Abiotic pathogen suppression: physiology and biology of aluminum toxicity to soilborne fungi

Abstract

An interdisciplinary approach was utilized to study the toxicity of aluminum (Al) to soilborne plant pathogens with the goal of developing a pathogen-suppressive potting medium containing non-phytotoxic, Al-organic matter complexes. Toxicological studies addressed the toxicity of monomeric Al species to Thielaviopsis basicola and Phytophthora parasitica and documented the sensitivity of these organisms to the metal. Until recently, research on Al-toxicity to fungi has only focused on the trivalent Al cation (Al³⁺) which is also considered the most phytotoxic Al ion. The toxicity of Al-hydrolysis species to fungi were tested by modeling in vitro test solution equilibria using GEOCHEM-PC and correlating the predicted values of Al-species activities with reduction in spore production of the two pathogens. Chlamydospore production of T. basicola was negatively correlated with Al³⁺ activity, whereas inhibition of sporangia production of P. parasitica was related to the activity of multiple monomeric Al species. Toxicity of Al to T. basicola was observed in solutions containing &#8805 20 micromolar Al. Sensitivity of P. parasitica to Al was observed at < 1.0 micromolar Al, suggesting that P. parasitica is more sensitive to Al than T. basicola. Using fluorescence microscopy, the localized accumulation of Al in pathogen tissues was detected using lumogallion, an Al-specific, fluorescent stain. Accumulation of Al was observed under various chemical conditions, ranging from salt solutions to more complex systems containing Al-peat complexes. An ecological approach was applied to study the dynamic interactions of soil chemical and physical properties with soil microflora for the suppression of P. parasitica in a medium amended with Al₂(SO₄)₃ and composted swine waste (CSW). Abiotic and biological mechanisms of pathogen suppression were incorporated into the CSW-amended medium. Al-mediated suppression resulted in reduction of sporangia production in medium exhibiting K-exchangeable Al levels > 2 micromolar Al. Biological suppression also resulted in reduction of sporangia production and this suppression was maintained after Al levels dropped below the threshold necessary for abiotic suppression. The incorporation of abiotic and biological control mechanisms into a potting media may facilitate suppression of a wide range of soilborne pathogens and enhance applicability of disease-suppressive media in a disease management strategy.