In vitro inhibition of several phytopathogenic fungi from avocado by soluble potassium silicate
Silicon is a bioactive element only recently implicated as having fungicidal properties. The present study examined water soluble liquid potassium silicate for activity against several types of avocado phytopathogenic fungi. In vitro dose responses towards soluble potassium silicate (20.7% silicon dioxide) were determined for Phytophthora cinnamomi, Phomopsis perniciosa, Pestalotiopsis maculans, Lasiodiplodia theobromae, Glomerella cingulata, Natrassia sp. and Collectotrichum gloeosporioides. Inhibition of mycelial growth was dose dependant with 100% inhibition observed at 80 ml (pH 11.7) and 40 ml (pH 11.5) soluble potassium silicate (20.7% silicon dioxide) per litre of agar, for all fungi tested in two of the replicate experiments with the exception of Natrassia sp. G. cingulata and C. gloeosporioides at 40 ml in one replication. For both replicate experiments, Phytophthora cinnamomi, Phomopsis perniciosa, Pestalotiopsis maculans, Lasiodiplodia theobromae, Glomerella cingulata, Natrassia sp. and Collectotrichum gloeosporioides were only partially inhibited at 5, 10 and 20 ml soluble potassium silicate per litre of agar. Percentage inhibition was, however, positively correlated with soluble potassium silicate concentrations. Soluble potassium silicate raised the pH of the agar from 5.6 to between 10.3 and 11.7 at concentrations of 5 and 80 ml soluble potassium silicate per litre of agar respectively. The effect of pH on fungal growth does not follow a clear trend for all fungi tested. In the absence of soluble potassium silicate, high pH caused only partial inhibition of mycelial growth. Soluble potassium silicate therefore suppresses fungal growth effectively in vitro and this is largely a fungicidal effect. INTRODUCTION It has long been known that silicon reduces the incidence of fungal diseases in various pathosystems (Fauteux, Remus Borel, Menzies Belanger, 2005) Numerous studies have shown increased resistance to fungal plant diseases as a response to silicon applications. These include increased resistance to powdery mildew (Uncinula necator) in grapes (Bowen, Menzies Ehret, 1992) powdery mildew (Sphaerotheca fuliginea) in cucumbers (Adatia Besford, 1986; Belanger, Bowen, Ehret, Menzies, 1995) powdery mildew (Erysiphe cichoracearum) in muskmelons (Menzies, Bowen Ehret, 1992) as well as Cladosporium spp. and Pythium spp. in cucumbers (Cherif, Asselin Belanger, 1994) It was proposed that amorphous silica deposition in the leaf apoplast prevent penetration of fungal hyphae (Carver, Zeyen Ahlstrand, 1987; Kunoh Ishizaki, 1975) Although this in part explains the prophylactic properties of silicon, monomeric silicon is also implicated in activation of several plant natural defences (Fauteux et al. 2005) such as fl avanoids phytoalexin mediated response to silicon applications (Fawe, Abou Zaid, Menzies Belanger, 1988) and prophylactic properties of silicon of soluble potassium silicate against phytopathogenic fungi affecting avocado. Numerous avocado diseases are caused by pathogenic fungi present in the soil or air within avocado orchards. In many instances the world has become reliant on chemical control of these diseases, with the fear of fungicide resistance ever increasing (Hardy, Barrett Shearer, 2001) Development of fungicide resistant strains of phytopathogenic fungi has been amply demonstrated. It is therefore imperative that alternative control measures should be investigated.The current study was therefore initiated to assess the in vitro effect of soluble potassium silicate on Phytophthora cinnamomi and six other pathogenic fungi affecting avocado. Table 1. Fungal pathogens of avocado tested for their in vitro sensitivity to soluble potassium silicate (20.7% silicon dioxide) in plant defence reactions to fungal attack (Cherif et al. 1994) Bekker, Kaiser, Van der Merwe Labuschagne (2006) demonstrated for the fi rst time that soluble potassium silicate has direct in vitro fungitoxic activity against a wide range of fungi. In the current study we are investigating the fungitoxic activity MATERIALS AND METHODS Fungi tested Isolates of seven fungal pathogens of avocado maintained on potato dextrose agar (PDA) were obtained from the Mycological herbarium of the Plant Protection Research Institute, Agricultural Research Council (ARC) Pretoria, and Westfalia Technological