Assessing Yield Potential of Cowpea Genotypes Grown Under Virus Pressure

Cowpea or Southernpea [Vigna unguiculata (L.) Walp.] is an important grain legume in many parts of the tropics. However, viral diseases, particularly Cucumber mosaic virus (CMV) and Blackeye cowpea mosaic virus (BlCMV), can be a limiting factor in cowpea production. We evaluated in replicated field plots and under virus pressure nine PIs (441919, 441925, 441917, 147071, 146618, 180014, 180355, 194208, 612607) and three commercial cultivars (Coronet, KnuckleHull-VNR, Pinkeye Purplehull), some of which had shown absence of symptoms for CMV and BlCMV in unreplicated, seed regeneration plots of the U.S. cowpea collection. Only 3% of all plots had plants infected with both CMV and BlCMV in 2003 and 2004. This percentage increased to 47% in 2005. The accession PI 441917 had the highest 3-year mean for grain yield. However, PI 147071, PI 180014, and ‘KnuckleHull-VNR’ had higher seed protein concentration than other genotypes, but their grain yield was significantly lower than that of PI 441917. The cultivar Coronet and PI 180355 attained midbloom and maturity earlier than the other genotypes. Overall, PI 441917 outperformed all other genotypes for grain yield, including virus-resistant PI 612607 and the cultivar KnuckleHull-VNR. This accession is in the process of being released as a virus-tolerant genotype and should be useful in cowpea breeding programs to help control yield losses by CMV and BlCMV.

(258) GreenPack-DG, a Pinkeye-type Southernpea with an Enhanced Persistent Green Seed Phenotype

The USDA–ARS has released a new pinkeye-type southernpea cultivar named GreenPack-DG. GreenPack-DG is the first pinkeye-type southernpea to be released that has a persistent green seed phenotype conditioned by both the green cotyledon gene (gc) and the green testa (gt) gene. The new cultivar was developed from a cross between Charleston Greenpack (green cotyledon phenotype) and the breeding line USVL 97-296 (green testa phenotype). Except for longer pods, GreenPack-DG is similar in appearance and maturity to Charleston Greenpack. Dry GreenPack-DG seeds have a richer and more-uniform green seed color than dry seeds of Charleston Greenpack. GreenPack-DG seeds are much less susceptible to color loss due to blanching when harvest is delayed than are seeds of green-cotyledon cultivars such as Charleston Greenpack. Color loss is a critical problem in production systems where preharvest desiccants are used to facilitate mechanical harvesting operations. The 7-day delay between application of the desiccant and initiation of harvesting operations can result in serious color degradation. Results of 3 years of replicated field tests at Charleston, S.C., indicate that GreenPack-DG yields are comparable to Charleston Greenpack yields. The new cultivar has excellent field resistance to blackeye cowpea mosaic virus and does not produce hard seeds. GreenPack-DG is recommended for trial by the frozen food industry as a replacement for Charleston Greenpack. Protection for GreenPack-DG is being sought under the Plant Variety Protection Act.

New Method for Screening Cowpea Germ Plasm for Resistance to Cucumber mosaic virus

Utilizing the Cucumber mosaic virus(CMV)-resistant cowpea germ plasm line, GC-86L-98, a new method of screening for resistance in the greenhouse followed by field screening was developed. A uniform source of CMV inoculum (freeze-dried infected cowpea tissue) was diluted to provide an infection rate in GC-86L-98 similar to that observed under field conditions. Plants of test lines were mechanically inoculated with this standard inoculum rate and assessed under greenhouse conditions. Lines considered equivalent in infection percentage with GC-86L-98 were then evaluated in field tests. Test line plants were exposed to virus from susceptible cultivar Coronet when plants were at the first or second trifoliolate leaf stage, and then leaf samples were assayed by direct antigen coating-enzyme-linked immunosorbent assay. Lines with infection percentages similar to or lower than the resistant control were considered resistant. A new line (PI 441917) with CMV resistance and several lines with Blackeye cowpea mosaic virus resistance were found. Newly discovered CMV-resistant lines will help to control the serious cowpea stunt disease caused by synergism of the two viruses.

PROGRESS REPORT: EFFORTS TO DEVELOP PINKEYE-TYPE SOUTHERNPEA CULTIVARS WITH AN ENHANCED PERSISTENT GREEN SEED PHENOTYPE

The USDA has developed four pinkeye-type southernpea candidate cultivars (Experimental designations: US-1090, US-1092, US-1094, and US-1096) that have a persistent green seed phenotype conditioned by both the green cotyledon gene (gc) and the green testa (gt) gene. Each of the candidate cultivars produces dry seeds that have a richer and more uniform green color than seeds of either green cotyledon or green testa phenotype cultivars. Seeds of these candidate cultivars are much less susceptible to color loss due to blanching when harvest is delayed than are seeds of green cotyledon phenotype cultivars. Color loss is a critical problem in production systems where pre-harvest chemical desiccants are used to facilitate mechanical harvesting operations. The 7-day delay between application of the desiccant and initiation of harvesting operations can result in serious color degradation. The results of four 6-replicate field trials indicate that the yield potential of each of the four candidate cultivars is equal to that of the green cotyledon pinkeye-type cultivar Charleston Greenpack. Additionally, each of the candidate cultivars is resistant to blackeye cowpea mosaic virus and do not produce hard seeds that are troublesome to frozen food processors. The seed shape, seed size, and seed eye pattern traits of the candidate cultivars are similar to those of Charleston Greenpack.