An In Vitro Co-Culture System for Rapid Differential Response to f. sp. Race 4 in Three Cotton Cultivars.

f. sp. race 4 (FOV4) is a devastating fungus pathogen that causes Fusarium wilt in both domesticated cotton species, (Upland) and (Pima). Greenhouse and field-based pathogenicity assays can be a challenge because of nonuniform inoculum levels, the presence of endophytes, and varying environmental factors. Therefore, an in vitro coculture system was designed to support the growth of both domesticated cotton species and FOV4 via an inert polyphenolic foam substrate with a liquid medium. A Fusarium wilt-susceptible Pima cotton cultivar, 'GB1031'; a highly resistant Pima cotton cultivar, 'DP348RF'; and a susceptible Upland cotton cultivar, 'TM-1', were evaluated for 30 days during coculture with FOV4 in this foam-based system. Thirty days after inoculation, disease symptoms were more severe in both susceptible cultivars, which displayed higher percentages of foliar damage, and greater plant mortality than observed in 'DP348RF', the resistant Pima cotton cultivar. This foam-based in vitro system may be useful for screening cotton germplasm for resistance to a variety of fungus pathogens and may facilitate the study of biotic interactions in domesticated cotton species under controlled environmental conditions.