Arboriculture & Urban Forestry 34(5): September 2008 alphitoides) when applied as a single therapeutic spray treatment.A short-term (20 day) reduction in leaf disease severity was, however, recorded with salicylic acid and potassium phosphite. Research by the author also indicated a single therapeutic spray treatment of the four SIR agents used in this investigation have no long- term effects on disease severity of other foliar pathogens such as apple scab (Venturia inaequalis) and Guignardia leaf blotch (Guignardia aesculi) (Percival, unpublished data). Induction of antioxidant and defensive enzymes such as su- peroxidase dismutase and peroxidase are recognized as initial SIR responses in plants (Kessmann et al. 1994). No such re- sponse was recorded in this investigation after RESISTIM ap- plication indicating no SIR-inducing properties. In addition, no significant role of betaine in the form of RESISTIM in enhanc- ing tree resistance against oak powdery mildew was recorded when applied as a single therapeutic treatment. Research in Australia and the United States have found phos- phites to be effective in the control of diseases, particularly those that belong to the oomycetes group such as Phytophthora spp., Pythium spp., and the Downy Mildew diseases (Jackson et al. 2000; Wilkinson et al. 2001; Miller et al. 2006). In addition, phosphites control other diseases that fall outside this group such as the bacterial disease Erwinia amylovora (apple fireblight). Potassium phosphite has been shown to provide a significant degree of control against Venturia inaequalis when applied as foliar sprays at 10 to 12 day intervals (MacHardy and Jeger 1983). Application of phosphites and salicylic acid as a single therapeutic spray treatment, however, appeared to offer a short-term reduction in disease severity up until day 20 after application, but by the cessation of the growing season, no long-term control of oak powdery mildew under field conditions was recorded. The use of the harpin protein (trade name Messanger) has been shown to reduce disease severity of Botrytis cinerea on leaves and fruit of pepper (Capsicum annuum L. var. cvs. ‘Demre’, ‘Yalova Charleston’, and ‘Sari Sivri’) as measured by leaf chlorophyll content (Akbudak et al. 2006). In this study, application of harpin protein provided a significant degree of protection against oak powdery mildew in the 2005 trial. No significant long-term effects, however, were recorded in the 2006 trial. Such a re- sponse reflects those recorded by Agostini et al. (2003) who evaluated the influence of harpin protein (Messenger) on en- hanced resistance of potted lemon seedlings against citrus scab (Elsinoe fawcettii), grapefruit against melanose (Diaporthe citri), and Dancy tangerine against Alternaria brown spot (Alternaria al- ternata pv. citri). In the case of Diaporthe citri and (Alternaria alternata pv. citri), no enhanced resistance effects were recorded after application of harpin protein. In the case of citrus scab, Messenger provided a significant degree of control in some stud- ies but not others. A number of reasons were postulated for this lack of consistency. Plant phenology has been shown to be im- portant, with harpin protein showing greater effectiveness when applied at budbreak before inoculum buildup. Efficacy was also influenced by the weather conditions that prevailed at the time of application with cool wet weather not conducive for efficacy (Agostini et al. 2003). The potential shelf life and batch-to-batch variation of biologic propagules in terms of reduced viability has been highlighted by Downer (2007) that may also partially ex- plain the lack of consistency shown by the harpin protein. These problems should not distract from the fact that a single spray treatment of harpin protein during the 2005 growing season pro- vided season-long control of oak powdery mildew comparable 277 with repeat three-weekly applications of a commercially avail- able triazole fungicide. Such a response indicates that further research to clarify phonologic and climatic influences on the efficacy of SIR-inducing chemicals may help develop important tree protectant compounds. In addition, results of this study in- dicate a short-term (20 day) reduction in leaf disease severity after spray application of harpin protein. A positive relationship between enhanced leaf peroxidase and superoxide dismutase activities and reduced oak powdery mil- dew were observed up to 20 days after systemic induction. By day 60 after SIR treatment, however, any induced protection was no longer manifest with disease severity reflecting those of con- trols. Such a result indicates repeat application of systemic in- ducement at 20 day intervals may be warranted to enhance re- sistance against oak powdery mildew over time. Repeat spray applications of penconazole resulted in a signif- icant increase in peroxidase activity in English oak. Likewise, repeat spray applications of penconazole resulted in higher leaf activities of superoxide dismutase compared with controls, if not significantly so, in all cases. Reductions in disease severity as a result of direct fungicidal properties of penconazole is through inhibition of the C4-demethylase reactions in sterol biosynthesis of fungi (Allingham 2005). However, all triazole-based fungi- cides have been shown to induce a suite of morphologic adap- tations to include promotion of leaf catalase and superoxide dis- mutase activity that allow plants to tolerate a range of environ- mental stresses (Kraus and Fletcher 1994). These antioxidant enzymes and pigments are vital in quenching high-energy reac- tive oxygen species (ROS) such as superoxide and singlet oxy- gen produced in response to abiotic stresses such as salinity, drought, and atmospheric pollutants (Apel and Hirt 2004). Con- sequently, a number of authors suggest that triazole-induced pro- tection from abiotic stress is an important factor in improving tree vitality that in turn allows the tree’s own natural defense mechanisms to reduce subsequent pathogen attack (Zhang et al. 1994; Fletcher et al. 2000). In conclusion, results of this investigation indicate that in the case of oak powdery mildew, a positive association was noted between increased activities of peroxidase and superoxide dis- mutase and reduced disease severity in the short term. Such benefits were, however, not manifest over a longer time period indicating repeat 20 day interval sprays are required. Further research evaluating the influence of repeat SIR treatments are ongoing against a range of foliar and root pathogens. LITERATURE CITED Agostini, J.P., P.M. Bushong, and L.W. Timmer. 2003. Glasshouse evaluation of products that induce host resistance for control of scab, melanose, and Alternaria brown spot of citrus. Plant Disease 87:69–75. Akbudak, N., H. Tezcan, B. Akbudak, and V. Seniz. 2006. Effect of harpin protein on plant growth parameters, leaf chlorophyll, leaf col- our and percentage rotten fruit of pepper plants inoculated with Bot- rytis cinerea. Scientia Horticulturae 109:107–112. Allingham, R. 2005. The effect of the growth retardant paclotutrazol on the in vitro growth and development of Betula and Populus species. MSc Thesis, Dept. of Environmental Management, University of Central Lancashire. Apel, K., and H. Hirt. 2004. Reactive oxygen species: Metabolism, oxidative stress, and signal transduction. Annual Review of Plant Biology 55:373–399. Berglund, T., and R. Ohlsson. 1995. Defensive and secondary metabo- lism in plant tissue cultures, with special reference to nicotinamide, ©2008 International Society of Arboriculture
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