Fusarium wilt of blackeyed cowpeas has been known in California since the 1930s, and breeding for resistance to this disease pathogen has been a continuous effort. During the 1960s and 1970s, California Blackeye 5 (CB 5) cowpea (Vigna unguiculata L. Walp.), a widely grown cultivar of the time, became increasingly diseased by Fusarium oxysporum f. sp. tracheiphilum (Fot) Race 3 (2) throughout the growing regions of California. University of California cultivars CB 46 and CB 88 (1) were developed for resistance to Fot Races 1, 2, and 3. CB 46 is currently the principal blackeye cultivar grown on the majority of the acreage in the San Joaquin Valley. In 1989, a new race we designate “Fot Race 4” was isolated from wilted plants at a single field site in Stanislaus County. In years prior to identification, Fot Race 4 had caused severe wilt of CB 46 and CB 88 in this field. Even though the new Fot Race 4 remained confined to a small area for a number of years, sources of host plant resistance to Fot Race 4 were identified, hybridized, and screened, resulting in new progeny with desirable commercial agronomic characteristics. As observed in Stanislaus County, F. oxysporum f. sp. tracheiphilum Race 4 has the potential to cause serious crop damage, depending on virulence and soil inoculum levels, which may vary from year to year. In 1997 and 1998, an entirely different area in the southern San Joaquin Valley, about 140 miles from the original site in Stanislaus County, was found to have plants infected with Fot Race 4. Diseased plants were collected from patches in three separate CB 46 or CB 88 field sites in Tulare County. About 30 cultures were isolated from the diseased plants, which showed stunting, yellowing, and vascular discoloration. In greenhouse fusarium dip tests CB 46, CB 88, CB 5, and several Fot Race 4 resistant breeding lines were inoculated with all the collected isolates and evaluated. CB 46, CB 88, and CB 5 proved to be susceptible to these isolates, showing typical Fot Race 4 symptoms. The Fot Race 4 pathogen was then reisolated from greenhouse-grown, diseased stem tissue of CB 46, CB 88, and CB 5. These findings emphasize the importance of vigilance and necessity of continual disease surveys. They serve as an early alert for the University of California breeding program, and validate local cooperation with University of California Extension Farm Advisors. As a result of this effort new cultivar candidates with resistance to Fot Race 4 are in the final phases of multi-year commercial testing. References: (1) D. M. Helms et al. Crop Sci. 31:1703, 1991. (2) K. S. Rigert and K. W. Foster. Crop Sci. 27:220, 1987.
Soil-free pathogenicity assay of Fusarium oxysporum f.sp. vasinfectum race 4 on Pima cotton (Gossypium barbadense) seedlings