SEEDLING RESPONSE OF 2 TREMBLING ASPEN (POPULUS-TREMULOIDES) FAMILIESTO INFECTION BY HYPOXYLON-MAMMATUM

Open-pollinated progenies from two trembling aspen (Populus tremuloide s) families with a 30-year field history of 91% (putatively susceptibl e, PS) and 45% (putatively resistant, PR) mortality, respectively, cau sed by Hypoxylon mammatum, were grown in a greenhouse for 3 and 9 mont hs. Seedlings were inoculated with two isolates of H. mammatum, repres enting high and low aggressiveness, through a stem wound. Beginning 4 days after inoculation host responses were examined on 312 seedlings p er family for a period of 5 weeks on 3- and 9-month-old seedlings. Sig nificant differences in host responses were observed between the two i solates, with the more aggressive isolate allowing less callus formati on, causing more tissue necrosis and producing longer cankers than the less aggressive isolate on both aspen families. Between-family differ ence was much greater than within-family difference for tissue necrosi s, callus formation and canker length for both seedling ages. When the 3-month-old seedlings were challenged with H. mammatum, the PR family showed greater tissue necrosis, longer cankers, and less callus forma tion than the PS family. However, when the more lignified 9-month-old seedlings of the same families were inoculated, the PR seedlings had m uch less tissue necrosis, shorter cankers, more callus formation and l ess seedling mortality than the PS family. The results indicate chat a rtificial inoculation of seedlings in a greenhouse can differentiate t wo aspen families at the seedling stage, but their correlation to fiel d performance is highly dependent. upon the seedling age. Host respons es of order seedlings may be a better indicator of field performance.