US mortgage applications fell 2.7% in the first week of September, the steepest weekly drop in just over a month, according to data from the Mortgage Bankers Association. The decline coincided with a sharp rise in mortgage rates, with the average 30-year fixed rate reaching a 15-month high of 6.58%. Yields on longer-term US Treasury securities surged at the start of the month as high energy prices, heavy debt issuance, and widening fiscal deficits reduced investors’ appetite for duration in fixed-income markets. As a result, adjustable-rate mortgages accounted for 8.5% of all applications, the highest share since June of last year. Applications to refinance existing mortgages, which tend to be more sensitive to short-term rate moves, fell 6%, while applications for home purchase loans were little changed.