Arboriculture & Urban Forestry 38(6): November 2012 black spot control, was conducted. Increases in horsechestnut and rose resistance were assessed by recording foliar pathogen severi- ty on a 1–6 scale from ten randomly selected leaves per plant: 1 = No pathogen attack observed; 2 = less than 5% of leaf area affect- ed with leaf blotch/black spot; 3 = 5%–20% of leaf area affected with leaf blotch/black spot plus some yellowing; 4 = 21%–50% of leaf area affected with leaf blotch/black spot and significant leaf yellowing; 5 = 51%–80% of leaf area affected with leaf blotch/ black spot, severe leaf yellowing; 6 = 81%–100% of leaf area infected with leaf blotch/black spot with complete leaf yellowing. RESULTS Irrespective of pathogen or concentration applied, none of the biostimulants used in this investigation provided a significant de- gree of Guignardia leaf blotch or black spot control compared to water-treated controls (Table 2). In the case of black spot and Guignardia leaf blotch, pathogen severity was reduced by 8.1% (Superthrive 0.5 ml per liter) and 11.9% (Fulcrum CRV 20 ml per liter) respectively. However, in some cases, biostimulant ap- plications increased the severity of blackspot and Guignardia leaf blotch by 6.1% (Fulcrum 10 ml per liter and 7.1% (Crop Set 20 ml per liter) respectively (Table 2). Only the synthetic fungi- cide Systhane (myclobutanil), irrespective of concentration ap- plied, provided a significant degree of pathogen control. In this 259 instance, pathogen severity was reduced 66.7%–71.4% for black spot and 73.4%–79.6% for Guignardia leaf blotch (Table 2). DISCUSSION None of the biostimulants tested in this investigation achieved a sufficient degree of pathogen control to warrant replacement of a conventional synthetic fungicide. Therefore, despite the claims of some manufacturers, results of this study did not iden- tify any reason to advocate the use of the biostimulants tested for plant protection purposes. Few independent peer-reviewed publications exist that have evaluated biostimulants for their plant protectant properties. The plant hormone/vitamin complex Superthrive, failed to inhibit germination of apple scab conidia, formation of appressoria, or reduce leaf scab severity compared to water-treated controls in a detached leaf bioassay under labora- tory conditions; the conclusion was that this product had limited potential as a scab protectant compound (Percival 2010). Reasons for this lack of pathogen control efficacy can be suggested by reference to the use of biostimulants for other purposes. For ex- ample, biostimulants have been advocated as a means to enhance transplant survival of trees and improve crop yield and quality; however, similar to the results of this study, little influence of biostimulants on these parameters was recorded (Kelting et al. 1997; Thalheimer and Paoli 2001). Contradictory to this, Thomp- Table 1. Selected biostimulants applied to horsechestnut (Aesculus hippocastanum) and Rosa “Pretty Polly” to control Guig- nardia leaf blotch and black spot, respectively. Product Maxicrop Original Resistim Bioplex Fulcrum CRV Redicrop Crop Set Superthrive Systhane Active ingredient Seaweed extract Betaine Seaweed + humic acid extract Molasses Seaweed (cytokinin activity) Lactobacillus fermentation product and B5 vitamins Vitimin B and Auxin (NAA) Myclobutanil (triazole) Supplier Maxicrop (UK) Ltd, Corby, UK Mandops UK Ltd, Eastleigh, Hampshire, UK United Agri Products Ltd, Alconbury Weston, UK Banks Cargill Agriculture Ltd, St Hughs, Lincoln, UK United Agri Products Ltd, Alconbury Weston, UK United Agri Products Ltd, Alconbury Weston, UK Bartlett Tree Research Laboratory, Charlotte, NC Barrettine, St Ivel Way, Warmley, Bristol Table 2. Foliar pathogen severity rating on foliar tissue of horsechestnut (Aesculus hippocastanum) and Rosa “Pretty Polly.” Treatment Concentration per liter Water (control) Maxicrop Original Maxicrop Original Resistim Resistim Bioplex Bioplex Fulcrum CRV Fulcrum CRV Redicrop Redicrop Crop Set Crop Set Superthrive Superthrive Systhane Systhane - 10 ml 20 ml 10 ml 20 ml 10 ml 20 ml 10 ml 20 ml 10 ml 20 ml 10 ml 20 ml 0.25 ml 0.50 ml 0.3 ml 0.6 ml Horsechestnut leaf blotch severity rating 4.9 ± 0.56a 4.7 ± 0.54a 5.1 ± 0.58a 4.8 ± 0.46a 5.2 ± 0.53a 4.9 ± 0.52a 4.7 ± 0.47a 5.2 ± 0.61a 4.8 ± 0.50a 5.0 ± 0.56a 4.8 ± 0.55a 5.0 ± 0.58a 4.6 ± 0.60a 4.5 ± 0.40ab 4.2 ± 0.33ab 1.3 ± 0.18c 1.0 ± 0.14c Black spot severity rating 4.2 ± 0.30a 4.3 ± 0.33a 4.0 ± 0.29a 3.8 ± 0.31a 4.2 ± 0.32a 4.0 ± 0.33a 4.0 ± 0.27a 3.7 ± 0.31a 3.7 ± 0.25a 4.4 ± 0.33a 4.3 ± 0.37a 4.5 ± 0.34a 4.1 ± 0.38a 4.0 ± 0.29a 4.2 ± 0.31a 1.4 ± 0.04b 1.2 ± 0.03b Note: 1 = No pathogen attack observed; 2 = less than 5% of leaf area affected; 3 = 5%–20% of leaf area affected with some chlorosis; 4 = 21%–50% of leaf area affected, significant leaf chlorosis; 5 = 51%–80% of leaf area leaves affected, severe leaf chlorosis; 6 = 81%–100% of leaf area with complete leaf chlorosis. All values mean of six plants. Values followed by the same letter are not statistically significant (P ≤ 0.05) according to LSD. ©2012 International Society of Arboriculture
November 2012
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