The burrowing nematode Radopholus similis is one of the most damaging pathogens on banana plantations. The role of phenolics in plant defense responses to the nematode was histochemically and ultrastructurally investigated in susceptible and partially resistant cultivars. Histochemical observations of healthy roots revealed that high levels of lignin, flavonoids, dopamine, cafeic esters, and ferulic acids were associated with a very low rate of nematode root penetration in the resistant cultivar. The presence of lignified and suberized layers in endodermal cells contributed to limit invasion of the vascular bundle by the pathogen. After infection, flavonoids were seen to accumulate early in walls of cells close to the nematode-migrating channel in both cultivars and in all tissues of the infected resistant roots including the vascular tissues. The labeling pattern obtained with the gold-complexed laccase and with anti-pectin monoclonal antibodies showed that phenolics were distributed in a loosened pectin-rich material surrounding the nematode. This study provides indications that constitutive phenolics in banana roots are associated with the limitation of host penetration and colonization by R. similis. Accumulation of flavonoids in response to infection was detected in the vascular tissues of susceptible plants and in all root tissues in the partially resistant plants.

Bananas cultivated for export all belong to Cavendish cultivars and are all recognized as very susceptible to nematodes, particularly to the burrowing nematode Radopholus similis and the lesion nematode Pratylenchus coffeae. Even if there have been many changes in the management of banana nematodes in large commercial banana plantations, chemical control still remains most often the last resort method to manage the nematodes, although the number of registered products is definitely declining. Therefore, nematode control though genetic improvement is gaining new interest worldwide. In this study, 55 banana accessions mostly diploids from the Musa acuminata genome group (AA) but including some triploid accessions (AAA), some diploids of the Musa balbisiana genome group (BB) and some interspecific hybrids (AAB, AB) were evaluated for resistance to four nematode species R. similis, P. coffeae, Meloidogyne incognita and M. arenaria. These experiments were conducted in a growth chamber under controlled conditions. All banana accessions were susceptible to nematode species, although many different levels of susceptibility were detected. This study confirmed the good resistance status to R. similis of some cultivars from the Pisang jari buaya and Pisang batuau subgroups and the partial resistance of 17 diploid accessions significantly different from the susceptible reference cv. Grande Naine. This study also showed that 12 diploid accessions exhibited a partial resistance to P. coffeae, including some usual or potential genitors belonging to the wild diploids subspecies burmannica (cvs. Long Tavoy 1 and 2) and burmannicoides (cv. Calcutta 4). No source of resistance to Meloidogyne spp. was found. These screening results, combining for the first time four nematode species, are discussed within the scope of banana breeding in order to produce parental diploid lines with single or combined nematode resistances and further develop triploids that can substitute existing susceptible commercial cultivars.