By Stephen Culp , Reuters The mood amongst U.S. homebuilders took an unexpected dour turn this month. The National Association of Home Builders' (NAHB) Housing Market index (USNAHB=ECI) slid five points to land at 45 - the lowest since April - defying analyst expectations that it would hold firm at a neutral 50, which is the dividing line between optimism and pessimism. Coming on the heels of a 7-point drop in August, the residential construction sector is feeling the stress of rising mortgage rates; the average 30-year fixed contract rate has been above the 7% level since early August, and applications for loans to purchase homes are down 27.5% from a year ago, according to the Mortgage Bankers Association. "High mortgage rates are clearly taking a toll on builder confidence and consumer demand, as a growing number of buyers are electing to defer a home purchase until long-term rates move lower," writes Robert Dietz, NAHB's chief economist. Home construction initiall...