SPECTRUM OF THE MA GENES FOR RESISTANCE TO MELOIDOGYNE SPP IN MYROBALAN PLUM

The Myrobalan plum, Prunus cerasifera, bears a complete-spectrum resis tance to the root-knot nematodes (RKN) Meloidogyne spp. in comparison to the main resistance sources in Amygdalus rootstocks that have more restricted spectra, as evidenced by a differential resistance test bas ed on the predominant species M. arenaria, M. incognita and M. javanic a and the population M. sp. Floride. Resistance to M. arenaria (A) in Myrobalan plum is controlled by the Ma major resistance genes that are completely dominant and confer a non-host behaviour that totally prev ents the multiplication of the nematode. The inheritance of resistance of this self-incompatible species to M. incognita (I), M. javanica (J ) and the population M. sp. Floride (F), considered as belonging to a new RKN species, was studied using G(1) hybrids from a diallel cross b ased on five parents, the two resistant P.2175 (Mal gene; heterozygous ) and P.1079 (Ma2 gene; homozygous) and three host parents, P.2032, P. 2646 and P.16.5 (recessive for both genes), completed with the G(1) ba ckcrosses P.16.5 x (P.2646 x P.1079), P.2646 x (P.16.5 x P.1079) and P .2175 x (P.2646 x P.1079). G(1) and G(2) clones obtained from softwood cuttings sampled from trees in the field experimental design, rooted in the nursery, and inoculated in containers (six replicates per clone ) under greenhouse conditions, were simultaneously evaluated for their host suitability to two to four of the RKN species, based on a 0-5 ga ll index (GI) rating under a high and durable inoculum pressure of the nematode, and then classified into resistant (R; GI less than or equa l to 0.2) or host (H; GI greater than or equal to 1.3) classes. The re sistance classification of each individual clone, evaluated to two (A/ J: 319 clones), three (A/J/I: 249 clones) and four (A/J/I/F: 161 clone s) RKN species, from segregating and non-segregating crosses involving either Ma1 or Ma2 or both or none, was identical whatever the species . The independence of the R/H classification from the tested RKN indic ates that the Ma1 and Ma2 genes control resistance to all of them, and it is assumed that these genes also control resistance to other minor RKN species. The relationship of the Ma genes with the putative genes involved in Amygdalus sources is discussed with the objective of intr oducing them into new interspecific rootstocks expressing a complete-s pectrum and high-level resistance.