272 lovora responsible for fireblight in apples. Manufacturers claim harpin proteins possess wide activity against a broad range of pathogenic viral, bacterial, and fungal diseases and, in addition, enhance resistance to insect attack (www.yardener.com/ HarpinProteinProducts.html). Enhanced leaf peroxidase activity has been associated with SIR against tobacco mosaic virus, alfalfa mosaic virus, and Phy- tophthora cinnamomi (Van Lelyveld and Brodrick 1975; Vegetti et al. 1975; Van Loon 1976). Induced systemic resistance against, for example, the pathogens Colletotrichum lagenarium and Clado- sporium cucumerinum is associated with an enhanced ability of the host plant to lignify at points of fungal penetration. Lignification involves the polymerization of hydroxycinnamyl alcohols medi- ated by peroxidase in the presence of hydrogen peroxide (Gross 1979; Dalisay and Kuc 1995). Likewise, treatment of the lower leaves of potato plants with hyphal wall components of Phytoph- thora infestans systemically activated enhancement of superox- ide dismutase resulting in a significant reduction of hyphal pen- etration and susceptible lesions (Chai and Doke 1987). Investigations into SIR-inducing compounds to control oak powdery mildew have received limited study. In addition, the majority of studies regarding the efficacy of SIR-inducing chemicals have been conducted primarily under laboratory and glasshouse conditions that do not reflect field or landscape en- vironments. Finally, assessments of SIR efficacy in the majority of cases reflect applications applied as preventive (before visible symptoms of diseases expression) rather than therapeutic (when disease expression is visible). The objectives of this project were to investigate 1) the efficacy of commercially available SIR compounds on powdery mildew infection of English oak under field conditions as therapeutic rather than preventive treatments; and 2) the role of leaf peroxidase and superoxide dismutase activity on powdery mildew infection. MATERIALS AND METHODS Plant Material and Experimental Design The trial site consisted of a 2.0 ha block of English oak (Quercus robur L.) at the University of Reading Shinfield Experimental Site, Berkshire, U.K. Planting distances were based on a 2 × 1m (6.6 × 3.3 ft) spacing. Trees were planted in early November 2001 and trained to produce a central-leader system to an average height of 1.5 ± 0.15 m (4.95 ± 0.5 ft) with mean trunk diameters of10±2cm(4±0.8 in). Weeds were controlled with glyphosate (Roundup; Green-Tech, Sweethills Park, Nun Monkton, York, U.K.) applied at 15 mL (0.45 fl oz) per liter (0.26 gal) of water four times during the growing season . Supplementary irrigation was applied during the 2005 and 2006 growing seasons as re- quired. Trees were fertilized annually through a broadcast appli- cation of granular 29N–7P–9K fertilizer (Bartlett BOOST; The Doggett Corporation, Lebanon, NJ, U.S.) applied at 40 g (1.4 oz) m2 (11 ft2) in early May. The soil was a sandy loam containing 4% to 6% organic matter, pH of 6.2; available phosphorus, po- tassium, magnesium, sodium, and calcium were 52.0, 659.1, 175.2, 49.4, and 2188 mg/L (0.0001, 0.005, 0.002, 0.0001, and 0.02 oz/ gal), respectively, at the time treatments began. Historically, the oaks suffered annually from powdery mildew infections. Consequently, before the trial began in 2005 and 2006, trees were inspected in September 2004 and 2005 and only those trees visually rated with 50% to 80% of leaves affected, representing severe foliar discoloration and powdery mildew in- ©2008 International Society of Arboriculture Percival and Haynes: Controlling Oak Powdery Mildew fection, were used in the 2005 and 2006 trial. The insecticide deltamethrin (Bandu; Headland Agrochemicals Ltd., Saffron Walden, Essex, U.K.) was applied every 3 months during each growing season beginning in May 2005 to September 2006 to control pests. All sprays were applied using a Tom Wanner Spray Rig sprayer at 40 mL (1.2 fl oz) deltamethrin per 100 L (26 gal) of water. Trees were sprayed until runoff, generally 0.25 L (0.07 gal) deltamethrin per tree. Systemic-Inducing Resistance Treatments Seven trees with 5% to 20% of leaves affected with powdery mildew, i.e., some yellowing but little or no defoliation, were treated per SIR product. This occurred on 23 July 2005 and 31 July 2006. During SIR spray treatments, polythene screens 1 m (3.3 ft) high were erected around each tree to prevent dispersal of sprays and possible cross-contact with other trees. The base of the tree was covered with a 0.5m× 0.5m(1.65 × 1.65 ft) polythene mulch to prevent potential soil percolation. The treatments (Messenger [herapin protein], Agri-Fos [potassium phosphate], Asprin [sali- cylic acid], RESISTIM [Betaine, amino acid derivatives], two fungicide [penconazole] regimes, and a water control) were ap- plied in seven randomized complete blocks with a single tree as the experimental unit, giving a total of 49 observations per re- sponse variable. Foliar sprays of each chemical were applied until runoff using a 10 L (2.6 gal) backpack sprayer at the manu- facturer’s recommended rate: RESISTIM (a.i. betaine): 10 mL (0.3 fl oz) per liter (0.26 gal) of water (Mandops UK Ltd., Eastleigh, Hampshire, U.K.); Topas (a.i. penconazole): 1.5 mL (0.045 fl oz) per liter (0.26 gal) of water (Syngenta Crop Protection UK Ltd., Whittlesford, Cambridge, U.K.); Messanger (a.i. harpin protein): 4 g (0.14 oz) per 100 m2 (1,111 sq ft) (EDEN Bioscience Corporation, N. Bothell, WA); Agri-Fos (a.i. potassium phosphite): 3 mL (0.09 fl oz) per liter (0.26 gal) of water (Shelton Technology Ltd., Boundary House, Charwelton, Northamptonshire, U.K.); and Salicylic acid: 3 mL (0.09 fl oz) per liter (0.26 gal) of water (Shelton Technology Ltd.). In addition, a comparative evaluation of a conventional spray program used within the United Kingdom for powdery mildew control was conducted. Trees were sprayed with the fungicide Topas (a.i. penconazole), a protectant conazole fungicide with antisporulant activity, at the recommended rate of 1.5 mL (0.045 fl oz) per liter (0.26 gal) of water. Penconazole treatments began at the same time as application of SIR products and every 3 weeks thereafter until the cessation of the experiment (27 September 2005 and 2006). Disease Severity Disease severity of whole trees was assessed visually on 27 September 2005 and 2006. Each tree was rated ona0to5 rating scale using a visual indexing technique and ratings on the scale: 0 no powdery mildew observed; 1 less than 5% of leaves affected and no aesthetic impact; 25% to 20% of leaves affected with some yellowing but little or no defoliation; 321% to 50% of leaves affected, significant defoliation, and/or leaf yellowing; 451% to 80% of leaves affected, severe foliar discoloration; and 5 81% to 100% of foliage affected with 90% to 100% defoliation. Leaf disease severity was assessed at days 10, 20, and 60 after application of the SIR-inducing chemical to determine the short- and long-term effects of each product using a similar indexing
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