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Autophagy is ubiquitously present in eukaryotes. During this process, intracellular proteins and some waste organelles are transported into lysosomes or vacuoles for degradation, which can be reused by the cell to guarantee normal cellular metabolism. However, the function of autophagy-related (ATG) proteins in oomycetes is rarely known. In this study, we identified an autophagy-related gene, <i>PlATG6a,</i> encoding a 514-amino-acid protein in <i>Peronophythora litchii,</i> which is the most destructive pathogen of litchi. The transcriptional level of <i>PlATG6a</i> was relatively higher in mycelium, sporangia, zoospores and cysts. We generated <i>PlATG6a</i> knockout mutants using CRISPR/Cas9 technology. The <i>P. litchii</i> Δ<i>platg6a</i> mutants were significantly impaired in autophagy and vegetative growth. We further found that the Δ<i>platg6a</i> mutants displayed decreased branches of sporangiophore, leading to impaired sporangium production. PlATG6a is also involved in resistance to oxidative and salt stresses, but not in sexual reproduction. The transcription of peroxidase-encoding genes was down-regulated in Δ<i>platg6a</i> mutants, which is likely responsible for hypersensitivity to oxidative stress. Compared with the wild-type strain, the Δ<i>platg6a</i> mutants showed reduced virulence when inoculated on the litchi leaves using mycelia plugs. Overall, these results suggest a critical role for PlATG6a in autophagy, vegetative growth, sporangium production, sporangiophore development, zoospore release, pathogenesis and tolerance to salt and oxidative stresses in <i>P. litchii</i>.