Ash dieback in Lithuania: disease history, research on impact and genetic variation in disease resistance, tree breeding and options for forest management

Abstract

Future research needs include studies of ash regeneration potential in forest stands, and revealing genetic diversity in and structure of the naturally regenerating F. excelsior populations. Moreover, studies of natural forest succession in dieback - affected but not further managed F. excelsior stands are highly desired. As options of effective management of the devastated stands and means for disease control are still quite limited, the research will focus on one of the most promising options to save ash - its selection and breeding for resistance against H. fraxineus. In the presented review we aimed to summarize the existing knowledge on ash dieback and possible options of its control (forest management and tree breeding) accumulated in Lithuania over the last 20 years of the ongoing epidemic. Due to this devastating disease Lithuania has lost more than a half of its forest stands where Fraxinus excelsior was a dominant tree species, and health condition of the remaining stands is still continuously det eriorating. According to data collected from permanent forest monitoring plots, about 9 % of F. excelsior trees die out every year. Several research projects have been dedicated to investigate reasons and consequences of the current ash dieback phenomenon; also, several tree breeding projects targeted to detect and breed disease - resistant F. excelsior genotypes, families and populations have been launched. The review presents most important results of the implemented research projects and most important findings from Lithuanian F. excelsior progeny and clonal trials which revealed substantial genetic variation and heritability of susceptibility /resistance traits and indicated a good potential of selection and breeding of dieback - resistant ash propagation material. In addition, we briefly present studies on communities of xylotrophic fungi residing in declining ash trees, and genetic diversity, population structure and virulence of H. fraxineus isolates of Lithuanian origin. Finally, we present a summary of researcher’s recommendations prepared for Lithuanian forest owners and managers for management of dieback - affected ash stands and for selection of F. excelsior genotypes least susceptible to H. fraxineus and their further use for establishment of resistant seed orchards and further tree breeding for resistance.