Department of Plant Pathology, University of Wisconsin, 1630 Linden Drive, Madison 53706-1598
Changes in monoterpene and phenolic compounds resulting from water stress and colonization by Sphaeropsis sapinea were examined for 9- and 11-year-old red pine trees in a plantation and 3-year-old seedlings in a growth chamber. Four treatments were assigned at random to individual trees in the field: no treatment, herbicide to kill surrounding weeds, supplemental water, and both herbicide and supplemental water. In the growth chamber, seedlings were either not watered (water stressed) or watered daily (nonstressed). Shoots were inoculated with agar plugs colonized with either S. sapinea isolates of morphotype A and B (field) or only isolates of morphotype A (growth chamber). Nine monoterpenes were detected in tissue extracts; the most common were α-pinene (59 to 74% of the total), β-pinene (13 to 33% of the total), and δ-3-carene (1 to 5% of the total). Shoots inoculated with isolates of morphotype A had more severe symptoms and produced higher concentrations of monoterpenes in both experiments compared with the controls. In the growth chamber, inoculations with isolates of morphotype A caused higher concentrations of phenolics compared with the controls. In the field experiment, monoterpenes increased in quantity only in shoots of stressed trees inoculated with isolates of morphotype A. Isolates of morphotype B caused few symptoms and did not alter monoterpene concentrations. Increases in monoterpenes do not appear to be involved in the response to infection by morphotype A in nonstressed trees, and the role of phenolics is unclear. However, these results are consistent with previous observations that monoterpenes may be involved in the differences in aggressiveness between morphotypes on red pine.