In Vitro Fungicidal Activity of Calcium and Potassium Salts on Several Commercially Significant Plant Pathogens

In vitro dose responses of several calcium and potassium salts were determined on some commercially significant plant pathogens, including: Helminthosporium solani, Fusarium oxysporum f. sp. pisi race 2, Colletotricum coccodes, Phytophthora cactorum, Phytophthora cinnamomi, Phytophthora erythroseptica, Phytophthora infestans, Phytophthora megasperma, Pythium ultimum, and Venturia inaequalis. Mycelial growth inhibition was both salt-specific and dose-related. Pythium ultimum was completely inhibited by 75 mg L or greater calcium propionate, but needed 300 mg L or greater of calcium acetate and 40 mL L or greater of potassium silicate for complete inhibition. Phytophthora infestans was completely inhibited by 150 mg L or greater calcium acetate, 150 mg L or greater calcium propionate, or 5 mL L or greater potassium silicate. Phytophthora cactorum was completely inhibited by 300 mg L or greater calcium propionate, but required 600 mg L or greater calcium acetate and 10 mL L or greater potassium silicate for complete inhibition. Phytophthora cinnamomi was completely inhibited by calcium propionate at 600 mg L or greater or by 10 mL L or greater potassium silicate. Only potassium silicate inhibited Phytophthora megasperma, Phytophthora erthroseptica, V. inequalis, and H. solani at concentrations of 5 mL L or greater, 20 mL L or greater, 40 mL L or greater, or 80 mL L or greater, respectively. Potassium acetate did not completely inhibit any of the pathogens in this study when tested at concentrations 1200 mg L or less. Agronomic and horticultural crops in the Pacific Northwest constitute a multimillion dollar industry. Both conventional and organic cropping systems in this region are affected by several fungal and fungal-like organisms of commercial significance (Table 1). Conventional farming systems in particular have become very reliant on synthetic fungicides, and with mounting pressure from the environmental lobby, efforts must be made to identify suitable alternatives. Unfortunately, most organic remedies currently available are limited in their efficacy and given the small array of acceptable products available to organic growers, every effort should be made to quantify efficacy of potential new products with alternative modes of action to those currently in use because this will help reduce the risks associated with pesticide resistance. Several studies have been undertaken in recent years identifying the fungicidal properties of many different inorganic salts (Biggs et al., 1997; Campanella et al., 2002; Hervieux et al., 2002; Olivier et al., 1999; Samelis et al., 2001). The following salts have shown good potential for use as fungicides: potassium silicate (Bekker et al., 2006), calcium acetate (Palou et al., 2002), and calcium propionate (Aguayo et al., 2008; Arroyo et al., 2005; Biggs, 1999, 2004; Biggs et al., 1997; Blogdett et al., 2002; Kortekamp, 2006; Mills et al., 2005; Suhr and Nielsen, 2004). However, most of these studies dealt with only some of the pathogens tested in this study and the remainder were postharvest fruit pathogens. In addition, no studies to date have evaluated the use of potassium acetate as a potential fungicide. Consequently, the present study was initiated to determine whether commercially important fungi and oomycetes in the Pacific Northwest could be suppressed before harvest using potassium acetate and other inorganic salts. This in vitro investigation is regarded as being a starting point for future field investigations and those compounds showing promise will be further tested in vivo. Material and Methods Fungal and oomycete isolates were obtained from culture collections maintained by Oregon State University and Washington State University (Table 1) for the purpose of testing the susceptibility of these fungal and oomycete isolates to different calcium and potassium salts. Isolates were chosen based on taxonomic diversity within the ascomycetes and oomycetes as well as economic importance as plant pathogens in the Pacific Northwest. For this study, Helminthosporium solani, Fusarium oxysporum f. sp. pisi race 2, Colletotricum coccodes, and Venturia inaequalis were subcultured and analyzed on Potato Dextrose Agar (PDA) (Becton, Dickinson & Co., Sparks, MD) prepared according to the manufacturer’s specifications. Cultures of Phytophthora infestans, Phytophthora megasperma, Phytophthora cactorum, Phytophthora erythroseptica, Pythium ultimum, and Phytophthora cinnamomi were maintained and analyzed on Corn Meal Agar (CMA) (Becton, Dickinson & Co.). Before inoculation, fungal and oomycetes isolates were subcultured onto these media and incubated at room temperature ( 25 C) until the diameter of the culture was nearly that of the entire plate. Reagents were obtained from Spectrum Chemicals and Laboratory Products (Gardena, CA) as follows: calcium acetate, anhydrous powder (FCC) [C4H6CaO4; Received for publication 24 Feb. 2011. Accepted for publication 25 Mar. 2011. We thank Dr. Lawrence Marais of Monterey Agricultural Resources for financial assistance, which helped support this study. Manager. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 46(6) JUNE 2011 913 DISEASE AND PEST MANAGEMENT