Abstract:
Tanoak (Lithocarpus densiflorus), along with some other members of Fagaceae, are susceptible to sudden oak death caused by the oomycete Phytophthora ramorum. Symptoms of the disease include dying crowns, bleeding cankers, and eventually death of infected trees. The cause of mortality is not well understood, but recent research indicates that stem water transport is reduced in infected trees as compared to healthy trees. Possible mechanisms for reduced hydraulic conductivity include the presence of fungal structures in xylem vessels or tyloses. In this study, the spatial and temporal development of tyloses was studied in relation to water conductivity of P. ramorum-infected sapwood. A log inoculation experiment and a field inoculation experiment using live tanoak trees were conducted. In the log experiment, tylosis development in xylem was compared at several sites in non-inoculated and P. ramorum- inoculated logs at two, four, and seven weeks. Inoculated logs showed no significant difference in tylosis frequency at two weeks, but had a significantly higher frequency of tyloses than the non-inoculated logs after four weeks. At seven weeks, the frequency of tyloses was lower than non-inoculated logs. In the field inoculation experiment, trees inoculated with P. ramorum and non-inoculated trees were harvested after five and 14 months. Sapwood tissue was excised from specific sections of each harvested tree and specific conductivity and tylosis
development were measured. Inoculated trees with xylem infections had a significantly higher frequency of tyloses as compared to wounded, non-inoculated trees. An increase in tyloses in tanoak sapwood was associated with a decrease in specific conductivity, suggesting that tyloses induced by the infection of P. ramorum may interfere with sapflow in the tree stem. At 14 months, tylosis development was affected in tissues further away from the inoculation site as compared to five months, paralleling the vertical spread of infection. This pattern suggests that large, mature cankers might contain numerous tyloses and could significantly impede the transport of water in diseased trees.
