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Phylogenetic, morphological, and pathogenic characterization of Alternaria species associated with fruit rot of mandarin in California

Fei Wang: United States Department of Agriculture - Agricultural Research Service

<div>Alternaria rot caused by<em> Alternaria</em> species is one of the major postharvest diseases of mandarin fruit in California. The aims of this study were to identify <em>Alternaria</em> species using phylogenetic analyses and morphological characteristics and test their pathogenicity to mandarin fruit. In 2015 and 2016, decayed mandarin fruit with Alternaria rot symptoms were collected from different packinghouses in California and isolations of the putative fungi were made. In total, 177 <em>Alternaria </em>isolates were obtained from decayed fruit which were classified into three species, <em>A. alternata</em>, <em>A. arborescens</em> and <em>A. tenuissima. </em>The identification was based on DNA sequences of the second largest subunit of RNA polymerase II (<em>RPB2</em>), plasma membrane ATPase and Calmodulin gene regions in combination with morphological characters. Of the 177 isolates, 88 isolates (49.7%) were identified as <em>A. alternata</em>, 53 isolates (29.9%) were <em>A. arborescens</em> and 36 isolates (20.3%) were <em>A. tenuissima</em>. Mycelial growth rate and sporulation varied among the selected isolates of the three species. Pathogenicity tests showed that all three <em>Alternaria</em> species were pathogenic on mandarin fruit at both 5°C and 20°C. Our results indicate that three <em>Alternaria</em> species, <em>A. alternata, A. arborescens, </em>and<em> A. tenuissima</em>, were responsible for Alternaria rot of mandarin fruit in California with <em>A. arborescens</em> causing fruit rot on mandarin being reported for the first time.</div>