Estimation of dispersal gradients of S- and P-type basidiospores of Heterobasidion annosum

Summary The number of airborne deposits of the root pathogen Heterobasidion annosum was counted with a wood disc method at distances 5–1000 m from one S‐type and one P‐type infection centre in stands of Picea abies in June, July and August, 1995. The level of background deposition in the area was determined in four healthy control stands. Based on intersterility group determination of deposits, only measurements within 100 m of the infection centres could be used in estimations of dispersal gradients owing to confusion with background deposition. A power law gradient model adequately described the reduction of deposits with increasing distance from the infection centres. Gradients were fairly similar in four directions around the infection centres as the wind speeds were low (3.2m/s) during the collection periods. The gradients were steeper around the P‐type centre, probably due to its lower spore production capacity. The background deposition level (2.8 colonies/m2× h) was reached at distances 1255 m and 98 m from the S‐type and P‐type centre, respectively.