DETERMINANT FACTOR OF OAK MORTALITY IN JAPAN: XYLEM DISCOLORATION AND DYSFUNCTION ASSOCIATED WITH BEETLE INVASION AND FUNGAL INFECTION
Kansai Research Center, Forestry and Forest Products Research Institute, Kyoto 612-0855 Japan
Background and objectives
The high mortality rate of deciduous oak trees, Quercus serrata and Q. crispula, during summer months is a serious problem in districts along the Japanese coast. Prior to wilting, massive attacks by the ambrosia beetle, Platypus quercivorus, into trunks and xylem discoloration are observed. An unidentified fungus has been detected in the beetle's mycangia and also in wilting oak xylem . Healthy oaks were killed by the inoculation with that fungus . I investigated the physiological and cytological reaction of the trees against invasion by the beetle and fungal pathogen. Mechanism of wilting and determinants of oak mortality are discussed.
Materials and methods
Healthy oak trees without beetles' attack, living trees with beetles' attack, and wilting trees were harvested at several forests from June to November. Water ascent in xylem was monitored on some unwilted trees by 1% Acid-Fuchsin injection before cutting. Effects of gallery formation by the beetle were checked on the cross section of stems at 1 m intervals . Anatomical studies were made on xylem sections cut from the areas around galleries and from normal sapwood. Distribution of fungi was checked on PDA with the same areas used for anatomical observation.
Results and conclusions
The beetles attacked many oaks that looked healthy and were making normal radial growth. Sapwood had discolored to dark-brown around the galleries whether the tree started wilting or not. The discolored area is formed as a protective reaction after infection or injury, and is called wound heartwood . Parenchyma cells are necrotic and vessels are dysfunctional in that area. The discolored area was at its maximum where beetle galleries were densely formed usually from the base to the breast height of trunks. At the start of wilting, sapwood had reduced to less than 12% of trunk cross-section and sap ascent had been completely stopped at the height of maximum discoloration; occasionally cambium was necrotic in the trunks.
The fungus associated with the beetle was closely associated with the discolored xylem. In the light-brown areas of discoloration, young hyphae were growing actively in large vessels elongating from galleries and invaded ray parenchyma cells through pits on the cell walls. Tyloses were budding from ray cells around such hyphal invasion. As a protective reaction, yellow substances that were assumed to contain phenols  were produced in ray parenchyma cells, and exuded into vessels, discoloring the xylem. These substances did not however seem effective in preventing fungal distribution, probably because the fungus extends along beetle's galleries before their accumulation. Extensive enlargement of the discolored and dysfunctional xylem-area following gallery elongation led to the mortality of the oak tree.
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