The rural areas of the Treasure Coast have a new source of mortgages. The U.S. Department of Agriculture has won over borrowers who find its Rural Development Home Loans, backed by the USDA, through Palm City-based East Coast Mortgage Lenders, an efficient, low-cost path to a mortgage. Under the program, the USDA essentially guarantees a mortgage with a bank, and that’s caught on with borrowers. The state office of the federal agency said 46 loans totaling $5.81 million have been issued in Martin County this year. There were no loans made in 2005, near the height of the building boom. Indian River County home buyers took out 34 loans worth $3.73 million for the year to date.