In reference lists, style like periodicals, with X University Extension Publication analogous to the journal name (in italics) and the publication number as the journal issue number (set roman). There is no need to specify either the place of publication or what type of Extension publication it is (fact sheet, manual, etc.).
For PNW publications, it is not necessary to cite all three universities in the cooperative. Include only the full name of the first university listed on the cover, which will be specified as the copyright holder in the indicia.
Angima, S., M. Noack, and S. Noack. 2011. Composting with Worms. Oregon State University Extension Publication EM9034. Oregon State University. catalog.extension.oregonstate.edu/files/project/pdf/em9034.pdf.
Fitzgerald, T. 2010. Gardening in the Inland Northwest. Washington State University Extension Publication MISC0304. Washington State University. pubs.wsu.edu/ItemDetail.aspx?ProductID=14405&SeriesCode=&CategoryID=&Keyword=Gardening in the Inland Northwest.
Neher, O.T. and J.J. Gallian. 2013. Powdery Mildew on Sugar Beet: Importance, Identification, and Control. Pacific Northwest Extension Publication PNW643. University of Idaho. cals.uidaho.edu/edcomm/pdf/PNW/PNW643.pdf.