields, with areas ranging from 0.26 to 0.50 ha; samples were obtained during the rainy season. The samples, represented by about 50 g roots, were collected within a rectangular grid measuring 7 × 12 m, 7 × 10 m or 5 × 13 m. Among the ten calculated semivariograms (four for M. javanica populations and six for P. zeae populations or the mixture between P. zeae + P. brachyurus), four could not be fitted to any model and presented a pure nugget effect; the spherical model showed the best fit to the semivariograms of data observed in the other six conditions. In those cases, values of range in semivariogram varied from 18 to 35 m, allowing the nematode aggregation area to be estimated at 2,110 m 2 , on average, suggesting that at least five sampling points per hectare would be necessary, on average to obtain a reliable estimate for the population of these plant parasites in a given area. Key words: Saccharum, Meloidogyne, Pratylenchus, spatial variability, population DISTRIBUIÇÃO ESPACIAL DE NEMATÓIDES FITOPARASITOS EM ÁREAS CULTIVADAS COM CANA-DE-AÇÚCAR RESUMO: Nematóides são importantes parasitos da cana-de-açúcar no Brasil e, em função disso, muitos estudos foram conduzidos, nos últimos anos, para avaliar a eficiência de medidas de controle. Entretanto, não há estudos sobre a distribuição espacial de nematóides em canaviais e esses estudos são essenciais para o desenvolvimento de planos de amostragem, visando aplicação em programas de manejo integrado. A distribuição espacial de Meloidogyne javanica, Pratylenchus zeae e de populações mistas de P. zeae e P. brachyuurs foi estudada em seis campos comerciais, com áreas de 0,26 a 0,50 ha, nas quais procedeu-se à amostragem na época chuvosa do ano. As amostras, representadas por cerca de 50 g de raízes, foram coletadas em grid retangular de 7 × 12 m, 7 × 10 m ou 5 × 13 m. Dos dez semivariogramas calculados (quatro para populações de M. javanica e seis para populações de P. zeae ou mistura de P. zeae + P. brachyurus), quatro não se ajustaram a qualquer modelo, apresentando efeito pepita puro; o modelo esférico foi o que melhor se ajustou aos semivariogramas dos dados observados nas outras seis situações. Nesses casos, o alcance variou de 18 a 35 m, permitindo estimar a área de agregação dos nematóides em 2110 m2, em média, sugerindo que, seriam necessários, em média, pelo menos cinco pontos de amostragem por hectare para obter uma estimativa confiável da população desses fitoparasitos em determinada área. Palavras-chaves: Saccharum, Meloidogyne, Pratylenchus, variabilidade espacial, populações INTRODUCTION Among the many species of nematodes found in association with sugarcane, there are three parasites for the Brazilian conditions: Meloidogyne javanica (Treub) Chitwood, M. incognita (Kofoid & White), and Pratylenchus zeae Gra- ham (Lordello, 1981; Novaretti & Téran, 1983: Moura et al., 1990). On average, these species cause productivity losses between 20 and 40% in the first cut of susceptible varieties, also reducing the productivity - and consequently the longevity of sugarcane ratoon stands (Dinardo-Miranda, 2006). Sci. Agric. (Piracicaba, Braz.), v.66, n.2, p.188-194, March/April 2009 Spatial distribution of plant-parasite nematodes Several studies have been conducted in order to evaluate the effectiveness of control methods such as chemical nematicides (Dinardo-Miranda & Garcia, 2002; Silva et al., 2006), resistant varieties (Dinardo-Miranda et al., 1996; 2003b; Garcia et al., 1997), organic matter (Novaretti & Nelli, 1985; Dinardo-Miranda et al., 2003a), and crop rotation (Rosa et al., 2003; Dinardo-Miranda & Gil, 2005). However, no studies report on the spatial distribution of nematodes in sugarcane fields. Such studies are indispensable for the development of sampling plans, aimed at their application in integrated management programs (Taylor, 1984; Southwood, 1978; Giles et al., 2000). The spatial distribution of nematodes in the field is frequently described as aggregate (Ferris & Wilson, 1987), which means that there is spatial dependence between the populations in the sampled points. In view of this, geostatistics is the most adequate tool for the study of these populations, since it quantifies the spatial dependence between field-collected samples to be quantified and used to construct maps (Leibhold et al., 1993; Roberts et al., 1993; Ellsbeury et al., 1998). The present study aimed to characterize the spatial distribution of M. javanica, P. zeae or mixed populations of P. zeae and P. brachyurus (Godfrey) Filipjev & Schuurmans Stekhoven in sugarcane, using geostatistical analysis, so as to provide guidance for sampling procedures in the field. MATERIAL AND METHODS The spatial distribution of M. javanica, P. zeae, and mixed populations of P. zeae and P. brachyurus was studied in six commercial fields, whose characteristics are presented in Table 1. Areas ranging from 0.26 to 0.50 ha were selected in each field. Samplings were obtained in these areas 189 during the rainy season, when populations usually peak due to favorable moisture and temperature conditions, as well as to abundant root growth. Samples were collected within rectangular grids measuring 7 × 12 m (areas 1 to 4), 7 × 10 m (area 5), or 5 × 13 m (area 6), (Table 1). Each sample was represented by about 50 g sugarcane roots, from which the nematodes were extracted by a combination of sifting and centrifugal flotation methods, in sucrose solution, according to Coolen & D´Herde (1972). Data were initially analyzed by descriptive statistics and mean, standard deviation, coefficient of variation, maximum value, minimum value, skewness and kurtosis were obtained using the Statistical Analysis System (SAS Institute, 1995). In order to obtain lower coefficients of variation, data were transformed to log(x+1), and basic statistics were recalculated. To verify the hypothesis of normality of the data, the Shapiro & Wilk (1965) test was carried out using the Statistical Analysis System (SAS Institute, 1995). After this, geostatistical analyses of t (...truncated)