Arboriculture & Urban Forestry 34(5): September 2008 Arboriculture & Urban Forestry 2008. 34(5):271–279. 271 The Influence of Systemic Inducing Resistance Chemicals for the Control of Oak Powdery Mildew (Microsphaera alphitoides) Applied as a Therapeutic Treatment Glynn Percival and Ian Haynes Abstract. A 2 year field trial was conducted using established English oak (Quercus robur L.) to assess the efficacy of four commercially available systemic-inducing resistance (SIR) compounds (salicylic acid, potassium phosphite, harpin protein, be- taine) applied as a single therapeutic spray treatment against the foliar pathogen oak powdery mildew (Microsphaera alphitoides). In addition, a comparative evaluation of a conventional spray program (3 week spray intervals) used within the United Kingdom for powdery mildew control was conducted using the fungicide penconazole. The SIR-inducing compound containing betaine and a single spray treatment of penconazole had no significant influence on disease severity and specific activity of peroxidase and superoxide dismutase in both the 2005 and 2006 trials. Salicylic acid and potassium phosphite had no significant long-term effect on disease severity, although a short-term reduction in disease severity was recorded that was associated with enhanced leaf peroxidase and superoxide dismutase activity. A single therapeutic application of the SIR-inducing agent harpin protein signifi- cantly reduced disease severity of powdery mildew in the 2005 trial. No significant effects, however, were recorded in the 2006 trial. Only repeat spray applications of penconazole significantly reduced disease severity of oak powdery mildew in the 2005 and 2006 trials. The fungicide penconazole appears also to posses marginal SIR properties. Key Words. Antioxidant pigments; defensive enzymes; fungicides; integrated disease management; pathogen control; plant health care; urban landscapes. Oak (Quercus spp.) are highly susceptible to the foliar disease powdery mildew (Microsphaera alphitoides Griffon and Maubl.). Conventional control of powdery mildew requires the application of 10 to 22 fungicide applications per season depend- ing on weather conditions (Hibbard et al. 1996; Sabri et al. 1997). The potential for plant pathogens to overcome or to de- velop insensitivity to conventional fungicides and increased leg- islative restrictions regarding the use and application of pesti- cides means new techniques of tree pathogen control are of environmental and economic importance (Agostini et al. 2003). Plants can defend themselves against pathogen infection through a wide variety of mechanisms that can be local, consti- tutive, or inducible (Franceschi et al. 2000). Inducible resistance mechanisms such as systemic-induced resistance (SIR) can be acquired by challenging a plant with a weaker strain of a specific pathogen or exposing a plant to natural and/or synthetic com- pounds (Percival 2001). Formulations of commercial SIR- inducing chemicals such as Messenger (Eden Bioscience Corp., N. Bothell, WA) in the United States, Bion (Syngenta Crop Protection UK Ltd., Whittlesford, Cambridge, U.K.) in Europe, Agri-Fos (Agrichem Ltd., Queensland, Australia) in Australia and the United States, and Oryzemate (Meiji Seika Kaisha, Ltd., Yokohama, Japan) in Japan now exist. Several studies have found these SIR-inducing compounds to be effective in reducing severity of key tree pathogens such as fireblight (Erwinia car- otovora), Phytophthora root rot (P. palmivora), powdery mildew (Sphaerotheca pannosa var. rosa, Phyllactinia spp. and Uncinula necator), and wilt disease of spruce (Ceratocystis polonica) (Chris- tiansen et al. 1999; Percival 2001). Importantly, the level of disease control achieved was comparable with currently used agrochemi- cals. Unlike conventional fungicides, a “one-off” SIR application has been shown to provide growing season protection (Kessmann et al. 1994). RESISTIM is a plant stimulant and fertilizer combined with natural plant betaines (www.mandops.co.uk/resistim.htm). Beta- ines are naturally occurring amino acid derivatives that function as osmoprotectants within plants (McNeil et al. 1999). Osmo- protectants serve to raise osmotic pressure in the cytoplasm and stabilize proteins and membranes when salt levels or tempera- tures are unfavorable for plant growth. Osmoprotectants, there- fore, play important roles in the adaptation of cells to adverse environmental conditions (Yancey 1994). Manufacturers claim RESISTIM is particularly effective on legumes such as peas and beans, strengthening them against diseases such as mildew (www.mandops.co.uk/resistim.htm). However, a definitive role for betaine as an inducer of plant defense responses has not been clearly shown. More likely, enhanced levels of betaine are as a consequence of pathogen invasion rather than as a cause of re- sistance (Berglund and Ohlsson 1995). Phosphites exhibit a complex mode of action, acting both directly on the pathogen and indirectly by stimulating plant host defense responses such as phenolic accumulation that in turn inhibit pathogen growth (Guest and Grant 1991). Likewise, sali- cylic acid is widely recognized as an inducer of SIR resistance (Enyedi and Raskin 1993) and is shown to be effective in reducing disease incidence against Phytophthora palmivora in cacao, Pseu- domonas syringae pv. Maculicola on cucumber and Arabidopsis, and Colletotrichum lagenarium on cucumber and tobacco necro- sis virus (Rasmussen et al. 1991; Kessmann et al. 1994; Okey and Sreenivan 1996). Harpin protein is a naturally occurring bacterial protein pres- ent in several plant pathogenic bacteria to include Erwinia amy- ©2008 International Society of Arboriculture
September 2008
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