Resistance of Picea abies seedlings to infection by Heterobasidion annosum in relation to drought stress

Norway spruce (Picea abies) seedlings, 2–4 years old, were subjected to different degrees of stress. Drought stress affected the predisposition to infection by Heterobasidion annosum in the same way in root wood, stem wood and stem bark. At high water potentials (−3 to −5 bar) resistance was high, at intermediate levels (‐ 5 to ‐ 15 bar) it was low but increased again at water potentials below ‐ 8 to ‐ 15 bar. Neither light nor oxygen deficiencies appeared to have adverse effects on resistance. Total length of unsuberized roots was a sensitive indication of strain on the seedlings.