Journal of Taiwan Agricultural Research

Fig. 5.　The morphology of colonies and chlamydospores of Ustilaginoidea virens. Nine isolates (RSB-1 to RSB-9) were cultured on potato sucrose agar for 4 wk, and the appearance of colony showed on the (A) front and (B) reverse sides. The morphology of chlamydospores under (C) the differential interference contrast microscopy and (D) the scanning electron microscope.

Author:Po-Yu Lai, Chao-Jung Wu, and Hui-Fang Ni*

Abstract:

Rice false smut (Ustilaginoidea virens), traditionally a minor disease in Taiwan, has recently become more prevalent due to the increase in organic farming practices and the impact of climate change. This disease not only affects rice yield and quality but also poses a significant threat to food safety due to the mycotoxins. Given the limited reports on the occurrence of this disease in Taiwan, this study was conducted by a comprehensive two-year, four-crop survey of rice false smut incidence in organic rice fields in Minxiong Township, Chiayi County, from 2022 to 2023. The results indicated that rice false smut was present in the first crop of 2022 and the second crop of 2023, with the highest incidence recorded in 2022. The highest hill infection rates ranged from 27.5% to 30.7%, with panicle infection rates between 9.6% and 10.6%. Most infected panicles had 1−2 smut balls, primarily located in the middle to lower parts of the panicle. Nine isolates of U. virens were obtained from the field and cultured on potato sucrose agar (PSA), showing variability in sporulation capacity, colony appearance, and mycelial growth rate. Six isolates produced chlamydospores within 2−4 wk, while the other three did not. Genetic sequencing of six isolates revealed identical ITS sequences, 99.69% similar to U. virens isolate 118-CLR-KA-40 (Accession no. OL744511), confirming the identification. Mating type analysis indicated all isolates were belonged to the MAT1-1 type. The study also evaluated the effects of temperature on mycelial growth and chlamydospore germination of the two stable isolates, RSB-5 and RSB-9, with optimal growth and germination at 25℃. Based on the fungicide efficacy tests of mycelial gtrowh and chlamydospore germination of the RSB-5 and RSB-9 isolates, pyraclostrobin, mancozeb, iminoctadine tris, and benomyl showed effective mycelial growth inhibition rates for both isolates, while kasugamycin exhibited the lowest inhibition rate. Additionally, pyraclostrobin, thiophanate-methyl, mancozeb, iminoctadine tris, and benomyl achieved 100% inhibition of chlamydospore germination, whereas kasugamycin + tricyclazole and kasugamycin had lower inhibitory effects. These findings suggest that a rotation of pyraclostrobin, thiophanate-methyl, mancozeb, iminoctadine tris, and benomyl during the booting stage of rice could effectively reduce the impact of rice false smut.

Key words:Rice, Ustilaginoidea virens, Disease control