Current ex situ collections of the wild rubber tree Hevea brasiliensis were gathered more than 25 years ago mainly from populations in Southwest Amazonia. Information regarding the structure of genetic diversity among these populations is fragmented and needs refinement. Having analyzed polymorphisms at 15 microsatellites loci throughout the Hevea genome, we report on the genetic diversity of 307 clonally propagated individuals from 19 different collection points. All analyzed loci were highly polymorphic, averaging 21.7 alleles per locus, and displayed heterozygote deficiency. Differentiation among populations was analyzed on a subsample of 220 individuals from 14 populations. The overall F ST is 0.12; although low, this value is significant and suggests moderate differentiation. Mato Grosso (Brazil) populations were genetically more distant from all other populations. A highly significant correlation between genetic and geographical distances over the whole area under study was shown by the Mantel test. The Bayesian algorithm used by STRUCTURE software allowed for the identification of three population clusters that match the boundaries of hydrographical basins of the main Amazon River tributaries. Differentiation among populations could therefore be jointly explained by both geographical location within the hydrographical Amazon network and by isolation by distance among populations belonging to distinct catchments. It is likely that some of these populations no longer exist due to severe deforestation. We provide suggestions to improve genetic resource management and sampling strategy in natural populations of H. brasiliensis.

The components of genetic resistance from the Hevea brasiliensis cultivar MDF 180 against South American Leaf Blight (SALB) caused by Microcyclus ulei were investigated by QTL mapping. MDF 180 has already been described as a cultivar with a high level of partial and long-lasting resistance. The resistance of progeny individuals from a cross between a susceptible cultivar and MDF 180 was assessed both under controlled conditions of inoculation by three M. ulei isolates and under natural infection in a field trial. Genetic maps of the two parents of this progeny were mainly established based on microsatellites and AFLP markers. No resistance QTL were found in the susceptible parent. In the resistant parent, we identified a qualitative gene responsible for the resistance against isolates from French Guiana and a major quantitative resistance factor determining the resistance against isolates from the state of Bahia (Brazil). The qualitative resistance gene was denominated M15md and was located in the linkage group g15. Four minor resistance QTLs were also identified, two of which showed an epistatic interaction with M15md. The durability of the resistance of MDF 180 is discussed in light of these data.

Cocoa black pod rot, a disease caused by Stramenopiles of the genus Phytophthora, and particularly by the pan-tropical species P. palmivora, causes serious production losses worldwide. In order to reduce the impact of these pests and diseases, preference is given to genetic control using resistant varieties and, to that end, breeders seek sources of resistance in wild cocoa trees. For instance, surveys of spontaneous cocoa trees in French Guiana between 1985 and 1995 led to the collection of abundant plant material forming a particular genetic group (the “Guiana” group). Following numerous one-off studies demonstrating the merits of this group as a source of resistance to Phytophthora, this article presents the results of a comprehensive study assessing the resistance of 186 “Guiana” clones in relation to the Guianan strain (GY 27) of P. palmivora. This study, undertaken in French Guiana, using an efficient methodology (ten series of tests and a statistical test adapted to the ordinal nature of the data) confirmed that the “Guiana” genetic group does indeed constitute an important source of resistance to P. palmivora, though with some variations depending on the demes of origin. Numerous clones (59) proved to be as resistant as the SCAVINA 6 resistance control, whilst nine were statistically more resistant. The “Resistant” and “Moderately Resistant” Guianan clones totalled 108 (58% of the total tested). Some of the clones more resistant than SCAVINA 6 could be incorporated into numerous cocoa breeding programmes, particularly those that also display other notable qualities. The same applies for numerous other clones equivalent to SCAVINA 6, especially the “elite”’ clones GU 134-B, GU 139-A and GU 285-A.

Research Summary• Genetic resistance components of the Hevea brasiliensis × H. benthamiana RO 38 cultivar to Microcyclus ulei disease were investigated by inoculating isolates which succeeded in partially or completely infecting genotypes of a mapping population.• Progeny of a cross between RO 38 and a susceptible cultivar was inoculated under controlled conditions with three isolates and scored for two resistance traits. Interval mapping and a nonparametric test were used to detect resistance quantitative trait loci (QTLs).• Eight significant QTLs were detected, all of them inherited from the interspecific parent. Among these QTLs, only one contributed to the partial resistance against a highly pathogenic isolate, and no QTL was detected for resistance against the most pathogenic isolate. As an unexpected result, a single isolate can thus completely bypass this polygenic resistance.• This complex situation, where no clear relationship can be established between number of resistance factors and qualitative vs quantitative or partial vs complete resistance, is discussed and suggestions as to the detection of new and sustainable resistance sources are proposed.