The serious delinquency rate on mortgages backed by Fannie Mae dropped to 4%, the lowest level since June 2009. The rate fell every month since the 5.59% peak in February 2009 except for July when it went unchanged. Fannie said the rate fell 3 basis points from August. Fannie purchases from lenders and servicers totaled $55.3 billion in September, up 24% from the previous month but down 33% from the same month last year. It is the highest volume since the $57 billion reported in March. Fannie’s gross mortgage portfolio dropped to $722.1 billion in September, down 10% from one year ago. Under the conservatorship agreements established in September 2008, the Treasury Department capped this portfolio at $900 billion at the end of 2009 and scheduled it to be wound down by 10% each year, reaching $250 billion by 2018. Write to Jon Prior. Follow him on Twitter @JonAPrior.