Defining the stages of infection of grapevine stems with the trunk disease Botryosphaeria dieback
K. BAUMGARTNER (1), R. Travadon (2), E. Galarneau (1), A. McElrone (1). (1) USDA ARS, Davis, CA, U.S.A.; (2) University of California, Davis, CA, U.S.A.
Detection of <i>Neofusicoccum parvum</i>, one of the causal agents of Botryosphaeria dieback of grapevine, is limited to the late stage of infection, when the wood canker is well-established. Early detection is hampered by the internal nature of the canker, which is revealed only by cutting into the wood. A detection tool for the early stage of infection, and from leaves instead of the trunk or cordon, would inform management decisions. We defined early and late stages of infection of inoculated vs. non-inoculated (both wounded and non-wounded) plants at 2-week intervals for 2 months, based on: recovery of <i>N. parvum</i> in culture, xylem occlusions as measured by light microscopy, and xylem embolism as measured by high-resolution x-ray computed tomography (HRCT). Incubation of 1.5 months was required for recovery at 2 cm from the inoculation site (late stage). In contrast, occlusions and embolized vessels, which were significantly more frequent in inoculated plants, were found at only 2 weeks post-inoculation and up to 4 cm from the inoculation site (early stage). Our findings show that these changes in xylem anatomy occur in advance, both in terms of time and position relative to the inoculation site, of colonization. The next step is to identify candidate genes, as identified by RNAseq and confirmed by qRT-PCR, the expression of which in grape leaves corresponds with the early timing of occlusions and embolisms in the woody stem.
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