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Christopher A. Clark
Professor

 

Research
My research is primarily directed at solving short and long-term disease problems of the Louisiana and U.S. sweetpotato industries as well as research aimed at better understanding the biology of sweetpotato pathogens and diseases they cause.  Efforts currently emphasize two problems: the role of viruses in cultivar decline, and efforts to improve postharvest disease control with reduced reliance on prophylactic use of fungicides.

 

Role of viruses in sweetpotato cultivar decline

 

It has long been recognized that sweetpotato cultivars decline in yield and quality performance over years after they are released and even the most popular cultivars are replaced about every 15-20 years.  The reasons for this decline have not been well defined but it is thought that accumulation of viruses and/or mutations in the planting stock is responsible.  Although sweetpotato is the seventh most important food crop worldwide and an economically important crop in a few states in the U.S., including Louisiana, it has not received the same research attention that many other crops have enjoyed.  As a result of limited investment in research, our understanding of the viruses that infect this crop lags well behind that for many other vegetatively propagated crops.  For many years, Sweet potato feathery mottle virus (SPFMV) was the only virus recognized to occur in sweetpotatoes in the U.S.  However, our field studies indicate that SPFMV by itself has minimal effect on yield of sweetpotatoes, even though it re-infects healthy plants in the field very rapidly.  Therefore, our research has been directed in part to determine what pathogens might be interacting with SPFMV to cause cultivar decline.  About 10 years ago, we found that two other potyviruses, Sweet potato virus G (SPVG) and Sweet potato virus 2 (SPV2) also occur commonly in sweetpotatoes in the southeastern U.S. (Souto et al., 2003).  More recently, Sweet potato virus C has been recognized as a distinct potyvirus species that is also a common component of the virus complex.  The whitefly-transmitted begomoviruses, Sweet potato leaf curl virus (SPLCV) and Sweet potato leaf curl Georgia virus, were found to occur in ornamental sweetpotatoes and some sweetpotato breeding lines.  In field plot studies conducted in 2000-2006, none of the virus combinations tested (SPFMV, SPVG, SPV2, SPLCV) produced the same yield depression obtained when using ‘naturally infected’ plants from farmers’ fields as inoculum (Clark and Hoy, 2006).  Studies are in progress to determine whether adding SPVC to the complex reproduces this yield effect.

 

 

Furthermore, when titers of viruses were determined using real-time quantitative PCR (Kokkinos and Clark, 2006), naturally infected plants had about 200-fold greater titers of SPFMV, SPVG, and SPV2 than artificially inoculated Beauregard plants grown under the same conditions: 

 

  

Relative titers of Sweet potato feathery mottle virus (SPFMV), Sweet potato virus G (SPVG), and Sweet potato virus 2 (SPV-2) as determined by quantitative real-time PCR.  ‘Artificial’ = plants graft inoculated with all three viruses, ‘Natural’ = plants exposed to infection in the field for 7 years.  Plants sampled for the test were grown in the same greenhouse conditions.

 

Naturally infected plants also have more severe and persistent symptoms than plants artificially inoculated with SPFMV, SPVG, and SPV-2:

 

  

Left = Beauregard sweetpotato graft inoculated with SPFMV, SPVG, and SPV-2 showing virtually no symptoms, right = Beauregard plant derived from a source that had been exposed to natural infection in the field for 7 years.

It now appears that sweetpotatoes are affected by a complex of viruses in the U.S. that is not yet fully characterized.  Effects on the crop are likely determined by the interactions among these viruses with their host and the timing of infection.  We are continuing to try and isolate and identify viruses from sweetpotato with the intention of determining which ones are critical to cultivar decline.

 As a service to the sweetpotato industry, we do meristem-tip culture for virus therapy and subsequent indexing to produce virus-tested plants of important cultivars and advanced breeding lines from the LSU AgCenter breeding program.  These virus-tested plants are provided directly or through a private company, Certis USA, to the foundation seed programs in Louisiana and Alabama.  Sweetpotato producers now have access to relatively clean planting material, but they must usually increase it on their own farms for at least one year before they have sufficient seed to plant their crop.  Re-infection with SPFMV can be very high (often 100%) during this first year on farms, so in collaboration with Drs. Tara Smith and Don Ferrin, we conducted a multi-year survey to determine the performance of sweetpotato seed from farmers’ programs to determine when it is necessary for them to replace their seed with foundation seed. 

 Everlyne Wosula’s PhD research provided a lot of information about the relationships among aphid vectors, potyviruses, and spread of viruses in sweetpotato crops.  A key finding of practical significance is that potyvirus spread occurred primarily during a 3-4 week period at about the time sweetpotato vines are rapidly growing in the field, which coincides with a peak of titer, especially of SPFMV.  She also surveyed the aphids in sweetpotato fields, determined the relative transmission efficiency of the most common aphids: Aphis gossypii, Rhopalosiphum padi, and Myzus persicae, and found that common morning glory species were actually better acquisition hosts for SPFMV than sweetpotato.  Future research in collaboration with Dr. Jeff Davis in the Entomology Department will endeavor to expand on this information to identify strategies to reduce re-infection of virus-tested sweetpotatoes in the field.

Integrated management of Rhizopus soft rot

 

The Zygomycete that causes Rhizopus soft rot on sweetpotato, Rhizopus stolonifer, is an ubiquitous organism.  Rhizopus soft rot develops on sweetpotatoes that have been cured and stored, infection occurring through wounds that are incurred during the process of packing the storage roots for shipment to market.  It can completely decay roots in 1-2 weeks and for this reason, packers have routinely used a fungicide, primarily dicloran (Botran®) applied to the roots after they are washed to control the disease.  Since sweetpotato is the most popular baby food and since it is also popular with people trying to follow a health-conscious diet, reliance on prophylactic application of a fungicide at the last stage in packing is undesirable.  We are therefore doing research to try and develop an integrated management approach to improve management of Rhizopus soft rot while reducing reliance on fungicides.

 

Resistance is often used for managing diseases, but there had been little effort to investigate resistance to postharvest diseases, especially in sweetpotato.  We developed methods for screening for Rhizopus soft rot resistance and found that Beauregard, the dominant cultivar in U.S. production from 1988-2008 had greater resistance than earlier cultivars (Clark and Hoy, 1994).  However, despite its resistance, Rhizopus soft rot can still occur on Beauregard, and we are working with Dr. Don LaBonte to develop lines with greater levels of resistance that are horticulturally acceptable.

 

 

A Sweetpotato IPM Project was funded from 2003-2007 by the USDA Risk Avoidance and Mitigation Program (RAMP) and Gerber Foods that involved plant pathologists and entomologists from the LSU AgCenter, North Carolina State University, Mississippi State University, and Auburn University in a major program to address problems with soil insects and postharvest diseases.  Factors investigated in relation to Rhizopus soft rot included the role of wounds incurred on packing lines, the effect of postharvest environment, and the effect of preharvest environmental factors and cultural practices on disease.  

Research was conducted in plots in over 20 fields per year on commercial sweetpotato farms in Louisiana, and a similar number were conducted by colleagues in North Carolina, all growing Beauregard.  Sweetpotatoes harvested from these plots were cured for 5 days, then stored at 60oF for 100-120 days and inoculated with R. stolonifer and Dickeya didantii (syn. Erwinia chrysanthemi) using standardized wounding techniques and uniform inoculum.  Data was also collected from each of these plots for various weather parameters, soil analyses, and cultural practices followed by the producer.  The results indicated that differences in the preharvest environment have dramatic effects on susceptibility of Beauregard sweetpotatoes to both diseases, but also indicates that the two diseases respond differently to the environment. 

 

Incidence of Rhizopus soft rot and Bacterial soft rot in Beauregard sweetpotatoes harvested from 21 different fields in Louisiana in 2006.  The roots were cured, stored, and inoculated under identical conditions.

 

Various statistical analyses of the data from these plots indicated that certain weather variables such as soil and air temperature and soil moisture and soil constituents such as calcium, phosphorus, potassium, and organic matter may influence susceptibility of Beauregard to Rhizopus soft rot.  Research is ongoing to verify the role of these variables and determine their interactions.  Dr. Arthur Villordon at the Sweet Potato Research Station has used the yield data from the same plots to help develop a growing degree day model for determining optimal harvest time of sweetpotato and is working on a model to determine optimal planting time.  Eventually, we would like to combine these into a production model that producers can use to best manage their practices.

We also are evaluating other materials and practices that can be used on packing lines to reduce Rhizopus soft rot.  This includes evaluation of commercial ‘reduced risk’ fungicides and biological controls.  We have also worked with Dr. Dave Picha to evaluate the potential for using hot water.  While some experiments indicated that hot water (52-54oC for 2-4 mins.) significantly reduced Rhizopus soft rot incidence, in some experiments, hot water treatments increased Rhizopus soft rot.  Planned studies will address how environmental variables, hot water treatments, and post-packing treatments affect wound healing of sweetpotatoes and Rhizopus soft rot susceptibility.

Streptomyces soil rot

Streptomyces soil rot disease, which is caused by the soil-borne actinomycete, Streptomyces ipomoeae was the most significant disease problem on sweetpotatoes in the U.S. prior to the development of resistant cultivars by the LSU AgCenter.  Early on we determined that this pathogen is one of very few prokaryotic pathogens with the ability to directly penetrate plants (Clark and Matthews, 1987).  We also found that strains of this pathogen collected from various locations in the U.S. and Japan were relatively homogeneous when compared using rep-PCR and plasmid profiling (Clark et al., 1998).  However, they could be divided into three groups based on inhibitory interaction phenotypes.  Gregg Pettis in the Department of Biological Sciences has an interest in molecular biology of Streptomyces species and taken this area of research into a molecular study of the inhibitory phenomena that involve production of a bacteriocin-like substance, ipomicin and is actively investigating the mechanisms of pathogenicity in this organism.

Development of Disease-resistant cultivars

The LSU AgCenter sweetpotato breeding program is a highly productive team effort involving scientists in the School of Plant, Environmental, and Soil Sciences, the Sweet Potato Research Station, and the Departments of Plant Pathology & Crop Physiology and Entomology.  Breeding lines from this program are screened for resistance to multiple diseases including Streptomyces soil rot, Fusarium wilt, internal cork, root-knot nematode, bacterial root and stem rot, Fusarium root rot, and others.  The LSU AgCenter  program has been the primary source of resistance to Streptomyces soil rot and has released a number of cultivars, some of which have been very widely adopted by the industry.  The most notable example is Beauregard which accounted for >80% of U.S. sweetpotato acreage during the period 1988-2006.  Since 1980, the following cultivars have been released all of which are resistant to Fusarium wilt and Streptomyces soil rot and have other disease resistance as well:

1980 - Travis (L74-62) - grown on limited acreage in LA for several years

1981 - Eureka (L74-131) - grown on limited acreage in CA for several years

1987 - Beauregard (L82-508) - grown on approx. 80% of US acreage 1990-2005, also grown in Australia and New Zealand.

1992 - Hernandez (L82-66) - grown on approx. 5-10% of US acreage since 1995, mostly in NC

1994 - L86-33 - used as a source of multiple disease and pest resistance in U.S. and international breeding programs

1995 - Darby (L87-59) - not grown commercially to any significant extent

2001 - L96-117 - Patent No. US PP 15,038 P2 awarded to LaBonte, D., Wilson, P., Clark, C., Villordon, A., and Cannon, J.

2002 - Bienville (L94-96) - Patent No. US PP 15,580 P3 awarded to LaBonte, D., Wilson, P., Clark, C., Villordon, A., Cannon, J., Story, R., and Hammond, A.

2007 - Evangeline (L99-35) – U. S. Patent Ser. No. 11/789,681 awarded to LaBonte, D., Villordon, A., Clark, C.

2007 - Muraski-29 (L01-29) – U. S. Patent Ser. No. 11/975,203 awarded to LaBonte, D., Villordon, A., Clark, C.

2011 – Bonita (L05-29) – Patent No. US PP22,719P3 issued to D. R. LaBonte, A. Q. Villordon, T. P. Smith, and C. A. Clark.

2012 – Orleans (L05-111) – Patent No. US PP23,761 issued to D. R. LaBonte, A. Q. Villordon, T. Smith, and C. Clark

2012 – 07-146 – Patent No. US PP23,785 P3 issued to D. R. LaBonte, A. Q. Villordon, T. Smith, and C. A. Clark

Left = the susceptible cultivar Centennial, and right = a resistant breeding line growing in the Streptomyces soil rot nursery.

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Department of Plant Pathology & Crop Physiology
302 Life Science Building
Louisiana State University
Baton Rouge, LA 70803 
(225) 578-1464
 

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