| Sugar beet is widely grown throughout the world and represents the second largest sugar crop used to produce sugar.Sugar beet root rot caused by Fusarium has occurred in all main production areas of sugar beet across China,which seriously affected the yield and the sugar content of sugar beet,leading to great economic losses;but there are few reports related to this disease in China.In the present study,the population composition of Fusarium isolates recovered from diseased sugar beet root on the basis of morphological and molecular characteristics,their sensitivity to three fungicides,and mycovirus in Fusarium isolates were studied to provide theoretical reference for controlling sugar beet root ror disease in the field.In this study,a total of 552 Fusarium isolates were collected from sugar beet roots exhibiting typical root rot symptoms in eight provinces or autonomous regions of China from 2009 to 2015 and the number of sampling locations was 206.Among the Fusarium species obtained,Fusarium oxysporum(42.39%)was identified as the most prevalent species,followed by F.equiseti(24.28%)and F.solani(14.67%).The other eight Fusarium species,including F.proliferatum(4.89%),F.tricinctum(3.80%),F.redolens(3.62%),F.brachygibbosum(3.08%),F.verticillioides(1.45%),F.graminearum(1.27%),F.nygamai(0.54%),and F.culmorum(0.36%)were less frequently isolated.Fusarium isolates,representing the different species and geographic origins were demonstrated to be capable of causing tip rot and vascular discoloration in the sugar beet roots,with the incidence and disease index were 84.2%-100.0%and 41.94-75.83,respectively.This is the first report that F.tricinctum,F.brachygibbosum,F.redolens,F.proliferatum,F.nygamai,and F.culmorum could cause sugar beet root rot in China and F.tricinctum,F.brachygibbosum,F.redolens,F.proliferatum,and F.nygamai incited this disease worldwide.The optimum temperature for mycelium growth of Fusarium tricinctum,F.equiseti,F.redolens,F.oxysporum,F.verticillioides,F.solani,F.nygamai,and F.brachygibbosum is 30℃,while the optimum temperature for mycelium growth of F.culmorum and F.graminearum is 25℃.Under the same temperature,mycelium growth rate of Fusarium brachygibbosum,F.culmorum,and F.graminearum were the highest,followed by F.equiseti,F.redolens,F.oxysporum,F.verticillioides,F.solani,and F.nygamai;compared with these Fusarium species,mycelium growth rate of F.tricinctum was relatively lower.Mycelia growth of the tested Fusarium isolates were effectively inhibited by the three fungicides,difenoconazole,prochloraz,and tebuconazole;inhibition effect of prochloraz(whose average EC50 was 0.0288-0.3195 μg/mL)on the Fusarium isolates was significantly higher than difenoconazole(whose average EC50 was 0.3312-4.6103 μg/mL)and tebuconazole(whose average EC50 was 0.0678-1.0362μg/mL).In addition,ten difenoconazole-resistant strains of F.oxysporum were obtained through fungicide adaptation method.There was no difference in the sequences of cyp51 gene from the wild sensitive strain and the resistant strains with different resistance factor,which showed that no point mutation was detected in the cyp51 gene of the resistant strains.The dsRNA of Fusarium isolates was extracted and detected.The results indicated that one species of virus belonging Partitivirus was found in Fusarium equiseti strain 15XJ23,which has three dsRNA segments;one species of virus belonging Chrysovirus was discovered in Fusarium equiseti strain HL28,which has four dsRNA segments.Fourty five mycovirus-related contigs were detected from fifty Fusarium strains which representing the different species and geographic origins using the high-throughput sequencing.It was revealed that there were many diverse mycoviruses with different kinds of nucleic acid in the fungus Fusarium,including some unclassified novel mycoviruses. |
