186 Percival and Banks: Water-retaining Polymer and Fungicide Combinations al. 2007a). Chlorophyll fluorescence Fv/Fm ratios are regarded as a highly sensitive measure of damage to photosystem II and therefore, indirectly the leaf photosynthetic apparatus. Conse- quently, Fv/Fm ratios have been used to quantify damage and/or impairment of the leaf photosynthetic apparatus following fun- gal invasion and colonization of the leaf surface (Percival and Fraser 2002). Guignardia leaf blotch is regarded as a foliar blight with enzymatic degradation damage of the leaf, chlorophyll molecule, and photosynthetic system as a consequence of infec- tion (Pastirčáková 2003). Fv/Fm ratios ≥0.75 are associated with healthy plants. Only trees sprayed with penconazole four times throughout the growing season had values higher than 0.75 at the cessation of each experiment. Affected leaves of controls with heavy infection and fungicide/water-retaining polymer treated trees displaying milder infection symptoms were characterized by decreased Fv/Fm, below the 0.75 threshold. The brown or reddish-brown lesions that cover a large portion or even the entire leaf indicate Guignardia leaf blotch exerts its influence over the host via vascular connections (Pastirčáková 2003). It is known that Guignardia leaf blotch produces hydrolytic enzymes that degrade the epidermis, cell wall and necrosis symptoms develop as intercellular mycelium spreads rapidly within the leaf (Schlösser 1983; Pastirčáková 2004). The consequence of such interaction distresses the photosynthetic mechanism directly or indirectly and impairs its ability to quench excitation energy. In normal situations, light capture is accompanied by photochemi- cal and non-photochemical quenching mechanisms that balance photon utilization for electron transport purposes and repair of oxidative damage or heat dissipation (Anderson et al. 1997; Cruz et al. 2004). When absorbed light energy exceeds the leaf’s capac- ity to use trapped energy through photosynthesis or dissipate it by heat, damage to PSII occurs. Results of this study indicate that symptom development in Guignardia leaf blotch infected foliage is associated with increased excitation pressure at PSII centers, followed by oxidative damage and irreversible destruction of cen- ters (Baker et al. 2007; Horton and Ruban 2005; Muller-Moule et al. 2004). Such action leads to loss of chlorophyll and chlorosis (visi- ble Guignardia blotch infection severity) as observed in this study. Penconazole, when applied four times during the growing season, proved 100% effective for Guignardia leaf blotch con- trol. The effectiveness of penconazole against several other fun- gal pathogens under laboratory and field conditions has been confirmed by other authors (Kenyon et al. 1997; Mmbaga and Sauve 2004; Percival and Boyle 2005; Schnabel and Parisi 1997). Results of this study support these conclusions with repeat pen- conazole sprays proving to be the optimal treatment in terms of reduced Guignardia leaf blotch severity, improved photosynthetic efficiency (Fv/Fm), and higher leaf chlorophyll content (SPAD). Results regarding the use of a water-retaining polymer alone on reducing Guignardia leaf blotch development conclude no signifi- cant benefit as observed Guignardia leaf blotch severity, leaf chlo- rophyll fluorescence Fv/Fm values, and leaf chlorophyll content SPAD values were in both the 2007 and 2008 trials comparable to water treated controls. Consequently, use of water-retaining poly- mer alone as Guignardia leaf blotch protectant compound appears limited based on results of this study and are not recommended. In conclusion, results provide evidence that use of commer- cially available fungicides products in combination with a water- retaining polymer applied at the time of planting as a root dip to bare rooted stock can be used to reduce Guignardia leaf blotch ©2013 International Society of Arboriculture severity over a single growing season. However, foliar sprays of an appropriate fungicide would be required the year after planting to keep Guignardia leaf blotch severity to acceptable levels. Further research is ongoing, evaluating fungicide and water-retaining poly- mer root dips against several other key fungal pathogens of trees. Acknowledgments. The author is grateful for fund- ing in part from the TREE Fund (Hyland Johns Grant). LITERATURE CITED Anderson, J.M., Y.I. Park, and W.S. Chow. 1997. Photoinactivation and photoprotection of photosystem II in nature. Physiol Plant 100: 214–223. Baker, N.R., J. Harbinson, and D.M. Kramer. 2007. Determining the limitations and regulation of photosynthetic energy transduction in leaves. Plant Cell Environment 30:1107–1125. Berger, S., A. Sinha, and T. Roitsch. 2007b. Plant physiology meets phytopathology: Plant primary metabolism and plant pathogen inter- actions. Journal of Experimental Botany 58:4019–4026. Berger, S., Z. Benediktyova, K. Matous, K. Bonfig, M.J. Mueller, L. Nedbal, and T. Roitsch. 2007a. Visualization of dynamics of plant- pathogen interaction by novel combination of chlorophyll fluores- cence imaging and statistical analysis: Differential effects of virulent and avirulent strains of P. syringae and of oxylipins on A. thaliana. Journal of Experimental Botany 58:797–806. Blaedow, R.A., W.R. Chaney, P.C. Pecknold, and H.A. Holt. 2006. Investigation of fungicidal properties of the tree growth regulator paclobutrazol to control apple scab. Journal of Arboriculture 32:67–73. Bonfig, K., U. Schreiber, A. Gabler, T. Roitsch, and S. Berger. 2006. Infection with virulent and avirulent P. syringae strains differentially affects photosynthesis and sink metabolism in Arabidopsis leaves. Planta 225:1–12. Chaerle, L., I. Leinonen, H.G. Jones, and D. Van Der Straeten. 2007. Monitoring and screening plant populations with combined thermal and chlorophyll fluorescence imaging. Journal of Experimental Bot- any 58:773–784. Cruz, J.A., T.J. Avenson, A. Kanazawa, K. Takizawa, G.E. Edwards, and D.M. Kramer. 2004. Plasticity in light reaction of photosynthesis for energy production and photoprotection. Journal of Experimental Botany 56:395–406. Davies, M. J., N. A. Hipps, and G. Kingswell. 2002. The effects of indole- 3-butyric acid root dips on the root development and shoot growth of transplanted Fagus sylvatica L. and Quercus robur L. seedlings. The Journal of Horticultural Science and Biotechnology 77:209–216. Doccola, J.J., P.M. Wild, I. Ramasamy, P. Castillo, and C. Taylor. 2003. Efficacy of arborjet viper microinjections in the management of hem- lock woolly adelgid. Journal of Arboriculture 29:327–330. Fraser, G.A., and G.C. Percival. 2003. The influence of biostimulants on growth and vitality of three urban tree species following transplant- ing. Arboricultural Journal 27(1):43–57. Grazia J. De., P.A. Tittonell, and A. Chiesa. 2004. Growth and quality of sweet pepper (Capsicum annum L.) seedlings as affected by sub- strate properties and irrigation frequency. Advances in Horticultural Science 18:181–187. Horton, P., and A. Ruban. 2005. Molecular design of the photosystem II light-harvesting antenna: Photosynthesis and photoprotection. Journal of Experimental Botany 56:365–373.
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