Gary A. Pederson, USDA-ARS, Griffin, GA and David Ellis, NCGRP/ARS/USDA, Fort Collins, CO
The vegetable seed industry has been concerned with inconsistent naming of pathogen strains and races which has led to confusion in disease resistance claims. In August 2007, an American Phytopathological Society adhoc committee in cooperation with the International Seed Federation (ISF) was established to develop a system for distribution of host and pathogen differential sets. This system would help standardize identification of plant pathogen races and strains to enable accurate testing to support claims of resistance. The committee initially focused on four vegetable crop diseases: 1) Pepper bacterial spot, 2) Melon fusarium wilt, 3) Spinach downy mildew, and 4) Tomato tobamovirus. White papers (www.worldseed.org/isf/differential_hosts.html) were written for each of the four diseases documenting differential sets and proper procedures for race or strain testing. Seeds of the differential sets were provided to the USDA, ARS, National Plant Germplasm System (NPGS) by committee cooperators after being tested for the appropriate disease reaction prior to deposit in the NPGS. The NPGS's Plant Genetic Resources Conservation Unit established the host differential sets for all four diseases to develop a standardized procedure and handled initial seed distributions. Future seed distributions will be handled by the appropriate NPGS curator and location for each crop. Seed regeneration of all differential sets will be handled by committee cooperators, rather than curators, in order to test for expected disease reaction of each seed inventory. The differential sets are only distributed for disease reaction research since these seed inventories have undergone extensive disease testing. Pathogen races or strains are being obtained from committee cooperators and will be maintained and distributed by the NPGS's National Center for Genetic Resources Preservation when available. A copy of an APHIS permit will be required by requestors seeking to obtain the pathogen sets. The committee hopes that this system will be adopted by the scientific community and expanded to other crops where differential sets can be effectively utilized to support disease resistance claims.