Larry Duncan

(100% Research)

1. Education:

• B.S., Michigan State University (Psychology), 1971
• M.S., Univ. of Georgia (Master of Pl. Protect. and Pest Mgt.), 1970
• Ph.D., University of California (Nematology), 1983

• Associate Professor (1989-present) University of Florida
• Assistant Professor (1985-1989) University of Florida
• Researcher (1983-1984) Office de Recherche Scientifique et
• Technique Outre-Mer, Dakar Senegal

• Nematode management in citrus. Our research approach is based on IPM concepts that emphasize:
  • knowledge of the biology and ecology of the target organism
  • quantitative host-parasite interactions and biomonitoring
  • development of new methods of control with current technology that are compatible with economic and environmental requirements.

  During the past 5 years, we have emphasized biomonitoring with respect to nematode spatial patterns in soil and sample size requirements.With respect to biology/ecology, we have investigated relationships between seasonal and annual patterns of nematode population change and those of citrus fibrous root chemistry, and edaphic (salinity) and climatic (soil moisture) factors. We have investigated interactions between nematodes and soil fungi. We are investigating nematode attraction to roots and chemicals in root exudates. We have several programs to assess crop loss due to nematodes.

  • Nematode management programs have been considered with respect to:
    • sanitation (identification of biotypes of citrus and lesion nematodes for regulatory purposes);
    • host plant resistance (identification of DNA markers for resistance, conventional screening);
    • effects of cultural practices on expression of the "disease spreading decline";
    • biological control (entomopathogenic nematodes to control root weevils);
    • chemical control (efficacy of registered and experimental chemicals, novel approaches).

  4. Accomplishments:

  • Quantitative Nematology and Biomonitoring:
  • Demonstrated that several commonly used methods to estimate sample size in nematology are biased toward conservative estimates.
  • Demonstrated that use of nematology data from small plot studies tend to underestimate sampling requirements.
  • Derived sample size estimates for two species of nematode parasites of citrus.
  • Developed novel method to assess crop-loss by nematodes on mature perennial hosts.
  • Demonstrated 22% yield loss due to citrus nematode in mature citrus.

  Nematode Biology/Ecology:

  • Conducted demographic surveys and experiments supporting the hypothesis that population densities of nematode parasites of perennial plants are regulated in part by seasonal and annual patterns of carbohydrate availability.
  • Demonstrated that intermittent soil salinity predisposes citrus roots to support higher densities of citrus nematodes.Nematodes increase salinity ions in leaves, reduce salinity ions in roots and reduce nutrient ions (most notably K) in leaves and roots. Citrus nematodes increase non-structural carbohydrate concentrations in roots.

  Nematode Management:

  • Characterized biotypes of citrus nematodes and coffeae lesion nematodes in Florida citrus orchards.Movement of both nematodes is restricted by quarantine regulations of nursery planting stock.New characterization of lesion nematodes permits discrimination of biotypes that are capable of infecting citrus from several others for which citrus is a non-host.
  • Demonstrated attraction to various organic salts by the citrus nematode.
  • Increased efficacy of several nematicidal chemicals using nematode attractants as baits.
  • Reduced root infection by nematodes using chemical attractants as confusants.
  • Developed first sample size estimates to characterize population density of entomophilic nematodes in soil.
  • Estimated survivorship of three entomophilic nematode species in citrus rhizosphere in Florida.
  • Established first efficacy estimates for two nematode species against citrus root weevils.
  • Identified four molecular markers of a gene for resistance to citrus nematodes.
  • Characterized relationship between ploidy level of citrus rootstocks and host resistance to burrowing nematodes.

  5. Grants and Contracts: ($376,500)

  • Citrus Research Advisory Council (1993-95),
  • USAID-Egyptian National Agricultural Research Program (1991-94),
  • Miles Corporation (1989-94),
  • ISK Corporation (1990-94),
  • Ecogen Corporation (1994),
  • Polk County Trade Show Commission (1991, 1994),
  • Dow Chemical Corporation (1990).

  6. Career Publications:

  • Books: 0
  • Refereed papers: 31
  • Chapters in Books: 5
  • Reviews: 2

  7.Selected Publications:

  1. Duncan, L.W.1986.Effects of bare fallow on plant-parasitic nematodes in the Sahelian zone of Senegal.Revue Nematol. 9(1):75-81.
  2. Duncan, L.W.1991.Current options for nematode management.Annu. Rev. Phytopathol.29:469-490.
  3. Duncan, L.W. and D.M. Eissenstat.1993.Responses of Tylenchulus semipenetrans to citrus fruit removal:Implications for carbohydrate competition.J. Nematol.25(1):7-14.
  4. Duncan. L.W., M.M. El-Morshedy, and R. McSorley.1994.Sampling citrus fibrous roots and Tylenchulus semipenetrans.J. Nematol. (inpress).
  5. Duncan, L.W., J.H. Graham, and L.W. Timmer.1993.Seasonal patterns associated with Tylenchulus semipenetrans and Phytophthora parasitica in the citrus rhizosphere.Phytopathology. 83(5)573-581.
  6. Duncan, L.W., R.N. Inserra, and J.H. O'Bannon.1994.A resistance-breaking population of Tylenchulus semipenetrans from Florida. Plant Disease (in press).
  7. Duncan, L.W., D.T. Kaplan, and J.W. Noling.1990.Maintaining barriers to the spread of Radopholus Citrophilus in Florida Citrus Orchards.Nematropica 20(1):71-88.
  8. Duncan, L.W., J.D. Toole, R.N. Inserra, W.S. Castle, and J.H. O'Bannon. 1994.A comparison of two sampling methods to detect Radopholus citrophilus in Florida citrus.Soil Crop Sci. Fla. Proc. 53(in press).

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