Progress 01/01/20 to 09/30/20
Outputs
Target Audience:Sweetpotato growers, packers, processors, and consumers. Changes/Problems:Because restrictions on travel and the number of people who could work simultaneously in a lab were put in place by LSU in response to COVID-19, less progress than anticipated was made on objective 3. The lab staff did a remarkable job of keeping everything else on the planned timetable. What opportunities for training and professional development has the project provided?Waana Kaluwasha is conducting PhD dissertation research on management of Rhizopus soft rot of sweetpotato. How have the results been disseminated to communities of interest?Results of the research were presented to the Louisiana sweetpotato industry at the Louisiana Sweet Potato Association annual meeting, at the Louisiana Statewide Sweet Potato Advisory Meeting, and at the LSU AgCenter Sweet Potato Virtual Field Day, and shared with growers and extension and research personnel from other states in presentations at the National Sweetpotato Collaborators Group. What do you plan to do during the next reporting period to accomplish the goals?Obj. 1. Research will continue on assessing wound healing of sweetpotato storage roots as affected by a large number of environmental and cultural practice variables, including variations in wound healing response among sweetpotato cultivars. Experiments will be repeated to evaluate integrating varietal resistance, biological controls, and re-curing on control of RSR. Obj. 2. Screening of advanced breeding lines from the LSU AgCenter sweetpotato breeding program for each of the diseases will continue. Obj. 3. On-site visits will be made in sweetpotato production areas to collect samples of sweetpotatoes and Ipomoea species growing in proximity to sweetpotato fields and these will be tested for viruses present. Samples will be solicited from collaborators in other states and again from the LSU AgCenter breeding program and tested similarly.
Impacts
What was accomplished under these goals?
Impacts: 1. Rhizopus soft rot (RSR) presents a constant threat to sweetpotatoes after they are removed from storage and packed for shipment to market as the pathogen, Rhizopus stolonifer is ubiquitous. The U.S. industry has relied on prophylactic use of fungicides, primarily dicloran or fludioxonil. The LSU AgCenter has developed breeding lines with improved resistance, but environmental variation leads to significant disease on occasion evenwith resistant genotypes. To evaluate environmental and cultural variables that affect RSR, we have hypothesized that it is critical to know how they affect wound healing since the pathogen requires a wound for infection. Avoiding situations that reduce the rate of wound healing and finding ways to encourage wound healing could be integrated with resistance to enable practical management of this disease without reliance on prophylactic use of fungicides. 2. Resistant varieties have effectively managed diseases that once plagued the U.S. sweetpotato industry (Streptomyces soil rot, Fusarium wilt, Fusarium root rot, and Southern root-knot nematode [SRKN]). Research on this project assures that new varieties continue to maintain the resistance needed to prevent these diseases from re-emerging. While sweetpotatoes vary significantly in their resistance to RSR, different pre-harvest environments, wound types, and post-harvest environments lead to significant Rhizopus soft rot in the resistant genotypes. Finding resistance that holds up well across environments will enable sweetpotato packers to confidently use resistance for managing this disease. Black rot is a disease that has re-emerged but for which no resistant varieties have been developed in the U.S. Black rot was successfully managed for nearly 50 years in the U.S., but re-emerged in the 2010s and was widely disseminated. Although it is relatively easy to avoid black rot, once it is introduced to a farm, it can be difficult to manage or eliminate as the pathogen may persist in soil for 3 to 5 years and contaminate packing lines and field equipment. Highly resistant cultivars, if found, could greatly help manage this problem. Guava root-knot nematode (GRKN), a pathogen recently introduced into sweetpotato production areas in the U.S., represents a very serious threat to future production. Although varieties that are resistant to southern root-knot nematode are susceptible to GRKN, there are horticulturally acceptable breeding lines in the LSU AgCenter program that are resistant to GRKN and provide promise of a potential management practice for GRKN in sweetpotato. 3. A complex of four potyviruses (Sweet potato feathery mottle virus [SPFMV], Sweet potato virus G [SPVG], Sweet potato virus C [SPVC], and Sweet potato virus 2 [SPV2]) cause up to 25-40% yield reductions in Beauregard sweetpotato. Clean plants are produced in six clean plant centers in the U.S. through the National Clean Plant Network to reduce the effects of these viruses. However, the viruses can re-infect plants very quickly once they are grown in the field. On the foundation seed farm, re-infection rates can be reduced from 15-40% to 1% and seed roots can be directly tested to determine whether or not they have been re-infected. Sweet potato leaf curl geminivirus has also been found several times in the U.S., but there is little information on how commonly it occurs in commercial sweetpotato production or whether there are other viruses not previously recognized that may also contribute to yield decline. With increased reliance on use of PCR tests for sweetpotato virus testing, it is important to gain information on whether variants of well recognized viruses occur in the field that might evade detection because of minor changes in nucleic acid sequences. These findings can provide a foundation for initiating a quality assurance effort in sweetpotato seed programs. 1. Experiments were conducted to compare the use of re-curing with or without use of the biological control product BioSave with application of the fungicide dicloran on five different cultivars at both 120 and 145 days in storage. Incidence of RSR following inoculation was higher for all treatments at 120 days than at 145 days. At 120 days across cultivars, RSR incidence was 46.4, 36.2, and 12.5% for the non-treated control, 24-hour re-curing, and BioSave dip, respectively. At 145 days, the incidence was 33.1, 39.9, and 1.5% for the same respective treatments. Another experiment examined the effect on wound healing of treating storage roots with either 0.625% NaOCl, dicloran, hot water, or no treatment. The bleach treatment significantly increased the depth of desiccated cells on the wound surface and reduced the lignification index indicating an inhibition of wound healing processes while the hot water treatment had a slight, similar effect and dicloran had no effect on these parameters. In the same experiment, only dicloran significantly reduced development of RSR. 2. A total of 43 different breeding lines from the LSU AgCenter sweetpotato breeding program were evaluated for resistance to one or more diseases. A smaller proportion than usual (22 of 46) were rated as adequately resistant to Fusarium wilt (equivalent or better than the Beauregard standard). Eight of 12 lines evaluated in a field nursery had resistance to Streptomyces soil rot equivalent to Beauregard. Seventeen lines were evaluated in greenhouse tests for resistance to SRKN and GRKN and 5 were resistant and 1 highly resistant to SRKN while 1 was resistant and 3 highly resistant to GRKN, but none were resistant to both nematodes. Four of 11 lines were resistant in post-harvest inoculations for bacterial root rot, better than the resistant standard Heartogold and much more resistant than most of the recent high-yielding cultivars typified by Beauregard which are very susceptible. Three breeding lines had negligible development of Fusarium root rot following post-harvest inoculations of storage roots and were more resistant than lines previously evaluated or the resistant standard Beauregard. Less black rot developed on stems of vine cuttings inoculated at transplanting in a greenhouse than was observed in 2019 and was insufficient to rate reactions. Of the 11 lines evaluated for storage root reaction to black rot in post-harvest inoculations in October 2020, all were susceptible except one rated as intermediate. In an experiment concluded in November, 2019, one line, 16-186 had minimal lesion development at 4 weeks after inoculation with the black rot pathogen, while 28 other genotypes were susceptible to varying degrees. Obj. 3. To initiate a long-term survey of viruses in sweetpotato and related host plants, storage roots of breeding line 17-171 were obtained from test plots in farmers fields in Alabama, Mississippi, and Louisiana. These were sprouted, the vines grafted to the indicator host Ipomoea setosa and symptomatic I. setosa leaves from these grafts were tested by multiplex PCR (Li et al., 2012) for SPFMV, SPVC, SPVG, and SPV2 and by qPCR (Ling et al., 2010) for SPLCV. All samples tested negative for SPLCV and positive for SPFMV and SPVC, and SPVG and SPV2 were detected in a minority of the samples. An unusual symptom was observed in I. setosa grafted with mericlones of breeding lines in late 2019 that tested negative for all viruses by the multiplex potyvirus PCR, the SPLCV qPCR, and a Sweet potato chlorotic stunt virus qPCR. An entity was mechanically transmitted from the source I. setosa to I. nil seedlings and maintained until June 2020 when mechanical transmissions failed. Tissue samples and nucleic acid extracts from each of the above sources were shared with Dr. Maher Al Rwahnih at UC Davis for high throughput sequencing analysis. Other sample collecting activities planned for 2020 could not be conducted due to travel restrictions in response to the COVID-19 pandemic.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2020
Citation:
Sweany, R. R., Picha, D. H., and Clark, C. A. 2020. Hot-water baths, biologicals and re-curing effects on Rhizopus soft rot during sweetpotato packing. Plant Pathol. 69:284-293.
- Type:
Journal Articles
Status:
Published
Year Published:
2020
Citation:
Ferguson, M. H., Clark, C. A., and Smith, B. J. 2020. Genotyping Xylella fastidiosa in Rabbiteye blueberry in Louisiana, USA. Eur. J. Plant Pathol. https://doi-org.libezp.lib.lsu.edu/10.1007/s10658-020-02017-6
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2020
Citation:
Clark, C., DeRobertis, C., and Rezende, J. 2020. Assessment of LSU AgCenter sweetpotato lines for resistance/susceptibility to Meloidogyne incognita and M. enterolobii. p. 9 in; National Sweetpotato Collaborators Group Progress Report 2019.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2020
Citation:
Kaluwasha, W., and Clark, C. 2020. Preliminary studies on Rhizopus soft rot relative to wound healing and effects of selected postharvest control treatments on healing processes in sweetpotato. p. 12-13 in; National Sweetpotato Collaborators Group Progress Report 2019.
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