There is a new book available through APS Press titled 'Diseases of Herbaceous Perennials'. The authors are all plant pathologists and include Mark Gleason (Iowa State University), Margery Daughtrey (Cornell University), Ann Chase (Chase Horticultural Research), Gary Moorman (Penn State University) and Darren Mueller (Iowa State University).
The book is full of over 700 color photos and would be a wonderful reference text for growers of perennials (as well as garden centers, horticulturists and Master Gardeners).
You are welcome to stop by the Adams County Extension office and do some browsing before deciding to buy. We also have an extensive resource library for diagnosing plant diseases and insect problems that are available for growers to utilize. Please call ahead to make sure that we're in the office!