Mortality of hybrid triploid aspen in Wisconsin and Upper Michigan

Abstract

In eight field trials in Upper Michigan and Wisconsin, mortality of hybrid triploid aspen (Populustremuloides Michx. × Populustremula L.) was 93% by age 25, which was nearly twice that of native diploid and triploid aspen. Symptoms of decline observed on individuals of the hybrid triploid families included branch dieback, thinning crowns, and eventually tree death. The fungus Lahmiakunzei (Koerb.) (syn. Parkerellapopuli (Funk)) was found associated with a number of families from one specific hybrid triploid cross (T-1-58 × Ta-10). Symptoms of rough, black bark developed on 3% of the trees from this particular cross 10 years after planting and increased to 92% at age 25. Neighboring hybrid diploids of P. tremuloides × P. tremula and native triploids and diploids neither exhibited rough bark symptoms associated with L. kunzei nor showed symptoms of decline like those observed on the hybrid triploids. Mortality of native triploids and diploids in these trials was 41% and 58%, respectively, which was primarily caused by hypoxylon canker. Thinning crowns caused by branch dieback and tree mortality without the presence of L. kunzei has been common on the male tetraploid parent (Ta-10) in clone banks and seed orchards in Minnesota and Wisconsin. This parent was collected from an oceanic region near Ekebo, Sweden, and it, along with its hybrid progeny, appear to be poorly adapted to the climate of Upper Michigan, Minnesota, and Wisconsin, resulting in the high mortality of the hybrid triploids in these trials. The origin of the fungus L. kunzei and the nature of its specificity to T-1-58 × Ta-10 hybrid triploid cross are unknown.