INFLUENCE OF SOIL ALUMINUM AND PH ON ARMILLARIA ROOT-ROT IN DOUGLAS-FIR IN WESTERN WASHINGTON

It was hypothesized that high extractable Al concentrations in acid soils in young Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantations in western Washington would induce root stress and increase susceptibility to Armillaria root disease. In contrast, however, soils from 5 to 35 year-old Douglas-fir stands with a high severity of Armillaria root disease (13-25 infected trees/ha) had significantly lower (p < 0.05) average water extractable Al concentrations (9.0 +/- 3.1 mug Al/g) than soils with low disease severity (0-12 infected trees/ha) (22.4 +/- 5.3 mug Al/g). Soil pH and 2N KCl extractable Al averaged 4.46 +/- 0.10 and 313 +/- 164 mug Al/g, respectively, in low severity sites and 4.73 +/- 0.20 and 149 +/- 69 +/- g Al/g, respectively, in high severity sites, but were not significantly different between low and high severity sites. To examine the specific effects of Al and pH, growth responses of isolates of Armillaria ostoyae (Romagn.) Herink from Douglas-fir plantations were examined in pure culture. Diameter growth on soil extract agar declined as water extractable Al increased. In addition, diameter growth decreased with increasing Al in non-buffered media. The separate effects of Al and pH, however, could not be determined with this experiment; Al levels ranged from 0-200 mg/L, but pH declined from 5.8 to 2.6. Diameter growth decreased rapidly with increasing Al levels in buffered media at pH 4, but less rapidly at pH 5 or 6. Without Al in the media Armillaria growth increased with decreasing pH. Both Al and pH influenced Armillaria growth in culture and had interacting effects.