RESPONSE OF WESTERN CONIFEROUS SEEDLINGS TO INFECTION BY ARMILLARIA-OSTOYAE UNDER LIMITED LIGHT AND NITROGEN

During the first 16 mo of growth, seedlings of five western coniferous species were subjected to one of three physiological treatments: balanced light and nutrients, light limitation and adequate nutrients, or nitrogen limitation and adequate light. Four-month-old seedlings were inoculated with one of three isolates of Armillaria ostoyae. After 1 yr, disease severity was significantly greater when light or nitrogen were limited than when they were balanced. Seedlings responded to light or nitrogen limitation by lowering the concentration of phenolic compounds and/or raising that of sugar in root tissue. Such changes lower the ratio of the energy required for phenol or lignin degradation (E(pd) and E(ld), respectively) to the energy from available sugar (E(as)), which may indicate decreased disease resistance in seedlings. Decreasing E(pd):E(as) ratios were correlated with increasing disease severity and may be important in explaining why pathogenic species of Armillaria preferentially invade stressed seedlings.