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 after they are released and even the most popular cultivars are replaced about every 15-20 years.  The reasons for the 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.  During the past 20 years, a number of other viruses have been identified from sweetpotato, but not much is known about their roles in cultivar decline (Valverde et al., 2007).  Therefore, our research has been directed in part to determine what pathogens might be interacting with SPFMV to cause cultivar decline.  We found that two other potyviruses, Sweet potato virus G (SPVG) and Sweet potato virus 2 (SPV-2) also occur commonly in sweetpotatoes in the southeast U.S. (Souto et al., 2003).  In addition, 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, however, none of these viruses or combinations of them has produced the same yield depression obtained when using ‘naturally infected’ plants from farmers’ fields as inoculum (Clark and Hoy, 2006).

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 SPV-2 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 the viruses and the sweetpotato host plant 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 are conducting 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.  We are also collaborating with Dr. Jeff Davis in Entomology to learn more about the dynamics of insect vector populations to identify the most effective sources of virus inoculum and the timing of virus spread in the field.

Sweetpotato Virus Disease (SPVD) is a disease that has long been known in Africa and more recently in South America and is caused by the synergistic interaction of SPFMV and the whitefly-transmitted crinivirus, Sweet potato chlorotic stunt virus.  SPVD causes much more dramatic and persistent symptoms in sweetpotato than are seen in the U.S. and can cause yield reductions on U.S. cultivars of 80-90%.

Left = virus-tested Beauregard, right = Beauregard with SPVD following inoculation with the russet crack strain of SPFMV and the US strain of SPCSV.

All the essential components for SPVD appear to occur in the U.S.: SPFMV is essentially universal in sweetpotatoes in the U.S., several species of aphid vectors for SPFMV, including Aphis gossypii and Myzus persicae are common and this virus spreads rapidly in the field; SPCSV has been found, but is not common, and its whitefly vectors Bemisia tabaci and Trialeurodes abutilonea also occur, but are not as abundant as the aphid vectors of SPFMV.  We are pursuing two general lines of research with SPVD: we are working with Dr. Don LaBonte to develop, improve and understand the nature of resistance to SPVD, and we are trying to understand why SPVD does not occur in the U.S. despite the presence of the components.  Charalambos Kokkinos and Cecilia McGregor used microarray technology to determine that >200 genes are differentially regulated in sweetpotato affected with SPVD whereas only 12 genes were differentially regulated by SPCSV and 3 genes by SPFMV (Kokkinos et al., 2006).  Douglas Miano at the Kenya Agricultural Research Institute Biotechnology Centre developed molecular markers for SPVD resistance as part of his PhD dissertation (Miano et al., 2008) and we are continuing to do research to understand how resistance affects replication and translocation of SPFMV and SPCSV in the plant.  We are also doing research into factors that affect SPVD symptom development, such as sequence of virus infection (McGregor et al., 2009) and a putative transmissible suppressor of SPVD.

 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 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.  Smart Spud® devices that measure impacts were used on packing lines in Louisiana and North Carolina to identify problem areas.  Results from these studies along with recommendations on other aspects of postharvest handling were summarized in an extension bulletin prepared by the participants from North Carolina and Louisiana (Edmunds et al., 2008).

During 2004-2006, 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 60^oF for 100-120 days and inoculated with R. stolonifer and Dickeya didantii (syn. Erwinia chrysanthemi) using a standardized wounding techniques and uniform inoculum.  Data were 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 (Villordon et al., 2009) 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 are also working with Dr. Dave Picha to evaluate the potential for using hot water.

Streptomyces soil rot

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.

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

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