Suppression of Fusarium solani f. sp. phaseoli on bean by aluminum in acid soils

The severity of bean root rot caused by Fusarium solani f. sp. phaseoli in vitro was studied with regard to exchangeable soil aluminum for 25 soil samples collected from northeastern Honshyu island, Japan. Of these, 24 were Andosols, typically acidic and of volcanic ash origin. Disease severity was assessed based on the number of lesions produced by the pathogen on a 6-cm section of bean stem buried and incubated for 8 days at 25 degrees C in artificially infested soil samples. The number of lesions differed considerably among soil samples. In all soils in which disease incidence was very low, macroconidial germination was strongly inhibited. The inhibition was observed in all soil samples with exchangeable aluminum contents of at least 0.4 meq/100 g of soil, although it is unclear if this concentration is the lowest limit for inhibition. When soil pH was 5.6 or lower, higher amounts of exchangeable aluminum were detected from soils in which the major clay mineralogy was chloritized 2:1 minerals, while no or limited amounts of aluminum were detected from soils in which the major clay mineralogy was allophane/imogolite. Macroconidial germination and disease incidence are thus closely related to clay mineralogy, which regulates the behavior of exchangeable aluminum.