Browsing by Author "Aarnio, Leena"

Birch wood discoloration called pith flecks is caused by a birch stem miner Phytobia betulae Kangas (Diptera: Agromyzidae), which is a very common species in Finland in both silver and downy birch (Betula pendula Erhr. and Betula pubescens Roth). There is a strong positive correlation between fast tree growth and abundance of P. betulae. Female flies lay their eggs into the young upper shoots of the birch, and the larvae start to move downwards in the stem in the newly differentiated xylem layer under the bark and phloem. The larvae form long vertical tunnels which the tree fills with brown parenchyma cells. In tangential cutting the tunnels look like brown stripes, but in cross-sectional cutting they are like small brown spots in the annual growth rings of the stem. The larval tunnels don’t affect the wood strength, but they cause an aesthetic disadvantage especially for veneer and furniture industry. So far the only reliable and accurate method for counting the larval tunnels has been to cut the trees, take experimental discs from breast height and count the tunnels by using stereomicroscope. The tree trials for forest tree breeding are extensive. To study the genetic susceptibility of birch to P. betulae a quicker and more feasible method would be needed for measuring the larval tunnels of P. betulae. The aim of this study was to investigate: 1) whether it is reliable to count the pith flecks by naked eye and how equivalent the different measuring methods are, and 2) if it would be possible to use an increment core for measuring pith flecks from a growing tree. Genetic susceptibility of silver birch families to P. betulae was studied from forest tree breeding field trials in which the larval tunnels were counted by naked eye. The research data was collected from the experimental field trials of Natural Resources Institute Finland in Parikkala and Punkaharju. Reliability of different measuring methods was studied in young trees by comparing the amount of larval tunnels counted by naked eye, from one sector of the disc and from an increment core to the amount of tunnels counted by using stereomicro-scope. The clones were ranked by their susceptibility to P. betulae from the lowest to the highest by using every method of the test. In aged trees with big diameters the larval tunnels were counted from one vertical slice and from one sector of the experimental disc. When studying the open-pollinated progeny of B. pendula with southern and central Finnish provenances the birch families were ranked by their susceptibility to P. betulae. In that data the larval tunnels were measured by naked eye. The order of the families was compared before taking the growth effect into account and after that, when the most and least susceptible families could be separated from the others. The results of the study revealed that the amount of the larval tunnels of P. betulae can be counted pretty reliably by naked eye and the counting can be done from one part of the experimental disc. Cutting the trees is still demanded, because the results of the increment cores were far too unreliable. In case of aged trees both counting the tunnels from one vertical slice and from one sector of the disc proved to be useful methods. When studying the birch families the results showed that fast tree growth alone does not increase the birch susceptibility to P. betulae, but there are also other factors affecting it. Heritabil-ity in the progeny trials on former agricultural land varied from 0.18 to 0.34. Measuring the pith flecks from large breeding data is too slow and laborious, and a quick method for counting the larval tunnels would be needed e.g. if a genetic marker would be searched for the trait in the future.