Variation in phloem resistance of Norway spruce clones and families toHeterobasidion parviporumandCeratocystis polonicaand its relationship to phenology and growth traits

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Forty clones of Norway spruce (Picea abies) from eight full-sib families and their parents as grafts in a seed orchard were wound-inoculated with two major pathogenic fungi, Heterobasidion parviporum and Ceratocystis polonica, and the lengths of the resulting phloem lesions were recorded six to eight weeks after the inoculations. Significant variation was found for the lesion lengths both between families and clones within families, with broad sense heritability estimates of 0.21 for H. parviporum and 0.51 for C. polonica. There was no positive correlation between phloem lesion lengths caused by H. parviporum and C. polonica, suggesting that the pathosystems differ in the genetic basis of phloem resistance. No strong relationships were found between the lesion lengths and tree heights and the day of growth start and growth cessation. However, the response to the inoculations was strongest when the shoot had passed the bud-burst stage and was elongating. Quite strong relationships were found for both fungi between the lesions lengths of the offspring and those of the male parents but not for the female parents. The effects of inoculation time for screening Norway spruce clones for differential resistance against H. parviporum and C. polonica and future research approaches are discussed.