Segmentation applied to weather-disease relationships in South American leaf blight of the rubber tree

International audienceSouth American leaf blight (SALB) is a severe threat to world rubber production. One way of controlling it is to set up plantations in zones not conducive to the disease. Such zones are known once a plantation has been set up, but few data are available on how climate affects the disease, especially in the Amazon region. With better knowledge of conditions that are favourable to SALB epidemics it would be possible to more accurately identify risk zones in Asia and Africa, continents that are still SALB-free. Based on a trial design involving detailed and frequent observations, and with a method rarely used in plant epidemiology, the segmentation method, the results presented in this article make it possible to list, in order of importance, climatic factors that influence disease severity under conditions where the climate varies little over the year

Numerous clones resistant to Phytophthora palmivora in the "Guiana" genetic group of Theobroma cacao L.

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

List of resistant clones (ordered according to their effect in the ordinal probit model), their average score after 10 series of tests, and their deme.

<p>The clones statistically more resistant than the SCA6 resistance control are scored “> SCA 6” and those equivalent to SCA 6 are scored “R”. The names of the clones in bold type are clones selected in the field for their high productivity and their low pod losses caused by rot diseases.</p

A rubber tree's durable resistance to Microcyclus ulei is conferred by aqualitative gene and a major quantitative resistance factor

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

Cross-representation of the clones and homogeneity groups.

<p>Each column corresponds to a clone and each row to a homogeneity group. Four statistically different groups are represented, from left to right: very resistant clones  =  more resistant than SCA6 (red); resistant clones  =  equivalent to SCA 6 (orange); moderately resistant clones  =  less resistant than SCA6, but more resistant than ELP 40-B (yellow); susceptible clones  =  not different from ELP 40-B (green). The clones indicated, with their average score, are the controls (SCA 6, ELP 40-B, OYA 2-B, GF 24) and those on the edge of groups (GU 150-A, ELP 35-A, T60/887).</p