Commercial guava orchards have been destroyed in the São Francisco river Valley, northeastern Brazil, by the Meloidogyne enterolobii nematode but to date no effective control has been identified. The objective of the present study was to obtain and assess interspecific hybrids between Psidium guajava × P. guineense for tolerance to M. enterolobii, for use as guava rootstock or in genetic studies. Crossings were made between P. guajava GUA 161 PE accession and the P. guineense ARA 138 RR and ARA 153 BA accessions. The crossings were made when the calyx ruptured on the flower buds, and two SSR microsatellite markers were used to confirm the hybridization. When the plants reached 15 to 20 cm in height, they were inoculated with a suspension containing 10,000 nematode eggs. Four months after inoculation, the soil was removed from the plants and individual evaluation was carried out for the number of galls, number of eggs and reproductive factor (RF). The plants in the two crossings were considered as hybrids when genotyped with the mPgCI 251 and mPgCI 252 loci. The 10 plants from the 161 PE × ARA 138 RR cross assessed were tolerant to the nematode, with gall index and RF equal to zero. Seven of the 10 plants of the GUA 161 PE × ARA 153 BA cross assessed were considered susceptible to the nematode. The results suggested variability for tolerance among P. guineense accessions and that tolerance to the nematode could be conferred by a dominant allele. Plants of the interspecific hybrid grew similarly to the guava trees at eight months of age in field and were highly compatible with 'Paluma' guava, indicating that this strategy can present excellent results in controlling M. enterolobii.

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ABSTRACT. This study aimed to investigate the genetic variability among guava accessions and wild Psidium species of the Embrapa Semiárido germplasm collection by using microsatellite loci to guide genetic resources and breeding programs, emphasizing crosses between guava and other Psidium species. DNA was extracted using the 2X CTAB method, and polymerase chain reaction products were analyzed on 6% denatured polyacrylamide gels stained with silver nitrate. The unweighted pair-group method using arithmetic average dendrogram generated from the distance matrix of the Jaccard coefficient for 183 alleles of 13 microsatellite loci was used for visualization of genetic similarity. The number of base pairs was estimated using inverse mobility method based on the regression of known-size products. Analysis of molecular variance was performed using total decomposition between and within guava accessions. The accessions showed similarity from 0.75 to 1.00, with the dendrogram presenting cophenetic value of 0.85. Five groups were observed: the first included guava accessions; the second, P. guineense accessions; the third, one accession of P. friedrichsthalianum; and the last 2 groups, P. cattleianum. The genetic similarity among P. SSR Psidium allelic database and accession divergence guineense and some guava accessions were above 80%, suggesting greater possibility to obtain interspecies hybrids between these 2 species. The genetic variability between the accessions was considered to be high (Φ ST = 0.238), indicating that guava genetic variability is not uniformly distributed among the 9 Brazilian states from where the accession were obtained. Obtaining a greater number of accessions by Brazilian states is recommended in order to have greater diversity among the species.

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