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Root and Crown Rot of Walnut in Chile primarily affected by Phytophthora species

Natalia Riquelme: Pontificia Universidad Católica de Valparaíso

<div>Between 2015 and 2017, a survey was carried out in orchards of walnut trees affected by root rot and crown rot, in five regions of central Chile. In each region, 9 orchards between 1 and 21 years of age were selected, the incidence and severity of damage associated with root and crown rot was analyzed. Roots samples from symptomatic trees were collected and cultured in semi-selective P5ARP medium to isolate pathogens from the oomycete class. From collected soil samples <em>Phytophthora</em> was recovered employing Rhododendron leaves bates. Pathogen identification was carried out by morphological analysis and ITS and beta-tubulin sequences in the rDNA. The most frequently isolated species was <em>Phytophthora cinnamomi</em>. Pathogenicity tests were performed with representative isolates of each Oomycete obtained. <em>Phytophthora cinnamomi</em>, <em>Phytophthora citrophthora</em> and <em>Pythium ultimum</em> proved to be pathogenic in <em>Juglans regia</em>; the test also showed that the <em>P. cinnamomi</em> and <em>P. citrophthora</em> isolates were more aggressive than <em>P. ultimum</em>. Multigene phylogeny based on the sequences of the nuclear ITS and beta-tubulin, and the mitochondrial <em>cox</em>1 genes, showed that all isolates of <em>P. cinnamomi</em> recovered from <em>J. regia</em> in Chile belong to the same clade as the isolates of the pathogen from other places and hosts. In conclusion, <em>Phytophthora cinnamomi</em> is the main <em>Phytophthora </em>species affecting walnut orchards in Chile.</div>