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J.R. Bohac

Sweetpotato, Ipomoea batatas is in the morning glory family, Convolvulaceae, genus Ipomoea, group Batatas. It has many wild Ipomoea relatives that serve as a reservoir of many needed pest and stress-resistance genes. A major barrier to introgression of useful genes is the ploidy gap—sweetpotato is a hexaploid and wild Ipomoeas are diploids and tetraploids. The wild species can be successfully crossed using 2n pollen or by first increasing ploidy by colchicine treatment. The ploidy of such hybrid offpsring can be determined by DNA flow cytometry. My objective was to develop a technique to determine DNA content in Ipomoea and values for DNA content for the major Ipomoea species using the EPIC flow cytometer with a UV detector. Nuclei were extracted and pretreated with cellulase and pectolyase before staining with propidium iodide (PI). A highly linear relationship was found between the DNA content determined by DNA flow cytometry and the ploidy of the closest sweetpotato relatives as determined by chromosome counts. These species were diploid I. trifida, tetraploid I. batatas, and hexaploid I. batatas. DNA content was most similar among other diploid Ipomoea species in the group Batatas and was significantly different in other Ipomoeas not in group Batatas.

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J.R. Bohac and S. Rajapakse

An Ipomoea accession from Indonesia, originally classified as I. trifida, was found to segregate in flower morphology. It was hypothesized to be either a very close relative of I. batatas (6x sweetpotato), or a hybrid between I. batatas (6x) and I. trifida (2x). Twelve seedlings of this accession were grown and precise measurements of sepal angle, corolla shape, and root morphology were taken. Samples were also compared on the DNA level using molecular markers. Based on morphological measurements, it was found that some individual seedlings of the unknown Ipomoea accession were not significantly different than I. batatas; others were not significantly different than I. trifida. The control I. batatas and I. trifida lines were significantly different from each other. DNA flow cytometry was used to determine that all seedlings were diploids with the same amount of DNA per cell. Overall plant morphology and molecular analysis confirmed that all of the seedlings were very closely related and the segregation in flower morphology was not due to a seed mixture. This data is consistent with the hypothesis that the accession is a hybrid between I. batatas and I. trifida.

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P. D. Dukes, J. R. Bohac and J. D. Mueller

A root-knot nematode (Meloldogyne incognita) project was initiated in a field of infested sandy loam (EREC) in 1991 and continued. There were ten sweetpotato entries consisting of six cultivars (Beauregard. Excel, Georgia Jet, Jewel, Red Jewel, and Sumor), three advanced lines (W-270, W-274, and W-279) and PI 399161 which were selected for their diversity in disease reactions and other traits. Each entry was planted in the same plots each year to monitor effects of continuous cropping, disease reactions, yield and population shifts of the pathogen. Marketable yields were reduced each year for Georgia Jet and Red Jewel, but not for Beauregard. Internal necrosis in the storage roots was most severe for Beauregard. Several of the highly resistant entries, especially Sumor and W-279, performed well each year, including high yields, good quality. and little or no nematode reproduction. This study demonstrates the considerable economic benefits of a high level of durable resistance to root knot in sweetpotato.

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J.R. Bohac, J. M. Schalk and P.D. Dukes

A two year study was conducted to evaluate the efficacy of insect resistance in sweetpotato cultivars from our breeding program in combination with an insecticide (fonofos) and/or a parasitic nematode (Steinernema carpocapsae). In the laboratory, use of the parasitic nematode resulted in 99% mortality of Diabrotica larvae. In both years, much higher control of damage by all insect classes was achieved by the use of resistant cultivars in combination with a nematode and/or fonofos treatment. Analysis of the first year's field data showed the parasitic nematode treatment gave good damage protection against the WDS (Wireworm, Diabrotica, Systena), sweetpotato flea beetle, but not grubs. In this same year, fonofos only gave good protection against WDS. In the second test year, fonofos gave good protection against WDS, but the nematode did not. High moisture conditions may have affected the efficacy of the parasitic nematode. Host plant resistance by sweetpotato cultivars appears to be less affected by variable field conditions and accounted for 64% of the total crop protection (compared to the check susceptible line).

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J.M. Schalk, J.R. Bohac, P.D. Dukes and W.R. Martin

This 2-year study was conducted to determine if soil insect damage could be reduced in sweetpotato [Ipomoea batatas (L.) Lam] by treatment with an insecticide (fonofos) and/or a parasitic nematode (Steinernema carpocapsae Weiser), in conjunction with sweetpotato cultivars that differed in susceptibility to soil insect damage. Analysis of field data for the first year showed that the parasitic nematode provided significant damage protection of sweetpotato from wireworms (Conoderus spp.), Diabrotica sp., Systena sp., and sweetpotato flea beetle (Chaetocnema confinis Crotch), but not from grubs (Plectris aliena Chapin; Phyllophaga ephilida Say). In this same test, fonofos used alone provided protection against wireworm-Diabrotica-Systena (WDS complex) damage. In the second test, the nematode did not provide soil insect protection for the WDS complex, but fonofos did reduce damage for these insects. Poor efficacy in the second test with the nematode probably was due to high rainfall, which saturated the soil. Resistant cultivars provided good protection for all three categories of damage. When used with the insect-susceptible check `SC 1149-19', the nematode or fonofos treatments provided better control for all insect categories in the first test. In both years, much higher control of damage by all insect classes was achieved by the use of resistant cultivars in combination with the nematode and/or fonofos treatment (64% higher crop protection than the susceptible check line). Chemical name used: O-ethyl-S-phenylethylphosphonodithioate [fonofos (Dyfonate 10G)].

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S.J. Kays, W.J. McLaurin, Y. Wang, P.D. Dukes, J. Thies, J.R. Bohac and D.M. Jackson

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D. Michael Jackson, Howard F. Harrison, Judy A. Thies, Janice R. Bohac and J.D. Mueller

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K.S. Ling, C.A. Clark, C. Kokkinos, J. R. Bohac, S.S. Hurtt, R. L. Jarret and A. G. Gillaspie

Sweet potato virus disease (SPVD) is the most devastating virus disease on sweetpotato [Ipomoea batatas (L.) Lam] world wide, especially in East Africa. However, weather it is present in the U.S. is unknown. SPVD is caused by co-infection of sweetpotato feathery mottle virus (SPFMV) and sweetpotato chlorotic stunt virus (SPCSV). Presence of two other potyviruses, sweetpotato virus G (SPVG) and Ipomoea vein mosaic virus (IVMV) has also been confirmed in the U.S. Sweet potato leaf curl virus (SPLCV), a whitefly (Bemisia tabaci) transmitted Begomovirus, also has the potential to spread to commercial sweetpotato fields and poses a great threat to the sweetpotato industry. The U.S. collection of sweetpotato germplasm contains about 700 genotypes or breeding lines introduced from over 20 different countries. Newly introduced sweetpotato germplasm from foreign sources are routinely screened for major viruses with serology and graft-transmission onto indicator plants (Ipomoea setosa). However, a large portion of this collection including heirloom cultivars or old breeding materials has not been systemically screened for these major sweetpotato viruses. In this study, a total of 69 so-called heirloom sweetpotato PI accessions were evaluated for their virus status. We used Real-time PCR to detect five sweetpotato viruses, including four RNA viruses (SPCSV, SPFMV, SPVG, and IVMV) and one DNA virus (SPLCV). A multiplex Real-time RT-PCR system was developed to detect three RNA viruses (SPFMV, SPVG, and IVMV). Preliminary data indicated that about 15% of these heirloom sweetpotato germplasm carried at least one of these viruses tested. Details on virus infection status will be presented.

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J.R. Bohac, P.D. Dukes Sr., J.D. Mueller, H.F. Harrison, J.K. Peterson, J.M. Schalk, D.M. Jackson and J. Lawrence

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J.R. Bohac, P.D. Dukes, A. Jones, J.M. Schalk, H.F. Harrison Jr., S.C. Charleston and M. G. Hamilton

Carolina Bunch is a sweetpotato cultivar that combines high yield, excellent flavor and appearance with multiple pathogen and pest resistances. It is ideal for home or market gardens, because of its short vine and bunch habit that allow for production of high yields in a limited space. The roots are fusiform with uniform shape and a smooth, bright, light copper skin and dark orange flesh. When baked, the roots have a smooth texture and are sweet, moist and have excellent flavor and appearance. This sweetpotato can be grown virtually without pesticides. It has very high levels of resistances to southern root knot and other species of nematodes, Fusarium wilt, feathery mottle virus, sclerotial blight in plant beds, and Streptomyces soil rot. It has good resistance to many soil insects including several species of wireworm, Diabrotica, Systena, and flea beetles. In the southern US, it yields better than `Jewel' in a growing season of 110-120 days. Foundation roots are available in limited quantities from South Carolina Foundation Seed Association, Inc, 1162 Cherry Hill Rd, Clemson SC 29634-0393.