I know that if you plant garic beneath plum trees it will help prevent peach leaf curl (Taphrina deformans).

To fonzie47 thanks for the advice. I'll put it into practise right away-thanks again.

If going for ornamental effect something that has complementary color at flowering time or fruiting time would be the ticket. I have seen red flowered cultivars of ornamental quince (Chaenomeles) suggested for planting with orchard plum (Prunus domestica cultivars). At flowering time you get a strong contrast in flower color. There also might be some potential for contrasting fruit color later, I suppose, although the quince fruits may ripen too late for this.