| In recent years, with the development and continually use of pesticides, the resistance of plant pathogens to the pesticide becomes more and more severe. There are two main reasons for this phenomenon: one is that the same pesticide is used continuously on the same area, the other is the concentration used for pesticides becomes higher and higher. Both of these promote the formation of drug-resistant pathogen. On the one hand, pathogen mutations which are resistant to one kind of pesticide would be induced after exposure to this kind of pesticide for a long time; on the other hand, with the selection pressure of a pesticide increase, drug resistance which has been existed in the pathogen will accumulate gradually and the drug-resistant population will become the dominant population. Thus the efficiency of pesticides will reduce. Valsa canker, caused by fungal pathogen of Valsa Fr. species is the most destructive disease of apple tree that causes serious economic losses. In this study, difenoconazole and azoxystrobin, which showed high control efficiency on other plant pathogens, were test for their sensitivity to Valsa spp. in vitro; the resistant risk of these two fungicides were assessed. The main results are as follows:1. Forty eight Valsa spp. isolates from different region in China were tested for sensitivity to difenoconazole. The results showed that EC50 values of difenoconazole on mycelial growth ranged from 0.000141 to 0.193μg/ml, with a mean of 0.035μg/ml. Most of the isolates were sensitive to this fungicide. However, one isolates HNXX47 exhibited reduced sensitivity to difenoconazole with EC50 of 0.193μg/ml, which was significantly higher than those of remaining isolates tested and thus was considered resistant to difenoconazole with resistance factor (a ratio of EC50 for a fungicide- resistant isolate relative to the mean EC50 for the sensitive isolates) of 5.96.2. Three species V. mali, V. malicola and V. persoonii exhibited different sensitivity to difenoconazole. V. malicola is the most sensitive species with EC50 of 0.014μg/ml followed by V.mali with EC50 of 0.037μg/ml, and V. persoonii was the least sensitive one with EC50 of 0.059μg/ml.3. Isolates from different regions did not show significant difference on the sensitivity to difenoconazole. 4. The fitness test of difenoconazole- resistant isolate HNXX47 showed that the mycelial growth rate of HNXX47 was slower than other sensitive isolates tested in this assay; after being subcultured on PDA for nine generations, EC50 of difenoconazole for difenoconazole- resistant isolate HNXX47 remained at the same level as that of the 0, 3, and 6 generation indicating that the resistance of this isolated may be stable.5. Forty eight Valsa spp. isolates from different region in China were tested for sensitivity to azoxystrobin. The results showed that EC50 values of azoxystrobin on mycelial growth ranged from 0.051 to 0.248μg/ml, with a mean of 0.155μg/ml. It indicated that the population of Valsa spp. was more sensitive to difenoconazole than azoxystrobin.6. Three species V. mali, V. malicola and V. persoonii exhibited different sensitivity to azoxystrobin. V. malicola is the most sensitive species with EC50 of 0.106μg/ml followed by V.mali with EC50 of 0.161μg/ml, and V. persoonii was the least sensitive one with EC50 of 0.214μg/ml.7. Isolates from different regions did not show significant difference on the sensitivity to azoxystrobin.
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