| Rice false smut is caused by Villosiclava virens that infects specifically rice stamen filaments. Since1990s rice false smut has become one of the major rice diseases in China due to the large scale cultivation of hybrid rice varieties and the improvement of water and fertilizer levels, and its outbreaks were also frequently reported in other countries. The disease not only reduces the grain yield and quality, but also threatens the grain safety as food for human and animals due to great amount of mycotoxins in the false smut balls.So far, there has been debate on life cycle of V. virens, especially about the overwinter form and primary infection source. In this study, sclerotia formation of V. virens in different geographical regions, the sclerotial germination behavior and the spore spreading had been carried out. And the potential alternative hosts in main rice-growing regions in China were also surveyed and identified. The main results are as follows:1. During2008and2012, sclerotial production was investigated in main rice-growing regions in China. We found that Sclerotia could be formed commonly in Fujian, Zhejiang, Liaoning and Shanxi, covering the sub-tropic and temperate zones in China. In South China, sclerotia usually produced on late-maturing varieties, especially on the super-rice cultivars. The maximal number of sclerotia in super-rice cultivar could reach150,000per acre in the seriously diseased fields. Ascospores capture tests in paddy fields showed that ascospores could be captured during the main growth period of rice, while the chlamydospores could captured just after disease occurrence. These indicate that sclerotia are most likely the exclusive source for primary infection of V. virens.2. Laboratory tests showed that relative low temperature is an important environmental factor for sclerotial formation.3. Sclerotia germination experiments showed that:(1) Sclerotia could germinate within1to3months under suitable conditions, and fruiting bodies and ascospores within the next month. Sclerotia could germinate many times and could product spore continuously while nutritional and environmental conditions allows. One sclerotium could produce about500,000ascospores on average, and each ascospore could germinate to produce6.2secondary conidia.(2) As the fruit bodies developed, sticky honeydews appeared and covered on their tops, which prevent the ascospores from ejecting. This suggests the ascospore spread may need other vectors, such as insects.(3) The ascospores usually germinated in the fruiting bodies to produce mycelial colonies and then produced small conidia in experimental condition.(4) Light is necessary for inducing the fruit body formation and sexual reproduction in sclerotia germination. In dark condition the sclerotia would germinated to produce loosely mycelial colonies and then formed conidia on them. However, after the recovery of light these mycelial colonies could return into sexual reproduction by bundling the loose hyphae into fruit bodies. The growth of fruiting bodies showed significant phototropism.4. Two common weeds in paddy fields, Digitaria marginata and Echinochloa crusgalli, were found to present the similar symptoms of smut diseases on a few individuals in Xiangshan (Zhejiang,2013and2014) and Chengdu (Sichuan,2013). Morphological observation of the spores and rDNA-ITS sequence analysis showed that the pathogens belonged to the Basidiomycetes, Ustilago syntherismae and Ustilago trichophora, respectively. Ultrastructural examinations revealed that the fungi were biotrophic parasites and could penetrate into host cells directly. The pathogens could be detected both in the stems and spike-stalks of D. marginata, implying it is a systemically infected pathogen. These suggest that there has been no alternative host of V. virens to be identified in China. And even if existing, it may not play an important part on the life cycle and disease occurrence in China. |
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