Short-term Effect of Thinning on Pinus sylvestris Damage and Sporulation Caused by Cronartium flaccidum

The short-term effect of thinning on cronartium rust on Scots pine (Pinus sylvestris L.) was investigated in a stand of Scots pine (artificially seeded and in the pole-stage) severely infected by the stem-rust fungus Cronartium flaccidum (Alb. & Schwein) G. Winter. The disease rate (the number of old and fresh infections) was recorded before thinning, after which all trees with a resin-top as well as all individual branches with sporulating lesions were removed from the thinned plots, while the comparison plots remained non-thinned. The numbers of sporulating lesions were recorded on the plots annually for 2-5 yrs after thinning. Thinning had no significant short-term effect on the year-to-year relative number of sporulating lesions. The relative number of sporulating lesions, however, increased in both thinned and nonthinned trees, varying significantly from year to year. Thinning had no significant effect on the year of formation of the shoot bearing annual sporulating lesions, the duration of sporulation or the growth of the fungus along the infected shoot in perennial lesions during the first 5 yrs after the treatment.