The root-knot nematode Meloidogyne incognita is widely distributed and a major pathogen of cotton (Gossypium spp.) worldwide. The objectives of this study were to assess the genetic variability and aggressiveness of Brazilian populations of M. incognita in cotton. Five populations of M. incognita and one isolate of M. enterolobii (outgroup) were used in the molecular analysis. Our results showed that only 2.7 % of the RAPD and AFLP fragments were polymorphic. Despite the existence of two races (races 3 and 4) and two esterase phenotypes (I1 and I2), a low genetic variability among populations was observed, which might be due to the mitotic parthenogenetic mode of reproduction of this pathogen. The aggressiveness/virulence among populations towards different cotton genotypes was also studied. None of the populations was virulent to the resistant cotton genotypes M-315 RNR, TX-25, CIR1343, Wild Mexican Jack Jones and CIR1348 (reproduction factor <1). Two populations of M. incognita from the states of Mato Grosso do Sul and Parana (Umuarama) (races 4 and 3, respectively) were highly aggressive to the susceptible control FM966 and virulent to the accessions LA-887 and Clevewilt-6 that showed moderate resistance to other populations tested.

A B S T R A C TIn Africa, peri-urban vegetable production systems supply perishable vegetables to the rapidly expanding urban centers. These highly intensive systems are characterized by high levels of pests and diseases and an excessive use of synthetic pesticides to reduce their population densities. Root-knot nematodes (RKN) are especially prevalent in these systems but are often not recognized, or diagnosed correctly. The limited ability to accurately identify these pathogens likely results in the inappropriate use and misuse of control measures, such as genetic resistance, crop rotation, or synthetic chemicals. Given the perceived importance of RKN, a species characterization study was conducted in peri-urban vegetable (amaranthus, cabbage, pepper, carrot, cassava, eggplant, okra, tomato) fields and some coffee plantations, in Benin, Kenya, Nigeria, Tanzania and Uganda. Meloidogyne spp. were characterized from 143 field samples using esterase phenotypes (EST) and SCAR markers. Five known species were identified: three phenotypes for M. javanica populations (EST J3, Rm: 1.0, 1.25, 1.4; EST J2a, Rm: 1.0, 1.4; EST J2b, Rm: 1.0, 1.25), two for M. incognita (EST I1, Rm: 1.0; EST I2, Rm: 1.05, 1.0), one for M. arenaria (EST A2, Rm: 1.2, 1.3), one for M. enterolobii (EST E4, Rm: 0.70, 0.75, 0.90, 0.95), one for M. izalcoensis (EST I4 Rm: 0.86, 0.96, 1.24, 1.30) and two unusual esterase phenotypes for two unknown species, named Meloidogyne sp.1 and sp.2. Combinations of species were detected from numerous locations. Genetic diversity was further studied using RAPD primers, by comparing a subset of the sampled populations from Africa and some populations from Brazil and El Salvador. The analysis identified separate clusters of the more common and minor species, with low variability observed for African and American populations. The SCAR markers correctly identified all Meloidogyne species with the exception of M. ethiopica, Meloidogyne sp.1 and Meloidogyne sp.2. For Meloidogyne sp.1, the SCAR markers corresponded wrongly to M. javanica and M. arenaria, and for Meloidogyne sp.2 to M. incognita. This demonstrates the shortcomings of using SCAR markers alone, which can generate erroneous results for RKN species. Further morphological and molecular studies are required to clarify the identity of these two atypical species.

Root-knot nematodes (RKN) cause important production losses of rice (Oryza sativa L.) in the world. Together with Meloidogyne graminicola Golden and Birchfield 1965, M. oryzae Maas, Sanders and Dede, 1978 and M. salasi López, 1984 have been causing damages in irrigated rice fields in Central and South America. In addition, six other RKN species may occur in rice fields in other regions of the world. Correct identification of Meloidogyne spp. is difficult but essential for the management of rice RKNs. The objective of this study was to develop some species-specific molecular markers for the diagnosis of South American RKN rice-related species. Isozyme phenotypes indicated the occurrence of some RKN species in the Brazilian samples, namely M. graminicola, M. oryzae, M. javanica, and two cryptic species designated as Meloidogyne sp. 2 and Meloidogyne sp. 3. Random amplified polymorphic DNA (RAPD) analysis of 16 isolates revealed interspecific genetic polymorphism between Meloidogyne spp., but isolates belonging to the same species (i.e., sharing the same esterase phenotype) always clustered together, whatever the species considered. Specific SCAR markers of 230, 120, and 160 bp were developed for M. graminicola, M. oryzae, and M. salasi, respectively. These SCAR markers may be potential molecular tools for application in routine diagnostic procedures subject to their validation with other rice RKN field populations in the world.