Effect of uninucleate Rhizoctonia on the survival of outplanted Scots pine and Norway spruce seedlings

One-year-old container-grown seedlings were planted in spring on clear cut areas: the Norway spruce (Picea abies) on a moist upland site (Myrtillus-type)and Scots pine (Pinus sylvestris) on a dryish upland site (Vaccinium-type). While still in the nursery, half of the seedlings of each species had been inoculated during the previous summer, with a uninucleate Rhizoctonia sp., a root dieback fungus. At outplanting all the seedlings appeared healthy and had a normal apical bud, although the height of the inoculated seedlings was less than that of the uninoculated control seedlings. At the end of the first growing season after planting, the mortality of inoculated Scots pine and Norway spruce seedlings was 25 and 69%, respectively. After two growing seasons the mortality of inoculated seedlings had increased to 38% for Scots pine and 93% for Norway spruce. The mortality of control seedlings after two growing seasons in the forest was 2% for Scots pine and 13% for Norway spruce. After outplanting the annual growth of inoculated seedlings was poor compared with the growth of control seedlings. These results show that, although Rhizoctonia-affected seedlings are alive and green in the nursery, the disease subsequently affects both their survival and growth in the forest.