Robert Carver, Clark, Christopher A.
INVESTIGATOR: Clark, C. A.
Plant Pathology & Crop Physiol
LOUISIANA STATE UNIVERSITY
BATON ROUGE, LOUISIANA 70893
BIOLOGY AND MANAGEMENT OF ECONOMICALLY IMPORTANT SWEETPOTATO DISEASES
CLASSIFICATION HEADINGS: R212 . Pathogens and Nematodes Affecting Plants; S1450 . Sweet potato; F1160 . Pathology
NON-TECHNICAL SUMMARY: Diseases reduce the profitability of sweetpotato in all phases of production and marketing. The greatest losses of sweetpotato to disease are the results of viruses that accumulate during vegetative propagation, which contribute to decline in performance of varieties over time, plant bed diseases such as sclerotial blight that kill plants needed for transplanting the crop, postharvest losses due to decay, and field diseases such as Streptomyces soil rot and root knot. It will determine what events that occur in the production field predispose sweetpotatoes to greater development of disease in storage. Expected outcomes include the development of new sweetpotato varieties with improved disease resistance, information to assist growers in determining where to plant seed crops and how to manage those seed crops to avoid reinfection with viruses, methods for treating sweetpotatoes with hot vapor or water on packing lines to prevent Rhizopus soft rot from destroying the sweetpotatoes during marketing without relying on use of fungicides, information on how to manage the crop in the field to reduce the incidence of postharvest disease, and methods to combine use of best quality seed and fungicides to prevent losses of plants to sclerotial blight. The cost or producing sweetpotatoes is very high and profits can be wiped out by reduced yields in the field or loss of crop in the field or in storage after all the expenses of producing the crop have been incurred. Growers must optimize their production and eliminate losses in the field and in storage to remain in business. New disease-resistant varieties and ability to produce seed with minimum loss to viruses are key components to optimizing production. Sweetpotato is increasing in popularity because it is considered one of the most nutritious crops available, but this also means that consumers are likely to be more health conscious and concerned about pesticide use. If Rhizopus soft rot resistance can be combined with heat treatments to alleviate the need for fungicide treatment, sweetpotato will be better able to compete with other commodities for the attention of consumers.
OBJECTIVES: 1. To incorporate multiple disease resistance into new sweetpotato breeding lines with the ultimate goal of generating multiple-disease- and pest-resistant cultivars for the Louisiana and U.S. sweetpotato industries. 2. To isolate and identify pathogens transmitted in sweetpotato propagating material that are involved in cultivar decline and develop management strategies to reduce reinfection of propagating material by these pathogens as it progresses from virus-tested foundation seed through stages of propagation by farmers. 3. To determine the etiology of the complex of end rot diseases that develop on sweetpotatoes during storage, to determine the environmental conditions that predispose disease development, and to develop management methods to minimize the impact of these diseases. 4. To determine the role of microorganisms in soft rots that develop on sweetpotatoes in the field following flooding. 5. To develop alternative strategies for integrated management of Rhizopus soft rot to reduce reliance on prophylactic use of fungicides. 6. To develop methods for managing sclerotial blight in plant beds on susceptible cultivars such as Evangeline.
APPROACH: The project will continue ongoing efforts to develop multiple-disease- resistant sweetpotato cultivars using established methods for screening. Different insect traps will be used to relate insect vector population dynamics to sweetpotato virus spread and potential reservoirs will be evaluated for their contribution in the field to sources of virus inoculum by determining vector behavior on these hosts. A combination of biological and molecular approaches will be used to identify, characterize, and develop detection methods for unknown viruses in sweetpotatoes. Outputs from this research will be used to modify procedures used by the LSU AgCenter Sweetpotato Foundation seed program and in collaboration with the sweetpotato extension specialist to modify production practices of sweetpotato certified seed growers. Systematic isolations from sweetpotato vines and roots prior to and after harvest will be conducted to both determine the organisms that are involved in inducing end rots and to determine the dynamics of microbial populations in response to flooding/hypoxia. Various hot vapor/water treatments will be evaluated for their potential in reducing Rhizopus soft rot in the post-packing environment. Outputs and outcomes of end rot and postharvest disease research will be used in an effort coordinated through the extension services of participating states and the Sweetpotato CAP Advisory Committee initiated as part of the project funded by USDA, SCRI "Participatory Modeling and Decision Support for Improving Sweetpotato Production Efficiency, Quality and Food Safety" to improve practices in the industry. The role of seed quality in development of sclerotial blight on Evangeline will be investigated using roots that have been flooded in the previous season and/or that are affected by circular spot. Fungicides with known efficacy in controlling Sclerotium rolfsii will be evaluated as potential adjuncts or replacements for dicloran.
KEYWORDS: sweetpotato; ipomoea batatas; disease; resistance; virus; flood tolerance; postharvest disease
PROGRESS: 2010/01 TO 2010/12
OUTPUTS: An annual output of this project is production of virus-tested tissue cultures that are provided directly or indirectly through Certis, USA, to the LSU AgCenter sweetpotato foundation seed program and the Auburn University sweetpotato foundation seed program. These serve as the nuclear stock for production of virus-tested foundation seed which is sold to growers throughout Louisiana and if supplies permit, to growers in other states. A three-year study was completed in 2009 and a publication prepared and published in 2010 which describes the value of virus-tested foundation seed to growers. This information was also extended to growers at the Louisiana Sweet Potato Association Annual Meeting and at a field day held at the Sweet Potato Research Station. Efforts to improve disease resistance in sweetpotato cultivars have contributed to the development by the LSU AgCenter sweetpotato breeding team of a new cultivar, Bonita, which was released in 2010 and will be available to growers as a cultivar for the white-fleshed sweetpotato market niche. Bonita has resistance to Streptomyces soil rot, root-knot nematode, and Fusarium wilt. PARTICIPANTS: Christopher A. Clark, PI; Mary W. Hoy and Rebecca Sweany, research associates; Everlyne Wosula and Washington da Silva, graduate students. TARGET AUDIENCES: Sweetpotato growers, packers, and consumers. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
IMPACT: 2010/01 TO 2010/12
Screening for resistance to important diseases has led to development of breeding lines with resistance to Streptomyces soil rot, root-knot nematode, Fusarium wilt, Rhizopus soft rot, and other storage diseases, and recently released cultivars such as Bonita combine resistance to these diseases. A simple test in which sweetpotato storage roots are submerged in sterile distilled water in sealed Mason Jars and incubated at 32 C showed that Beauregard developed soft rot [flood-induced soft rot (FISR)] more rapidly than Evangeline, which correlates with anecdotal observations of their performance in flooded fields. FISR was delayed when roots were surface disinfested with NaOCl prior to incubation, suggesting that inoculum was on the surface of the roots. However, when roots were placed in polyethylene bags and incubated at 32 C, Evangeline roots developed end rots more rapidly than Beauregard. End rot development was either enhanced or unaffected by surface disinfestation, suggesting that the pathogens were already inside the roots. Etiology and epidemiology of FISR and end rots are subjects of continuing investigation. Using real-time PCR measurements of Sweet potato leaf curl virus (SPLCV) titer and field measurements of the effects on yield, we have found that available sweetpotato germplasm include lines with resistance to virus accumulation and tolerance to effects on yield. Should SPLCV become more prevalent in the field, high-yielding, orange-fleshed cultivars are available that have resistance and tolerance to the predominant strain of SPLCV. A second year of field monitoring found that sweetpotato potyviruses were not transmitted to trap plants in plant beds except very late in the bed production, but were transmitted during the last 60 to 80 days of the field season. Aphids were present throughout the season and species present are being identified. Aphids transmitted Sweet potato feathery mottle virus (SPFMV) at a greater rate from plant from natural, mixed infections than from plants with single infections and from Ipomoea hederacea more readily than from sweetpotato. Preliminary greenhouse studies indicated that stylet oils have potential for reducing transmission of sweetpotato potyviruses and the epidemiological studies will be used in future efforts to identify times when interventions such as stylet oil might be used to protect sweetpotato seed and production crops from reinfection with these viruses.
PUBLICATIONS (not previously reported): 2010/01 TO 2010/12
1. Villordon, A., Clark, C., Smith, T., Ferrin, D., and LaBonte, D. 2010. Combining linear regression and machine learning approaches to identify consensus variables related to optimum sweetpotato transplanting date. HortScience 45:684-686.
2. Clark, C. A., Smith, T. P., Ferrin, D. M., and Villordon, A. Q. 2010. Performance of sweetpotato foundation seed after incorporation into commercial operations in Louisiana. HortTechnology 20:977-982.
3. Clark, C. A., Ferrin, D. M., Hoy, M. W., and Smith, T. P. 2010. Evaluation of fungicides applied at different times for control of sclerotial blight on sweetpotato, 2009. Plant Disease Management Reports 4:V040.
4. Villordon, A., Solis, J., LaBonte, D., and Clark, C. 2010. The Bayesian choice: towards a decision-theoretic approach of determining irrigation frequency and amount in sweetpotato production. National Sweetpotato Collaborators Group Progress Report, 2009:12.
5. Wosula, E., Davis, J. A., and Clark, C. A. 2010. Aphid population dynamics and potyvirus movement in Louisiana sweetpotato fields. National Sweetpotato Collaborators Group Progress Report, 2009:13.
6. Villordon, A., and Clark, C. 2010. Some results from simulated flooding studies with in situ and detached storage roots: methods and potential pitfalls. National Sweetpotato Collaborators Group Progress Report, 2009:15.
Name: Clark, C. A.