| Rice is the second most widely grown cereal crop and the staple food for more than half of the world's population. Rice blast, caused by Magnaporthe grisea (Hebert) Barr.(Pyricularia grisea (Cooke) Sacc.), causing great damage to rice production in Fujian province, is one of the most widely spread and destructive diseases of rice in the world. The use of resistant varieties and chemical fungicides were considered as the best measures to control this disease. As the basic research, we should make clear both the composition and the distribution of physiological races and resistance to common fungicides of M. grisea. For this purpose, this paper studied on the composition of physiological races and the resistance to Tricyclazole, Kitazin P and Isoprothiolane of M. grisea in Fujian Provice from 2007 to 2008.The race of 148 isolates was identified by Chinese race differentials, CO39 near-isogenic lines (NILs) and LTH-NILs. All of isolates could be classified respectively into 20, 18 and 23 physiological races by those 3 sets of differentials, among which, ZB13, I34.1 and J76.2 were determined dominance races with occurrence frequencies of 26.52%, 50.76% and 44.70%, respectively. Meanwhile, the occurrence frequencies of the sub-dominant races ZC13, I35.1 and J70.2 showed a tendency of increasing. Diversity indexes of isolates in those 3 sets differentials were 0.8700, 0.6836 and 0.7716, respectively, among which Chinese race differentials exhibited the most potential for M. grisea isolates in Fujian. The results implied that the composition of physiological races remained stable and the replacement of the dominantrace had not been found yet.In this paper, the sensitivity of M. grisea to Tricyclazole was tested by inoculating spore suspension on rice seedlings. The EC50 of 131 strains ranged from 1.9452 to 65.6144μg/mL and the average EC50 was 12.8448μg/mL. The EC50 of the most insensitive isolate from Putian, Qiulu couty was 33.73 times that of the most sensitive one from Zhangzhou, Nanjing couty. Specifically, almost all (98.68%) the EC50 of tested strains ranged from 0 to 48.0μg/mL and a continuous unimodal curve was obtained with only two particular values beyond it. The reason behind this might due to the individual variation among strains. Finally, we concluded that M. grisea in Fujian had not showed obvious resistance to Tricyclazole yet because the average EC50 for all the strains demonstrated no significant differences among different physiological races and various collection sites.We determined mycelium growth rate to measure the sensitivity of 135 strains to Kitazin P. Results showed that the EC50 of 45 sensitive strains ranged from 1.7386 to 12.8067μg/mL and the sensitive baseline was 7.6206μg/mL. On this basis, the resistant index of other 90 strains from the fields ranged from 0.24 to 6.23 and there also were some low- and medium-resistant strains. The resistant index of the strains from Zhangzhou and Ningde city were higher than that from other cities and the highest resistant isolate appeared in Zhangzhou City with its resistance index of 6.23. The occurrence frequency both of low- and medium-resistant strains in ZC13 was 55.56% and 3.7%, respectively. However, only low-resistant strains existed in ZB13 with occurrence frequency of 54.29%.The sensitivity of 148 strains to Isoprothiolane were also measured by mycelium growth rate method. Results showed that the EC50 of sensitive strains tested to Isoprothiolane ranged from 0.3518 to 2.4984μg/mL with the sensitive baseline of 1.2794μg/mL. According to the results of resistance level detected in fields, the resistant index ranged from 2.01 to 6.37 and the low- and medium-resistant strains had already existed in the field but the resistance level was not high.. With the highest resistance of 6.37 in Longyan city, there were more resistant strains in Longyan and Ningde city. In addition, the occurrence frequency of resistant isolates from ZB13 (62.50%) was less than in ZC13 (67.87%).The biological characteristics of the strains with different resistant indexes to Isoprothiolane were investigated. Results showed that there were no significant differences between the sensitive isolates and the resistant ones.in terms of mycelium growth rate, sporulation capacity and spore germination speed. The results in cross-resistance experiments indicated that M. grisea in Fujian province showed significant cross-resistance to Isoprothiolane and Kitazin P. Therefore, these two fungicides are recommended to use alone in fields and should never be applied alternatively or as mixtures in order to avoid cross-resistance. In addition, the results also indicated that M. grisea in Fujian province had no cross-resistance between Tricyclazole with Isoprothiolane and Kitazin P.
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