Investigation on the mechanisms of wilting and host defenses in mass mortality of oaks

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Mechanisms of wiltingWater conductivity was conspicuously reduced near the inoculation site of oak seedlings inoculated with the pathogenic fungus Raffaelea quercivora, while the conductivity did not change in the distal part. Wilt development was delayed in both inoculated seedlings and excised seedlings when water was supplied to the distal part, suggesting local water blockage as the wilt mechanism. Transpiration decreased in the early stage of wilting after the fungal inoculation. No relationship was observed between axial spread of wood discoloration / fungi and wilt development. Although water content decreased in very adjacent area to the inoculation site, the area without water conduction extended further from dry zone, suggesting that no marked cavitation occur in this disease. Tyloses were observed in vessels which passed through the area without water conduction as the factor of sap ascent blockage.Defense responsesAlthough antifungal activity of extracts did not increase in … More reaction zone barrier, deposits lining or occluding xylem elements were remarkable. Lignin-like compound and pectin were major constituents in fiber region and in small vessel and tracheid region, respectively. Phenols, quinones and lipids were also observed. Fiber occlusion in the xylem was observed 3 days after the inoculation, and spatially continuous barrier was formed after one week. Tyloses, suberization and phenols accumulation became remarkable after 2 weeks. Lignin content increased at the reaction zone barrier. Most of increased lignin was attributed to condensed lignin, probably originated from guaiacyl-rich lignin.Characteristics in the responses of susceptible and resistant treesTo investigate the differences in defense responses among Fagaceae species, seedlings of several species were inoculated with R.quercivora. Only Quercus serrata and Q.crispula seedlings wilted. Differences in susceptibility coincided with tangential spread of wood discoloration, but not with the axial spread. Less