| Summary: | ABSTRACT Citrus gummosis is an important disease because it causes the death of young plants, resulting in their replacement, a costly operation. Its causal agents are different species of Phytophthora. Dissemination of these microorganisms occurs mainly by contaminated nursery trees. Differences in aggressiveness, competitiveness and response to fungicides varied among species of this pathogen. Therefore, this study aimed to examine diversity among citrus isolates of Phytophthora from two different origins in São Paulo State. Thirty isolates from citrus nursery trees from 13 municipalities and 11 from seven commercial orchards were analyzed. Interspecific diversity was performed by species-specific primers and sequencing of the ITS region. Subsequently, intraspecific diversity was carried out with 16 RAPD primers and clustering analysis of UPGMA, using the Dice coefficient. Our data identified only P. nicotianae from either substrates or commercial fields. Isolates clustered into two genetically distinct populations. Genetically similar isolates were also found. The existence of these clonal lineages among isolates from geographically distinct nursery trees suggests an efficient dissemination mechanism. This was the first study to examine the diversity of Phytophthora in citrus from substrates of nursery trees in Brazil.
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