Accessions of Gossypium spp., some of them never previously tested, were evaluated for resistance to a local isolate of Meloidogyne incognita race 3 in greenhouse experiments. Nematode infection was characterized based on the galling and egg mass indexes and the reproduction factors (RF). Root-knot nematode reproduction among the newly tested accessions varied from highly susceptible (AS0188, AS0189) to moderately resistant (MT123 no. 3), and some accessions showed highly reduced nematode reproduction (CIR1343, CIR1348, Fai Mui). Histological observations of two resistant accessions (G. barbadense CIR1348 and G. hirsutum TX-25, respectively) showed that resistance occurs through a two-stage mechanism in the first accession and through a single-stage mechanism in the second. Parasitism is blocked early after second-stage juvenile (J2) penetration or during its initial tissue migration (CIR1348) and the development of later-stage juveniles into female adults is suppressed at a later stage (TX-25 and CIR1348). Fluorescence and bright light microscopy showed that root cells surrounding nematodes exhibit a hypersensitivity-like reaction, with the accumulation of presumably phenolic compounds and the presence of necrotic cells that limit the development of nematodes and the formation of giant cells. Underdeveloped giant cells with degenerated cytoplasmic content were found in small numbers in CIR1348 and in large numbers in TX-25, along with deformed nematodes. The full characterization of the defence mechanisms of novel sources of resistance to the root-knot nematode in cotton constitutes a first step towards their use in crop improvement.

Nine accessions of wild Coffea arabica from Ethiopia were evaluated for resistance to Meloidogyne paranaensis. Two wellcharacterized susceptible and resistant cultivars were used as comparative controls. The experiments were conducted in a growth chamber using a clonal population of M. paranaensis (esterase phenotype P1) originating from Brazil. Resistance and susceptibility to the nematode were evaluated using the number of nematodes (eggs and J2) per plant, number of nematodes per gram of root and the reproduction factor (RF). All wild coffee accessions expressed a resistance response to M. paranaensis similar to that of the resistant control Nemaya (RF < 1.0). These results provide coffee breeders with material whose resistance can be transferred into commercial cultivars. Keywords: coffee, root-knot nematode, pathogenicity. RESUMO Avaliação da resistência de cafeeiros silvestres (Coffea arabica) a Meloidogyne paranaensisForam avaliados quanto à resistência a Meloidogyne paranaensis, nove acessos de cafeeiros silvestres incluindo dois cultivares bem caracterizados como testemunhas de suscetibilidade e resistência. Os experimentos foram realizados sob condições controladas em câmara de crescimento, utilizando uma população clonal de M. paranaensis (fenótipo de esterase P1), proveniente do Brasil. A resistência e a suscetibilidade ao nematóide foram avaliadas com base no número total de nematóides por planta (ovos + J2) e por grama de raiz e no fator de reprodução (FR). Todos os acessos mostraram resposta à infecção por M. paranaensis similar à da testemunha resistente (FR < 1,0). Com esses resultados, novos materiais, cuja resistência pode ser transferida aos cultivares comerciais, ficam disponíveis para os fitomelhoristas. Palavras-chave: café, nematóide das galhas, patogenicidade.