Effect of calcined lime (CaO) on chlamydospores of the rice false smut pathogen, Villosiclava virens, and their root attachment and infection

Abstract

Rice false smut caused by Villosiclava virens is a devastating disease. The smut balls contain chlamydospores, which fall onto paddy soils to become the primary inoculum. After transplanting, the chlamydospores subsequently germinate and infect rice roots. The application of a CaO‐containing fertilizer to paddy soils can inhibit the development of rice false smut disease; however, the underlying mechanism is unknown. In this study, we evaluated the suppression of chlamydospore behaviour due to CaO. Specifically, we clarified the effects of the following on chlamydospore morphology and germination: (a) pH, (b) calcium (Ca) concentration, (c) Ca concentration (pH adjusted to 6.5), and (d) CaO‐added soil solution. Germination was suppressed at pH 4 and 10, in contrast with the normal germination at pH 6, 7, and 8. Treatments with more than 10 mg/L Ca melted the outer layer of chlamydospores and suppressed germination regardless of whether the pH was adjusted to 6.5. A saturated CaO solution induced bursting of chlamydospores. Suppressed germination and a melted outer chlamydospore layer were also observed, even though the soil buffering effect initially prevented the CaO‐mediated pH increase. Furthermore, the chlamydospores within 15 mm from the CaO small lump exhibited suppressed germination in soil. In addition, there was no effect of CaO treatment on chlamydospore attachment to rice roots and hyphal invasion of rice roots in in vitro inoculation tests. These results suggest that Ca concentration is an important factor for inhibiting the occurrence of rice false smut disease.

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

“…In contrast, in the CSS and NCSS tests, the chlamydospores gradually collapsed because SiO 2 − (negative ion) may have inhibited the process of chlamydospore transparency and collapsed by Ca 2+ (positive ion). These results were mostly consistent with those of the CaO test (Nakajima et al, 2021), showing no gradual collapse or transparency. In addition, the percentage of transparent chlamydospores was consistent with that obtained through the CaO test (Fig.…”

“…4e, f). Chlamydospores are covered by an outer wall with spikes (Kim & Park, 2007) which are degraded under highly alkaline conditions (Nakajima et al, 2021). In this study, we indicated that both high pH and Ca 2+ might contribute to the gradual degradation of the outer chlamydospore walls causing their collapse.…”

“…3). In the calcium oxide (CaO) test (Nakajima et al, 2021), the chlamydospores rapidly became invisible and burst with desquamated outer walls. In contrast, in the CSS and NCSS tests, the chlamydospores gradually collapsed because SiO 2 − (negative ion) may have inhibited the process of chlamydospore transparency and collapsed by Ca 2+ (positive ion).…”

“…Soil extract (SE) and CSSE CSSE was prepared using the method described by Nakajima et al (2021) with minor modi cations. 1.5 g of the slag described above was added to 500 g of the soil and mixed thoroughly using a spatula.…”

“…To observe the morphology, the method described by Nakajima et al ( 2021) was used with minor modi cations. We mixed 0.1 ml of the chlamydospore suspension with 0.9 ml of CSS, NCSS, SE, or CSSE and incubated it at 25°C in the dark (Nakajima et al, 2021). For evaluating the morphology and germination of chlamydospores in these solutions, ve circles with an inner diameter of 1.8 mm per solution were observed using an inverted microscope (CKX41, Olympus, Japan) at 6, 24, 48, and 72 h after mixing.…”