Identification and pathogenicity of Phytophthora species in pear commercial orchards in Argentina

ʻBartlettʼ pear cultivar accounts for 40% of the pear production area of Rio Negro and Neuquen provinces, Argentina. During fall and spring 2014–2016, symptoms associated with Phytophthora rots were observed in declining commercial orchards. The aims of this survey were: (i) to identify the Phytophthora species associated with these irrigated commercial orchards, using different isolation strategies; and (ii) to investigate the pathogenic potential of Phytophthora isolates on pear fruit and tree. Several strategies were used to isolate Phytophthora spp. from soil/roots and collar rot. Fifty-two isolates were grouped by morphological and cultural characters, corresponding mainly to P. cactorum and other species of Clade 6. P. cactorum was the main species obtained from collar wood and root/soil by fruit bating. The identity of the species was confirmed by sequencing of the internal transcriber spacers (ITS, ITS1, and ITS4 primers) and comparison with sequences available in GenBank and Phytophthora Database (http://phytophthoradb.org). Sequences of P. inundata, P. rosacearum, P. lacustris, P. termophila and P.cactorum were deposited in GenBank. Pathogenicity tests were made for each species on 1-year-old shoots and fruit of ʻBartlettʼ and ʻd’Anjouʼ cultivars, and then on 2-year-old rootstock roots of ʻBartlettʼ cultivar. All Phytophthora species were pathogenic, with different levels of severity among isolates. P. cactorum was the most aggressive species on pear rootstock roots and 1-year-old shoots.