Spatial distribution and genetic variability of reniform and lance nematodes in cotton agroecosystems

Rotylenchulus reniformis (reniform nematode) and Hoplolaimus columbus (Columbia lance nematode) are two of the main plant-pathogenic nematode species affecting cotton production in the United States. For the selection and implementation of effective management practices it is important to understand distribution patterns and population genetic structure of these two nematodes in cotton agroecosystems.;In a three-year field study, the spatial distribution of reniform nematode was characterized in two commercial fields in South Carolina to determine the effects of relations with soil texture and crop rotation on reniform nematode distribution (Chapter 1). Results of this project suggest that population densities of reniform nematode are correlated with sand content, showing nematode densities peaked when sand content was around 60 % and declined when sand content increased above 60-65%. The clustered distribution found for reniform nematode suggests that site-specific management may be appropriate for this nematode, especially when individual fields show high structure in soil texture and population densities. Parallel to this study, in one of the fields, the population dynamics and spatial distribution patterns of Columbia lance nematode as influenced by soil texture and the presence of reniform nematode were examined under a cotton-corn-soybean rotation scheme (Chapter 2). Results demonstrate that Columbia lance nematode and reniform nematode are found in spatially distinct areas in the field. Results also indicate that for any given cotton soil sample is not likely to have both species of nematodes at levels above threshold and that concomitant infestations of Columbia lance and reniform nematodes will probably result in one species being the key pest, depending on sand content. Following these studies, lance nematode species associated with soybean in South Carolina and North Carolina were identified and for the recovered populations the genetic structure was examined (Chapter 3). Soybean is a susceptible host to lance nematodes and resistant cultivars are commonly prescribed rotations for management of reniform nematode in cotton agroecosystems. This study shows that H. columbus is the predominant species associated with soybean in South Carolina, whereas H. stephanus was the prevalent species in North Carolina. The populations of H. columbus examined with a mitochondrial and a nuclear markers showed lack of genetic structure for this species with only one haplotype shared for all populations. However, H. stephanus populations showed higher variability with mostly one specific haplotype per sampling location. Finally, morphology, phylogenetic and genetic structure approaches were undertaken to delimit Hoplolaimus species associated with a variety of hosts in other states in the United States (Chapter 4). High genetic structure associated with geographic locations was found in mostly all Hoplolaimus species collected. A new species is reported and another one (H. stephanus) with the fewest reports seems to be the most prevalent and common for the populations sampled. Results of these projects may improve the effectiveness of nematode management in cotton and provide tools to determine potentially damaging infestations of different Hoplolaimus species.