Progress 10/01/04 to 09/30/05
Outputs
The first field test of greenhouse selected tetrazygs (hybrid tetraploid rootstock candidates produced by crossing tetraploid somatic hybrid parents) was planted in spring 2005 in a commercial block with high Diaprepes pressure to validate the greenhouse screening program. Hamlin sweet orange grafted on replicates of 14 new rootstock candidates with a range of performance in the greenhouse tests were included. Two additional challenge/recovery assays were conducted. The first assay included 23 tetraploid and 9 diploid hybrids pre-selected for adaptation to high pH, calcareous soil inoculated with Phytophthora. All selections were subjected to Diaprepes force-feeding challenges. Hybrids selected for reduced mechanical damage were replanted in a 'Winder' Phytophthora mix to assess their root system recovery potential. Two hybrids performed extremely well (a diploid hybrid of Ambylcarpa mandarin x Hirado Buntan pummelo, and a tetraploid hybrid of Nova + HB pummelo x
Succari sweet orange + Murcott), and 4 other hybrids showed promise. These hybrids are being propagated for field evaluation. The second test was conducted in the same manner and one promising hybrid was identified. This hybrid is also being propagated for further study. More than 150 hybrids were selected from 2004 crosses and propagated by rooted cuttings for the greenhouse Challenge/Recovery Assays to be conducted during 2006. New crosses were also conducted during 2005 to generate more promising germplasm for subsequent testing. Field studies of the spatial variability of D. abbreviatus and soil and environmental characteristics showed that the abundance of Diaprepes adults was highest in high elevation areas and was negatively correlated with soil Mg and Ca. These results suggest possible relationships between Diaprepes, tree decline, soil liming, periodic flooding, and drainage. Neonate larval behavior during entry into soil indicate that as soil moisture increases to near
saturation (16-18%), soil penetrability generally decreases. Physical factors such as a 'sticky' soil surface impair larval entry, whereas a dry, loose surface soil layer improves larval entry. These results differ from earlier reports suggesting that neonates fail to penetrate dry soil. Field and laboratory studies demonstrated that augmenting numbers of entomopathogenic nematodes (EPN) in soil can initiate a food web cascade in which the prevalence of nematode antagonists and competitors temporarily increase at the expense of endemic EPN and natural biological control of insect pests. This non-target effect was more pronounced for steinernematid than heterorhabditid EPN species. Application of composted manure as a mulch beneath trees decreased the prevalence of nematophagous fungi, increased the prevalence of endemic EPN, and increased the mortality of sentinel weevil larvae. Eleven isolates of Heterorhabditis indica from diverse habitats in Florida were >99% congruent for the
internal transcribed spacer regions (ITS1 and ITS2) of rDNA. Assays comparing the ability of the isolates to move, persist, and kill weevil larvae in soils of different textures are ongoing.
Impacts
Rootstock tolerance to the Diaprepes-Phytophthora complex is unavailable and may be the only effective management tactic in habitats that are highly conducive to the pest-disease complex. Our program to select tolerant tetrazyg lines is promising in this respect. Reported correlations between regional edaphic conditions and weevil-fungus pressure suggest the involvement of physical factors and the possibility that cultural practices may be useful for managing the complex. The relationships between Diaprepes, tree decline, soil liming, periodic flooding, and drainage is of interest in this regard. EPN prevalence and species diversity are also regionally defined in Florida and growers may one day benefit from augmenting with regionally adapted EPN. The augmentation experiments suggest that augmenting the endemic EPN community with appropriate EPN species and use of some types of mulches may mitigate temporary non-target effects. They demonstrate the importance of timing
EPN augmentation events based on patterns insect recruitment into the soil in order to avoid creating non-target effects during periods of intense weevil recruitment.
Publications
- Barbercheck M. and L. W. Duncan. 2004. Abiotic factors. In: Nematode Biology. R. Gaugler and B. Bilgrami, Eds. CAB International, St. Albans, UK, p. 309-343.
- El-Borai, F. E., L. W. Duncan, and J. F. Preston. 2005. Bionomics of a phoretic association between Paenibacillus sp. and the entomopathogenic nematode Steinernema diaprepesi. Journal of Nematology 37:18-25.
- Georgis, R., G. Belair, L. W. Duncan, P. S. Grewal, A. M. Koppenhofer, L. A. Lacey, M. Samish, P. Torr, and R. W. H. M. van Tol. 2005. Successes and failures of entomopathogenic nematodes. Biological Control (in press).
- Grosser, J. W. 2004. What's New In Citrus Rootstock Improvement: Introducing 'Mandelos' and 'Tetrazygs'. Citrus Industry Magazine 85(2):22-24.
- Grosser, J.W. and Gmitter, F.G. Jr. 2005. 2004 SIVB Congress Symposium Proceedings 'Thinking Outside the Cell' -- Applications of somatic hybridization and cybridization in crop improvement, with citrus as a model. In Vitro Cell. Dev. - Plant 41:220-225.
- Li, H., J. P. Syvertsen, C. W. McCoy, R. J. Stuart, and A. W. Schumann. 2004. Soil liming and flooding effects on Diaprepes root weevil larval survival and citrus seedling growth. Proceedings of the Florida State Horticultural Society 117:139-143.
- Li, H., J. P. Syvertsen, R. J. Stuart, C. W. McCoy, and A. W. Schumann. 2004. Using overlay patterns of soil organic matter content, electrical conductivity and Diaprepes root weevil populations for delineating management zones in citrus. Proc. 7th Int. Conf. on Precision Agric., Minneapolis, MN. p. 1-10. Title: Progress in the Development of New Rootstocks Tolerant of the Diaprepes/Phytophthora Complex.
- Li, H., J. P. Syvertsen, R. J. Stuart, C. W. McCoy, A. W. Schumann, and W. S. Castle. 2004. Soil and Diaprepes abbreviatus root weevil spatial variability in a poorly drained citrus grove. Soil Science 169:650-662.
- Li, H., R. J. Stuart, J. P. Syvertsen, S.H. Futch, C.W. McCoy, and A.W. Schumann. 2006. Associations of Soil pH, Water, Magnesium and Iron with Citrus Tree Decline and Diaprepes abbreviatus Root Weevil Distributions in Two Fields. Journal of Experimental Botany (in press).
- Li, H., J. P. Syvertsen, C. W. McCoy, R. J. Stuart, and A. W. Schumann. 2006. Water stress and root injury from simulated flooding and Diaprepes root weevil feeding in citrus. Soil Science (in press).
- McCoy, C. W., W. S. Castle, J. H. Graham, R. J. Stuart, J. P. Syvertsen, and A. W. Schumann. 2004. Grove research: foliar sprays reduce Diaprepes-induced tree injury. Citrus Industry 85:23-25.
- McCoy, C. W., W. S. Castle, J. H. Graham, J. P. Syvertsen, A. W. Schumann, and R. J. Stuart. 2004. Pesticide suppression of Diaprepes abbreviatus (L.) (Coleoptera: Curculionidae) promoted differential growth and survival of 'Hamlin' orange trees budded to five rootstocks in a Phytophthora infested grove. Proceedings of the Florida State Horticultural Society 117:167-173.
- McCoy, C. W., R. J. Stuart, M. E. Rogers, H. N. Nigg, and W. S. Castle. 2006. Management of adult Diaprepes root weevil with foliar sprays, 2004. Arthropod Management Tests (in press).
- Shapiro-Ilan, D. I., L. W. Duncan, L. A. Lacey, and R. Han. 2005. Orchard crops. In: Nematodes as Biological Control Agents. P. Grewal, R-U Ehlers, and D. Shapiro-Ilan, Eds. CAB International, St. Albans, UK (in press).
|