104 Percival et al.: Film-Forming Polymers to Control Guignardia Leaf Blotch and Powdery Mildew controls were recorded in trees treated with Bond, Designer, and penconazole (Figures 5 and 6). In the case of Bond and penconazole treatment, increases in Fv/Fm and SPAD values were significantly higher than controls. In the case of De- signer, values were higher than controls, but not significantly so (Figures 5 and 6). Differences in the magnitude of the degree of protection conferred were recorded. Based on re- ductions in leaf disease severity values and higher leaf Fv/Fm ratios and SPAD measurements, efficacy of protection was in the order penconazole > Bond > Designer (Figures 4, 5, and 6). DISCUSSION The data show that two (Bond, Designer) of the four film- forming polymers provided significant control of powdery mildew and Guignardia leaf blotch infection of English oak and horsechestnut, respectively, in field experiments. Al- though film-forming polymers have been shown to provide a significant degree of control of many foliar diseases such as powdery mildew, leaf spot, rusts, and gray mold, results in the majority of cases were derived from cereal crops, primar- ily under glasshouse conditions (Ziv and Hagiladi 1984; Elad et al. 1990; Zekaria-Oren and Eyal 1991; Walters 1992; Sutherland and Walters 2002). Results of this investigation show that film-forming polymers have disease control poten- tial under field conditions. In addition, the application of the film-forming compounds to both tree species in the field produced no phytotoxic effects on leaf tissue. For example, higher Fv/Fm chlorophyll fluorescence ratios on Bond- and Designer–treated trees and Fv/Fm ratios of Spray Gard- and Nu-Film-P–treated trees comparable with controls suggest that the compounds had no detrimental effects on rates of photosynthesis and leaf expansion. Chlorophyll fluorescence Fv/Fm ratios have been shown to be highly sensitive to dam- age to the leaf photosynthetic apparatus in response to a range of stimuli such as salt, heat, and chemical and atmospheric pollutants and are consequently widely used as a noninvasive measure of tree vitality (Haldimann and Strasser 1999; Popo- vic et al. 2003; Percival 2004). In general, disease severity of powdery mildew and Guig- nardia leaf blotch was reduced by 15% to 54% and 32% to 54%, respectively, following application with the film- forming polymers Bond and Designer. Such results are in agreement with previous work showing control of powdery mildew infection of barley under glasshouse and field con- ditions by 70% to 85% and 50% to 65%, respectively, using menthene-based film-forming polymers such as Ethokem, Bond, and Vapor Gard (Sutherland and Walters 2002). Simi- lar control percentages of powdery mildew infection of wheat by menthene and wax emulsion-based film-forming polymers in the field were recorded by Ziv and Frederiksen (1987). In the case of Zekaria-Oren and Eyal (1991), greater than 80% suppression of leaf rust (Puccinia recondita f. sp. tritici) was ©2006 International Society of Arboriculture achieved using the film-forming polymers Bio-Film, Foli- cote, and Vapor Gard under glasshouse conditions, with little difference in efficacy recorded between preventive and thera- peutic film-forming polymer treatments. Many studies suggest that the ability of film-forming poly- mers to control foliar pathogens is due to enhancement of natural defenses at the leaf surface (Gale and Poljakoff- Mayber 1962; Ziv and Frederiksen 1983). Alternately, Oss- wald et al. (1984) reported that adherence of spores to the polymer-coated surface prevented development of germ tubes and was a primary factor in reducing disease severity. Other work suggested that film-forming polymers increase thick- ness, hardness, and resistance to enzymatic degradation of the epicuticular wax layer, thus increasing impenetrability of the leaf surface (Elad et al. 1990). Zekaria-Oren and Eyal (1991) found that film-forming polymers affected successful orien- tation of germinating urediniospores and development of ap- pressoria. They suggested this could be due to suppression of stimuli at the leaf surface by the formation of a physical barrier between the invading pathogen and the host plant. It has also been suggested that film-forming polymers may sub- tly alter the topography of the leaf surface, thus interfering with fungal recognition of host penetration sites—that is, the potentially invading fungus is unaware that it is on a host plant (Zekaria-Oren and Eyal 1991). Although certain film- forming polymers may be biologically inert and act by purely physical mechanisms, there is evidence that suggests that some may have direct fungicidal or fungistatic properties (Elad et al. 1989, 1990; Zekaria-Oren and Eyal 1991). Work by Sutherland and Walters (2002) found that the film-forming polymers Bond and Vapor Gard reduced germination of pow- dery mildew conidia and the subsequent formation of appres- soria and haustoria. Such a response was in accordance with previous work that showed that the film-forming polymers Bond and Vapor Gard reduced mycelial growth of leaf spot of oat (Pyrenophora avenae Ito and Kuribayashi) and rice blast (Pyricularia oryzae Cav.) in vitro. Microscopic observations reported changes in fungal morphology to include swollen shorter fungal cells, granulation of cytoplasm, and collapsed empty cells (Sutherland and Walters 2001). Importantly, all available information suggests that film-forming polymers achieve disease control via multiple defense mechanisms; this in turn makes it highly unlikely that a pathogen can readily develop resistance to such a control measure. In this investigation marked differences were observed in the degree of control obtained between film-forming poly- mers. In the case of Nu-Film-P and Spray Gard, no beneficial effects were obtained. Both Nu-Film-P and Spray Gard (poly- 1-p menthene and di-1-p menthene, respectively) have been used for many years primarily as antitranspirants to decrease water loss and wilting of commercially important forestry transplants, as well as extending pesticide efficacy. However, the recent generation of film-forming polymers (Bond, De-
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