Código PHP:
$rubroselec = odbc_exec($conexion,'SELECT descricao FROM pub."grupo-fornec"');
$rubro=odbc_fetch_array($rubroselec) ;
<SELECT NAME="rubro" id="rubro" >
<?php foreach ($rubro as $rubr){ ?>
<option value="<?php echo $rubr['descricao']; ?>"><?php echo $rubr['descricao']; ?></option>
<?php } ?>
</SELECT>