Secreted effector that acts as an elicitor of hypersensitive response (HR) specifically on plants carrying defense protein RPP39 (PubMed:22359513). The allele ATR39-1 is recognized by RPP39, whereas the ATR39-2 allele is not recognized (PubMed:22359513). The RxLR-dEER motif acts to carry the protein into the host cell cytoplasm through binding to cell surface phosphatidylinositol-3-phosphate. ATR39 has 2 polymorphic alleles that are highly conserved among Hyaloperonospora arabidopsidis isolate. This conservation is in stark contrast with other characterized H.arabidopsidis effectors ATR1, ATR5 and ATR13, which are highly divergent. The presence of amino acids 'Glu-168' and 'Val-169' in ATR39-2 blocks recognition by host defense protein RPP39. Belongs to the RxLR effector family.