IN-VITRO DETECTION OF CORNUS-FLORIDA CALLUS INSENSITIVE TO TOXIC METABOLITES OF DISCULA-DESTRUCTIVA

Dogwood anthracnose, caused by the fungus Discula destructiva Redlin, is a severe disease of flowering dogwood (Cornus florida L.) and Pacif ic dogwood (C. nuttallii Aud.), Disease control is inadequate in nurse ries and landscapes and absent in the forest. and resistant cultivars are not commercially available. The ability to select tissues insensit ive to culture filtrates from D. destructiva in vitro offers a novel a nd important approach for the selection of dogwood genotypes that are resistant to or tolerant of this devastating fungus. Embryo-derived do gwood callus cultures were established on Murashige and Skoog medium a mended with benzyladenine (BA) and either 2,4-dichlorophenoxyacetic ac id (2,4-D) or naphthaleneacetic acid (NAA). Selection for insensitivit y to D. destructiva metabolites was done by placement of individual cu ltures on media amended with progressively higher concentrations of a partially purified cult;re filtrate (PPCF) containing low-molecular-we ight compounds. Following this selection process, cultures were challe nged in a dose-response format with PPCF to determine whether the sens itivity of the callus to the culture filtrate had changed. During the selection period, the fresh weight of callus grown on medium containin g 2,4-D and amended with PPCF was always less than that of callus grow n on medium amended with the same concentration of potato-dextrose bro th (PDB, negative control). Fresh weight of callus was greater on medi um containing NAA amended with PPCF than on medium with the same conce ntration of PDB. Callus selected in the presence of NAA showed decreas ed sensitivity to toxic metabolites at higher concentrations of cultur e filtrate. The in vitro system described may assist in the identifica tion of disease-resistant germplasm important to the longterm survival of flowering dogwood.