The effects of forest structure and site characteristics on probability ofpine twisting rust damage in young Scots pine stands

The effects of forest structure and site characteristics on the susceptibil ity of Scots pine (Pinus sylvestris L.) stands to pine twisting rust (Melam psora pinitorqua (Braun) Rostr.) and on the relative amount of the disease in a pine stand were analysed in 109 young Scots pine stands on mineral soi ls in northern Finland. The data included the occurrence of pine twisting r ust in pine leader shoots, the numbers of trees by species, the mean height and mean age of the pines and site characteristics. The susceptibility of a pine stand, defined as the probability of rust occurrence, was modelled w ith logistic regression and the relative amount of pine twisting rust in a susceptible stand with multiple linear regression. The models were linked t ogether to assess the relative amount of pine twisting rust in pine stands. The results show that the susceptibility of the stands to the disease is b est explained by the presence or number of aspen, site fertility, soil ston iness and the number of pines per hectare. A combination of forest soil sto niness and site fertility has a strong effect on the probability of pine tw isting rust occurring in a pine stand. The relative amount of rust in infec ted pine stands is best explained by the number of aspens and the number of willows (soil characteristics) in stands where aspens are present. When th e relative amount of disease in the stands was calculated with the combined model, failure to correctly classify infected stands as such led to more s erious mistakes than failure to classify healthy stands correctly. (C) 2001 Elsevier Science B.V. All rights reserved.