258 Banks and Percival: Evaluation of Biostimulants on Leaf Blotch and Black Spot Arboriculture & Urban Forestry 2012. 38(6): 258–261 Evaluation of Biostimulants to Control Guignardia Leaf Blotch (Guignardia aesculi) of Horsechestnut and Black Spot (Diplocarpon rosae) of Roses Jonathan M. Banks and Glynn C. Percival Abstract. Biostimulants are classified as materials that are neither a fertilizer nor a pesticide, but when applied to a plant will enhance their health, growth, and protection. Manufacturers claim biostimulants have underexploited potential in providing protectant properties to plants against pathogen attack. This study evaluated the efficacy of seven commercially available biostimulants against the foliar pathogens Guignardia aesculi, leaf blotch of horsechestnut (Aesculus hippocastanum L.) and Diplocarpon rosae black spot of roses (Rosa “Pretty Polly”). None of the biostimulant products tested in this investigation achieved a sufficient degree of pathogen control to warrant replacement of or supplementation with conventional synthetic fungicides. Key Words. Aesculus hippocastanum L.; Bio Control; Biostimulant; Diplocarpon rosae; Guignardia aesculi; Integrated Pest Management; Plant Health Care; Pathogen Suppression. Foliar pathogens such as Guignardia leaf blotch (G. aesculi) of horsechestnut (Aesculus spp.) and black spot (Diplocarpon rosae) of roses (Rosa spp.) can result in serious economic losses for growers and vendors of ornamental plants as heavy infections can distract from plant aesthetic values (March- ant et al. 1998; Pastirčáková et al. 2009). New techniques of pathogen control are warranted due to the decreasing num- ber of synthetic chemical controls coupled with greater plant pathogen insensitivity to conventional fungicides and public demands to reduce pesticide use, stimulated by greater aware- ness of environmental and health issues (Percival et al. 2009). Products referred to as biostimulants may be of future interest to those involved in the organic management of plant patho- gens (Whipps 2001). Biostimulant formulations include com- pounds, such as acrylamide, amino acids, bacteria, carbohy- drates, endo- and ectomycorrhizal fungi, humic acids, marine algae, wing of bat, nitrogen-fixing bacteria, plant hormones, sea kelp, vitamins, yucca extracts, and other substances that vary according to the manufacturer (Ferrini and Nicese 2002; Percival 2010). Manufacturers claim biostimulants, gener- ally, conform to two modes of action: i) Activating a plant immune response, commonly known as systemic induced re- sistance (SIR); ii) Acting as fertilizers, despite the fact that their constituents differ from typical N:P:K fertilizers; there- fore, their primary role is not direct nutrition (Thomson 2004; Percival 2010) but may be involved in promoting beneficial physiological processes or mycorrhizal associations known to be involved in plant defense (Azcon-Aguilar et al. 2002). Biostimulants may also be less susceptible to fungicidal insen- sitivity (Tronsmo 1991) and because of their natural constitu- ents are considered less toxic to the environment and humans. ©2012 International Society of Arboriculture Previous research has shown applications of SIR products alone can result in resistance-induced yield increases of up to 367% (Burr et al. 1978), while applications of biostimulants exhibiting SIR activity reduced rooting and cut fungicide applications to nearly zero (Thompson 2004). Consequently, this study was undertaken to determine the effectiveness of seven different biostimulant prod- ucts against Guignardia leaf blotch of horsechestnut, caused by G. aesculi and black spot of roses, caused by Diplocarpon rosae. MATERIALS AND METHODS Pot experiments using four-year-old stock of Rosa “Pretty Polly” (susceptible to black spot) and Aesculus hippocastanum L. (sus- ceptible to Guignardia leaf blotch) were used. Twelve months prior to experiments, plants were potted into 5 L plastic pots filled with soil (loamy texture, 24% clay, 45% silt, 31% sand, 3.1% organic carbon, pH 6.2), supplemented with the controlled release nitrogen-based N:P:K (29:7:9) fertilizer Bartlett BOOST (The Doggett Corporation, Lebanon, New Jersey, U.S.) at a rate of 1 g/kg soil. Following potting, plants remained outdoors sub- ject to natural environmental conditions and were watered as re- quired. The experimental design used was a completely random- ized block design in which pots were re-randomized on a weekly basis. Eight plants per treatment were spaced at 0.5 m to prevent competition for light. Plants were sprayed until runoff four times during the growing season (May 3, June 4, July 5, and August 4, 2009) with a range of commercially available biostimulant products (Table 1). The lowest concentration used (Table 2) was based on the manufacturer’s recommended rate of application. In addition, a double strength concentration of each product was also evaluated. A comparative evaluation of the fungicide Syst- hane (i.e., myclobutanil), commercially used for leaf blotch and
November 2012
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