Today President Trump will address the American Farm Bureau’s 100th annual convention in New Orleans. But any promises of help will be too late for many farmers. Had he set out to ruin America’s small farmers, he could hardly have come up with a more effective, potentially ruinous one-two combination punch than tariffs and the shutdown.The trade wars collapsed farmers’ markets. Now, with farmers down, he’s kicking them with a partial shutdown that has effectively slammed the door on farm payments, loans and more. It’s hurting rural Americans — those who formed a big part of the base of Mr. Trump’s support in 2016.