Monitoring Sensitivity Of Magnaporthe Oryzae To Isoprothiolane And Pyraclostrobin In Middle And Lower Reaches Of Yangtze River

Rice blast,caused by Magnaporthe oryzae,is one of the most important diseases of rice leading to serious grain loss each year.At present,chemical control is the most reliable measure to control rice blast for its high efficiency of both control effect and economic.The region of middle and lower reaches of Yangtze River is the main rice producing areas in China,and unrational application of fungicides became a big problem in this area.It may lead to risk of fungicide resistance that long history of less variety of fungicide applied in the area.It is necessary to assess the efficiency of fungicides and application for control of rice blast disease and to carry on the research on sensitivity of M.oryzae to isoprothiolane and pyraclostrobin in middle and lower reaches of Yangtze River.In this study,tests both in laboratory and field were carried out to compare the indoor toxicity and field control effects of different fungicides on M.oryzae.The results indicated that the tricyclazole is most efficient fungicide for controlling neck blast,and broad-spectrum fungicides,such as azoxystrobin,trifloxystrobin·tebuconazole and azoxystrobin·difenoconazole showed better control effect in leaf blast.For the application period,initial heading stage is the critical period of prevention of neck blast,and it was necessary to perform an additional fungicide spray at full heading stage.On the condition of suitable fungicides,a spray before heading has a certain prevention effect against neck blast.In 2016-2017,895 strains of M.oryzae were isolated from samples which collected from 12 rice growing regions of the middle and lower reaches of Yangtze River.The sensitivity tests were carried out to establish the sensitivity baseline of isoprothiolane and pyraclostrobin of M.oryzae in the regions.The sensitivity of 116 of the M.oryzae strains of isoprothiolane was determined by mycelial growth inhibition method.The EC₅₀ value of isoprothiolane was 0.928-8.495 mg/L,and the highest EC₅₀0 for the least sensitive isolate was 9.154 times higher than the most sensitive one,and the sensitivity baseline to isoprothiolane of M.oryzae in the areas was 3.254±1.561 mg/L.Considering that SHAM can inhibit by-pass oxidation of fungus,which could disturb the sensitivity test for QoI fungicides,the sensitivity,colony growth rate and by-pass oxidation contribution value of pyraclostrobin with SHAM or not were tested and compared,and 50 mg/L was determined as the best concentration of SHAM.The sensitivity of 106 M.oryzae strains to pyraclostrobin were determined by mycelial growth inhibition method.The results showed that the EC₅₀ value was 0.003-0.027 mg/L,and the highest EC₅₀ for the least sensitive isolate was 9 times higher than the most sensitive one.The sensitivity baseline to evaluate the M.oryzae to pyraclostrobin in the area was 0.012±0.005 mg/L.The resistance of M.oryzae to isoprothiolane and pyraclostrobin were monitored by discriminating concentration.The discriminating concentration value of the resistance monitoring of isoprothiolane and pyraclostrobin in the middle and lower reaches of Yangtze River was determined by the minimum inhibitory concentration.The discriminating value of isoprothiolane was 60 mg/L and the discriminating value of pyraclostrobin was 1.4 mg/L.In this study,the resistance of rice blast fungus to isoprothiolane and pyraclostrobin was detected by the discriminating concentration.Five lower resitance strains of M.oryzae to isoprothiolane were found in the fields,and the resistance frequency was 0.559%.Field resistant strains of M.oryzae to pyraclostrobin were not detected.There was no cross resistance between isoprothiolane and other fungicides,such as pyraclostrobin,azoxystrobin,iprobenfos,and tebuconazole,however,risk still exist for cross resistance between isoprothiolane and iprobenfos,which has the similar mechanism of action.Results of genetic stablilty testing showed that the character of isoprothiolane could not be stablely inherited,so resistant strains were not easy to became dominant in field which reducing the risk of outbreaks.Resistant strains of M.oryzae to pyraclostrobin could not be found in the field for the fungicide not be large-scale applied in rice production yet.In this study,one moderate-resistance strain and four low-resistance strains to pyraclostrobin were obtained by indoor drug induction,and preliminary studies of their biological characteristics were performed.The results showed that the fitness of some resistant mutants decreased significantly comparing with the parental strains for reduction in rate of growth and sporulation.Analysis of cross resistance showed that there was no cross resistance between pyraclostrobin and fungicides with different mechanism of action,such as isoprothiolane,azoxystrobin,tebuconazole and carbendazim,but there was orthogonal cross resistance between pyraclostrobin and azoxystrobin which is also belong to the QoI fungicides,which means that long term application of strobilurin fungicides without alternation could lead to resistance risks.However,the resistance level of allresistance mutants to pyraclostrobin were not stable and was decreasing to sensitive finally after several generations.