Effects of soil drenching of water-soluble potassium silicate on commercial avocado (Persea americanaMill.) orchard trees infected withPhytophthora cinnamomiRands on root density, canopy health, induction and concentration of phenolic compounds

Avocado root rot, caused by Phytophthora cinnamomi Rands, remains a major constraint to avocado production worldwide. In the current study effects of successive soil drench applications of soluble potassium silicate on canopy health and root density of 13-year-old Persea americana Mill. trees infected with P. cinnamomi were investigated. Soil drenching with 20 l per tree of a 20 ml l-1 soluble potassium silicate solution (20.7% silicon dioxide) resulted in significantly higher root density when compared to untreated control trees, and trees injected with potassium phosphonate (Avoguard®) during most but not all evaluation dates. Three successive drenches of soluble potassium silicate resulted in the most significant increase in root density. A similar effect was seen on canopy health. In general, total soluble phenolic concentrations were significantly higher between March 2005 and January 2006 in those trees drenched three times with soluble potassium silicate per growing season (up to 72.62 µg l-1) compared to trees injected twice with potassium phosphonate per growing season (up to 68.77 µg l-1) and untreated control trees (51.62 µg l-1). This evidence suggests that multiple or even continuous applications of soluble potassium silicate to avocado trees will be required to effectively suppress Phytophthora cinnamomi over the entire growing season.