Temporal And Spatial Dynamics Of Avirulence Genotypes In The Populations Of Pyricularia Oryzae

Rice blast caused by the ascomycete fungus Pyricularia oryzae is one of the most devastating crop diseases worldwide.The most effective and economical methods for controlling the blast is to grow resistant rice cultivars.However,investigation on the dynamics of avirulence genotypes in P.oryzae populations is lacking,it is difficult in practice for farmers and breeders to deploy resistant rice cultivars without knowing avirulence genotypes in a P.oryzae population.It is therefore a key step to study the dynamics of avirulence genotypes in P.oryzae populations for deploying the blast resistance genes and for breeding blast-resistant cultivars.In this study,the author investigated temperal and spatial dynamics of avirulence genotypes in P.oryzae populations and its relation with field resistance of rice cultivars in three counties during 2010 to 2016,and obtained major results as follows.Firstly,the author collected a total of 300 single spore isolates from the diseased leaves of the Pil2,Pib,Pik or Pizt monogenic rice lines and 314 single spore isolates from the diseased leaves of Lijiangxintuanheigu(LTH)that were grown in the Agricultural Technology Extension Station of Donggang,Liaoning Province.The pathotypes for each of these isolates were determined based on their infections on the IRRI 24 monogenic rice lines with the wounding inoculation of detached rice leaves.The results showed that none of the isolates were avirulent to their original monogenic lines,whereas some numbers of the isolates from LTH were avirulent to each of the four monogenic lines,indicating that there exists selection pressure in the monigenic lines on P.oryzae.The author also collected 45 isolates from different diseased leaves of neighbouring LTH bunches,and identified them into 45 pathotypes.Further,the author collected 25 isolates from different lesions of the same leaves and found that most of them belonged to different pathotypes.In addition,the author collected two to six single spore isolates from the same lesions and confirmed that these isolates from the same lesions were identical pathotypes.Taken together,the author concluded that isolates from different leaves of LTH could be suitable for investigating avirulence genotypes of P.oryzae populations.The author then collected P.oryzae isolates from LTH that was grown in Dongang Agricultural Technology Extension Station of Liaoning province,Farm 856 of Heilongjiang Province and Conghua of Guangdong Province and determined their pathotypes by inoculating the IRRI 24 monogenic rice lines.During the past continuous six years,the author obtained 693 isolates from Dongang,556 isolates from Farm 856 and 352 isolates from Conghua.Clustering analysis showed that the isolates from Dongang,Farm 856 and Conghua could be classified into 674,504 and 328 distinct pathotypes,respectively,indicating that there exist huge numbers of pathotypes in P.oryzae populations.However,none of any single pathotypes occupied>2%of the total isolates in each of the three populations,indicating that there exists any predominate pathotypes in the P.oryzae populations.At the same time,the author compared the pathotypes from the three populations and found that isolates from the same counties were more closely clustered and were separated from those from different counties,indicating that isolates from the same populations have closely related pathotypes.Further,the author performed comparative analyses on avirulence genotypes and their temporal changes in each of the three populations for the past continuous six years,and obtained results showing that for every year and in each of the three populations,there were certain numbers of isolates that were avirulent to each of the monogenic lines.In particular,there exist predominant avirulence genotypes in each of the populations.For example,Avr-Pil,Avr-Pi20,Avr-Pi3,Avr-Pi5,Avr-Pi7,Avr-Pi9,Avr-Pikh,Avr-Pikm,Avr-Pikp,Avr-Piks and Avr-Piz5 were detected in more than 50%of the total isolates from Donggang population;whereas in the Farm 856 population,Avr-Pill,Avr-Pi12,Avr-Pi9,Avr-Pita2 and Avr-Pizt were detected in more than 50%of the total isolates.However,for the Conghua population,Avr-Pikh,Avr-Pikm and Avr-Pi9 were present in more than 50%of the total isolates.From this comparison,it is evident that the three different population of P.oryzae have distinct predominant avirulence genotypes,suggesting that the rice blast resistant breeding for different counties in China should implement distinct blast-resistance genes in according with predominant avirulence genotypes in the pathogen populations.To verify the above suggestion,the author combined data of the field resistance of the monogenic lines and the frequencies of avirulence genotypes in Dongang and Conghua and found that monogenic lines with cognate resistance genes to the predominant avirulence genotypes in the populations showed better performance of field resistance to the blast disease.There evidently exists a negative linear correlation between the disease indices of monegenic lines and the frequencies of cognate avirulence genotypes in the population.Notably,strong field resistance was recognized in monogenic lines that were incompatible with more than 70%isolates in the population.However,if an avirulence gene was shared by less than 40%of isolates,monogenic lines wtih the corresponding resistance genes were still susceptible in the field.Further,the author measured the anti-race spectrums of some commercialized rice cultivars and obtained results showing that the anti-race spectrums of Longjing 29,Longjiang 31 and Suijing 8 to the Farm 856 population were higher than 70%but Kongyu 131 was resistant against less than 40%isolates of the same population.Besides,the anti-race spectrum of Yanjing 456 to the Donggang population were higher than 70%approximately in six years while Liaoxing 1 and Yanfeng 47 were resistant against under 20%isolates of the above-metioned population.Kongyu 131 had been planted as major rice cultivars in Heilongjiang Province,which caused the epidemic of rice blast from 2005 to 2007.The rice planting area of Liaoxing 1 and Yanfeng 47 had been accounted for 80%,which led to the epidemic of rice blast in 2010.This study provided important experimental data to establish a novel efficient strategy for breeding blast-resistant rice cultivars and for deploying the resistance genes in accordance with dynamic changes in avirulence genotypes of P.oryzae to control blast in Donggang,Farm 856 and Conghua.