In vitro isolation and propagation of mammatoxin-resistant aspen

by Wann, Steven R.

Hypoxylon canker (Hypoxylon mammatum Wahl. Miller) is considered to be the most serious disease of quaking aspen (Populus tremuloides Michx.). H. mammatum has been shown previously to produce a host-selective toxin that putatively has been identified as a determinant in the disease. This has suggested that " mammatoxin " might be substituted for the pathogen in order to screen varieties of aspen for disease resistance. A method is described in which mammatoxin was used in vitro to screen tissue cultures of aspen for resistance to the necrotic effects of the toxin. By a novel organogenesis procedure, plants were regenerated and established in soil from surviving cultures. After 6-32 weeks of growth, challenge of these propagules with mammatoxin in a leaf puncture bioassay revealed that the resistance expressed in vitro was maintained in the plant. The origin of the mammatoxin resistance was demonstrated to reside in the seedling populations cultured. By establishing shoot cultures from a single cotyledon and growing the remainder of the seedling, the mammatoxin response of the propagules could be compared to that of the donor plant. When cultured on toxin-free medium, 16% of the ortet-ramet families exhibited significant intraclonal variation in the mammatoxin response. However, all propagules regenerated from toxin-resistant cultures were resistant to mammatoxin, and corresponded to toxin-resistant seedlings. The tissue culture screening system allows for convenient roguing of seedlots and subsequent propagation of individuals expressing mammatoxin resistance. -2-