The present study deals with the occurrence of the rust, Pucciniastrum padi (Kunze & Schm.) Diet., in the shoots and cones of Norway spruce (Picea abies (L.) H. Karst.) in the forest area of the training and experimental farm of Helsinki University at Viikki (60’10’ N; 25’ E). The most important task was to clarify the correlation between the occurrences of the disease in spruce and the abundance of the alternative host of the disease, bird-sherry (Prunus padus L.).
Infected shoots were encountered in a 17-year-old planted seedling stand of spruce. In this stand 8.4% of the seedlings were infected. The density of bird-cherry trees was in the stand higher than in the surrounding areas. The number of infected shoots was the greatest in those places where the density of bird-cherries was highest and already at a distance of some ten metres form the bird-cherry stands the degree of infection decreased considerably. The portion of infected cones in the whole material of this study was 19.5%.
The dependence of the frequency of disease on the abundance of bird-cherries at different distances from the spruce stand was studied by means of regression analysis. For this reason, the percentage infected cones were determined by sample plots and the abundance of bird-cherry trees from six zones (0–50, 50–100, 100–150, 150–200, 200–300, and 300–500 m) around each sample plot. The results showed that the dependence between the degree of infection of cones and the abundance of bird-cherry in the surroundings only reached the closest zone. There were also infected cones at greater distance, for instance, 200–300 m from the bird-cherries about 10% of the cones could be infected. Both the infected cones and shoots were longer than the healthy ones.
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