Arboriculture & Urban Forestry 36(1): January 2010 Arboriculture & Urban Forestry 2010. 36(1): 41–46 41 Effect of Systemic Inducing Resistance and Biostimulant Materials on Apple Scab Using a Detached Leaf Bioassay Glynn C. Percival Abstract. A detached leaf bioassay was used to evaluate several systemic inducing resistance agents, a range of biostimulant products and a conven- tional triazole fungicide (myclobutanil) on apple scab (Venturia inaequalis) development under laboratory conditions. None of the biostimulant prod- ucts (seaweed extract, betaine, molasses, humic acid, yucca extract, and plant hormone/vitamin complex) evaluated in this study inhibited germination of apple scab conidia, subsequent formation of appressoria or reduced leaf scab severity compared to water treated controls. All SIR agents used in this investigation (potassium phosphonate, potassium phosphite, harpin protein, salicylic acid, salicylic acid derivative) inhibited germination of apple scab conidia, subsequent formation of appressoria and reduced leaf scab severity. The synthetic fungicide myclobutanil resulted in the greatest levels of germination inhibition, reduced appressorium development and leaf scab severity. Results suggest application of an appropriate SIR product may provide a useful addition to existing methods of apple scab management; however, use of biostimulants as scab protectant compounds appears limited. Key Words. Fungicides; Integrated Disease Management; Pathogen Control; Plant Health Care; Urban Landscapes. Apple scab caused by Venturia inaequalis (Cooke) G. Wint., is one of the most serious diseases of ornamental and fruiting apples (MacHardy 1996). Presently, this disease is controlled mainly by synthetic fungicides applied frequently throughout the growing season (Holb et al. 2005). Identification of fungicide insensitive V. inaequalis isolates emphasise the need to evaluate nonfungicidal approaches to scab control (Stanis and Jones 1985; Schnabel and Parisi 1997). Genetically inherent constitutive and inducible de- fence responses protect trees against insect and pathogen attack (Krokene et al. 2008). Inducible resistance mechanisms such as systemic induced resistance (SIR) can be acquired by exposing plants to organic and/or synthetic compounds such as inorgan- ic potassium and phosphate salts, compost water extracts, low molecular weight proteins, and unsaturated fatty acids (Percival 2001). Developments in plant protection technology have led to the commercial availability of a range of SIR products that have been shown to reduce disease severity in many economically important grasses, crops, and woody plants. These SIR products may offer a potential environmentally benign alternative to syn- thetic fungicides (Percival and Haynes 2008). These compounds have no direct action on pathogens and are generally of lower mammalian toxicity than synthetic fungicides. Inoculations with the pathogen following treatment with a SIR product fail to cause disease, or the degree of disease severity is reduced (Van Loon et al. 2006). Among the SIR elicitors, harpin protein (Messenger) benzothiadiazole (Bion), potassium phosphite (Agri-Fos), potas- sium phosphonate (Phytogard), salicyclic acid (DSP), salicylic acid analogs (Rigel), and Probanazole (Oryzemate® ), are well- known and have shown to successfully reduce severity of fungal bacterial and viral diseases (Kato et al. 1984; Kessmann et al. 1994; Gorlach et al. 1996; Bécot et al. 2000; Bokshi et al. 2003). Products sold as biostimulants differ from traditional N:P:K fertilizers in that their active ingredient consists of a range of or- ganic compounds such as plant hormones, humic acids, marine algae extracts, sea kelp, vitamins, and other chemicals that vary according to the manufacturer (Fraser and Percival 2003; Barnes and Percival 2006). Most manufacturers claim biostimulants pos- sess wide activity against a broad range of pathogenic viral, bac- terial, and fungal diseases and in addition enhance resistance to insect attack (Mandops 2009; Bio-Plex 2009; Maxicorp 2009). There are, however, few independent scientific studies to support these claims (Grubinger 2005; Portillo et al. 2007). A laboratory based system for the rapid evaluation of resis- tance among apple genotypes and determining pathogenesis of V. inaequalis isolates was developed using a detached leaf bioassay (Yepes and Aldwinckle 1993a; Yepes and Aldwinckle 1993b). This detached leaf bioassay has been used to success- fully evaluate the efficacy of several film forming polymers on apple scab development and suppression. Polymers display- ing the greatest degree of scab suppression were carried for- ward for large scale field-testing, with the degree of scab con- trol shown under field-testing reflecting those obtained under laboratory conditions (Percival and Boyle 2009). Consequently, studies to date indicate adoption of this detached leaf bioas- say provides a means of identifying scab protectant properties that commercially available products may, or may not possess. Aims of this study were to evaluate the efficacy of five SIR and seven biostimulant products against apple scab us- ing a detached leaf bioassay under laboratory conditions. MATERIALS AND METHODS Plant Material and Product Treatment The apple trial site consisted of a 0.75 ha (1.9 acres) block of apple (Malus × domestica cultivar (cv.) Golden Delicious inter- ©2010 International Society of Arboriculture
January 2010
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