American Airlines has lowered guidance for the current quarter, and the carrier’s revenue challenges are probably going to persist past the summer given how many low- and ultra-low-cost rivals are now popping up at American’s top hubs, Seaport analyst David McKenzie says in a research note. Spirit Airlines and Frontier Airlines are shifting growth to Dallas Fort-Worth and Charlotte, American’s top two hubs, and pricing is starting to soften industrywide, the analyst says. “In short, AAL’s plans for high-single-digit growth this summer are running into challenges and proving premature,” he says, downgrading the stock to neutral. Shares fall 15% to $11.45.