Disclosed are: a plant disease control agent and a plant disease control method. Specifically disclosed are: a plant disease control agent comprising D-tagatose as an active ingredient and a plant disease control agent comprising at least one member selected from the group consisting of saccharides and substances each having a bactericidal and/or fungicidal activity. The plant disease control agents are effective for the control of cucumber, grape and cabbage downy mildew, damping-off in seedlings caused by Pythium spp, cucumber and barley powdery mildew, wheat stem rust, potato and tomato late blight, and others.
