| From metaphase of the 1990s to beginning of this century, the symptom "small grains and erect panicles"(SGP) in rice was widespread in most Oryza sativa cultivars in Jiangsu Province, which generally caused yield loss ranging from 10% to 30%, even as high as 50% in heavily infected regions. It was estimated that the occurrence of the phenomenon 'small grains and erect panicles' in rice totaled 3.3x105 ha during 2001-2003 in Jiangsu Province, resulting in at least 500 million kilogram of yield loss and 750 million RMB of economic loss. Nematodes were found in all rice cultivars samples with small grains and erect panicles. The application of 16% prochloraz cartap and 95 % Cartap nematicides could control the occurrence degree of small grains and erect panicles effectively. According to morphological characters, nematode might be conspecific with Aphelenchoides besseyi Christie, 1942. The leaf white- tip symptom appeared in some rice cultivars in field.After primary identification the structural characters of nematode body by light microscope, the A.besseyi nematodes separated from rice with SGP symptoms and grain produced scanning electron microscopy (SEM) and observed morphological characters of A. besseyi. There was no prominent difference between two nematode colonies by SEM. The lengths of rDNA-ITS PCR amplification product were about 750 bp. The sequence alignment result showed that the two nematode colonies were isogenous with no colonies differentiation phenomenon. So the small grains and erect panicles symptom appeared in Jiangsu Province were foreign to nematode colony. There might be the positive correlation between SGP symptom and heredity background of rice cultivars. Rice cultivar Wuyunjing No.7 inoculated A. besseyi nematodes collected from grain manifested small grains and erect panicles proved this point of view.In order to definitude the relationship between A. besseyi and the degree of small grains and erect panicles, the distribution of the nematode on the SGP was examined. Nematode on small grains and erect panicles tended to occur more towards the middle of the panicles, about 50% of gross nematode number in rice panicles. Nematode number per seed distributed in 1-20 range, about 47.7% in rice samples. There were about 10.8% seeds without nematode distribution in panicles with SGP symptom.A. besseyi was extracted from small grains and erect panicles and cultured artificially, then inoculated into bud and leaf sheaths of seedlings of two Oryza sativa cultivars, namely Zhendao No.2 and Wuyunjing No.7 in a greenhouse. The effect on rice growth, in particular the small grains and erect panicles, was revealed by the extent of the disease, seed expansion stages, nematode load and nematode mortality. In contrast to healthy seedlings, the height, length and the numbers of spikelets of unhealthy panicles of Zhendao No.2 were decreased by 6.7%, 16.4%, 13.5%, respectively. Before anthesis, nematodes were attracted to the leaf sheath and apical meristem, nematode load increased by 40%; after anthesis nematodes occurred in spikelets principally and the number increased by 90.8%.The percentage of infected seeds and nematode load were highest in plump seeds and lowest in empty seeds. Nematode mortality on grain with normal endosperm was lower than seeds with abnormal endosperm. Results indicated that A. besseyi was the pathogen in rice with the symptoms of small grains and erect panicles. Wuyunjing No.7 manifest only the small grains and erect panicles symptoms and not the symptom of leaf white-tip. These symptoms of small grains and erect panicles are new symptom records for disease caused by A. besseyi on rice. |
