Converter slag causes gradual collapse of chlamydospores of the rice false smut fungus and reduces hyphal invasion into rice roots

Rice false smut has become a serious threat to rice production. Durable chlamydospores of the fungus are the primary inoculum in paddy soil. Mixing converter slag into paddy soil can decrease disease incidence, but the mechanism is unknown. Here we first confirmed the suppressive effect of converter slag mixed into a paddy field: 3t/ha suppressed disease incidence by 75.6%. We then tested a converter slag solution (CSS), neutralized CSS (NCSS), and converter-slag-amended soil extract (CSSE), all at 3t slag/ha. In the CSS test, chlamydospores rapidly became transparent, then gradually collapsed, but NCSS slowed chlamydospore collapse. More germ tubes were abnormal in the CSS treatment than in the NCSS. The percentages of transparent, collapsed, and normally germinating chlamydospores in the CSSE treatment were intermediate to those obtained with CSS and NCSS. Hyphal invasion into rice roots in CSSE was also suppressed compared to that in control. These results suggest chlamydospores gradually collapse when exposed to cations and the pH is neutralized by soil, thus reducing hyphal production and infection of rice roots, contributing to disease suppression in fields.