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Detecting the grapevine wood-canker pathogen Neofusicoccum parvum (Botryosphaeria dieback) based on host markers of infection
E. R. GALARNEAU (1), S. Czemmel (2), R. Travadon (3), G. Cramer (2), A. J. McElrone (1), K. Baumgartner (1). (1) USDA-ARS CPGRU Davis, Davis, CA, U.S.A.; (2) University of Nevada-Reno, Reno, NV, U.S.A.; (3) University of California-Davis, Davis, CA, U.S.A.

Botryosphaeria dieback (causal fungus <i>Neofusicoccum parvum</i>) is a wood-canker disease that limits vineyard productivity. Early detection is hampered by the internal nature of the canker and absence of leaf symptoms. A detection tool for the early stage of infection, and from leaves instead of wood, would inform control practices. We defined the early stage of infection of inoculated vs. non-inoculated (both wounded and non-wounded) plants, based on light microscopic investigation of: 1) fungal colonization at and beyond the inoculation site, and 2) plant anatomical responses to wood colonization. Then we identified differentially-expressed genes from newly-expanding leaves that were specific to the early stage of infection, based on RNA-seq. Fungal colonization occurred in the xylem, mostly the fibers, and was restricted to the inoculation site until 2 months post-inoculation (MPI). Also at 2 MPI, a significantly higher percentage of xylem vessels fully-occluded by gels and xylem tissue with lower starch content were quantified in inoculated plants, but not in wounded or non-wounded plants. RNA-seq analysis of leaves identified 2,340 differentially-expressed candidate genes. qPCR analysis showed genes that were significantly induced and maintained at high levels in the early stages of infection were associated with plant defense responses, such as: 1) altered gene regulation, 2) cell wall modification, and 3) adaptation of amino acid biosynthesis.

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