86 Percival and Noviss: Potassium Phosphite and Myclobutanil for Pear Scab (Venturia pirina) Suppresion Arboriculture & Urban Forestry 2010. 36(2): 86–92 Evaluation of Potassium Phosphite and Myclobutanil Combinations for Pear Scab (Venturia pirina) Suppression Glynn C. Percival and Kelly Noviss Abstract. The purpose of this study was to investigate the use of potassium phosphite (Pi) alone and in combination with a reduced dose of synthetic fungicide (myclobutanil) to control pear scab (Venturia pirina) under field conditions. Irrespective of Pi and myclobutanil concentration, no leaf or fruit phytotoxic effects were observed throughout the 2006 and 2007 experiments. In both field experiments, Pi alone significantly reduced the in- cidence and severity of V. pirina on leaves and fruit compared to water-treated control with the degree of scab reduction similar to that of a reduced dose of myclobutanil. The efficacy of Pi at 20 ml per liter water in inhibiting V. pirina scab incidence and severity was superior than that of Pi at 10 ml per liter water and a reduced dose of myclobutanil in virtually all monthly assessments. Combining a reduced dose of myclobutanil with either Pi at 10 ml or 20 ml per liter significantly improved the efficacy of scab control compared to stand alone applications of each product at most monthly assessments. Data analyzed with Limpel’s formula indicated a positive synergistic effect between Pi and a reduced dose of myclobutanil. Greatest reductions in V. pirina incidence and severity on leaves and fruit were, however, achieved by stand alone applications of myclobutanil at manufac- turers recommended strength. Irrespective of year, crown volume, number of fruit per tree, and total fruit yield were higher in Pi and myclobutanil treated trees irrespective of concentration applied compared to water-treated controls. A combined mix of Pi with a reduced dose of myclobutanil proved effective in increasing crown volume, number of fruit per tree and fruit yield compared to stand alone applications of each treatment. Great- est increases in crown volume, number of fruit per tree and fruit yield were achieved by applications of myclobutanil at the manufacturers recom- mended strength. In virtually all cases, Pi combined with a reduced dose of myclobutanil induced positive synergistic effects on crown volume and fruit yield greater than their additive effects alone. Mean fruit weight per tree were in all cases higher in Pi and myclobutanil treated trees irrespective of concentration applied compared to water-treated controls, however, these differences were not statistically significant in all cases. Key Words. Fungicides, Integrated Disease Management; Orchard Management; Pathogen Control; Plant Health Care; Synergism; Urban Landscapes. Pear scab caused by Venturia pirina causes significant economic losses annually in many countries where pears are grown for human consumption (Villalta et al. 2004). In addition, ornamen- tal pear species are planted into urban landscapes (e.g., streets, public recreation areas, car parks) for aesthetics such as flow- ers, bark, berry, and leaf color. Repeated pear scab infection can result in tree mortality and/or undesirable reductions in aesthetic appearance. Controlling pear scab requires the frequent applica- tion of synthetic fungicides each season depending on weather conditions (Washington et al. 1999). Public demands to reduce fungicide use, stimulated by greater awareness of environmental and health issues, as well as development of fungicide tolerant strains of scab has placed greater emphasis on the development of reduced fungicide control strategies (Gozzo 2003; Fobert and Després 2005; Ilhan et al. 2006). Likewise, increased legislative restrictions regarding the registration, use and application of pes- ticides has led to a situation within Europe whereby more fun- gicides are withdrawn on an annual basis than released onto the commercial market, which in turn increases selection pressure for fungicide resistance in surviving populations (Anonymous 2009). For these reasons, research has focused on the development of a range of novel plant protection products that have limited or no known adverse effect on the environment and human health (Garbelotto et al. 2007). Pertinent examples include application ©2010 International Society of Arboriculture of inorganic calcium and boron fertilizers, biologically inert film forming polymers, systemic inducing agents, and biostimulants (Sutherland and Walters 2002; Ippolito et al. 2005; Rolshausen and Gubler 2005; Akbudak et al. 2006; Percival et al. 2006; Per- cival et al. 2009). The consensus of research opinion in the ma- jority of cases however, concludes that most of these novel plant protection products are generally less effective and consistent than standard synthetic fungicides for pathogen control (Agos- tini et al. 2003; Krokene et al 2008; Percival and Haynes 2008; Percival et al. 2009). This has led to the suggestion that a more appropriate role for these types of products would be in combina- tion with a reduced dose of synthetic fungicide to achieve control comparable or significantly higher than stand alone applications of fungicides at full dose (Bécot et al. 2000; Van Loon et al. 2002; Il- han et al. 2006). This in turn would reduce potential environmental impacts and extend the working life of existing fungicide products. One family of potential plant protection products are inorgan- ic potassium and phosphite salts. When applied to plants either as a foliar spray or soil drench, phosphites exhibit a complex mode of action, acting both on the pathogen (direct) and by stimulating plant host defense responses (indirect), such as: the accumula- tion of phytoalexins, hypersensitive cell death, cell wall ligni- fication and fortification and formation of lytic enzymes that inhibit pathogen growth (Guest and Grant 1991; Garbelotto et al.
March 2010
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