Study On Alfalfa Paraphoma Root Rot,a New Disease In The World

Alfalfa disease is the main factor restricting alfalfa production,and the alfalfa root rot not only seriously affects the yield and quality of alfalfa,but also accelerates the decline of alfalfa grassland.Chifeng City,Inner Mongolia was known as China's “grass capital”.In order to realize the impact of alfalfa disease on production in Chifeng,we investigated the local alfalfa diseases and discovered a new disease in the world,and the pathogen's biology,physiology,pathogenicity,infection cycle and disease resistance to 30 alfalfa varieties were studied.The main results are as follows:1.There were eight kinds of diseases in the area,which were powdery mildew(Leveillula leguminosarum),rust(Uromyces striatus),leaf spot disease and black stem disease caused by Phoma medicaginis,brown spot caused by Pseudopeziza medicaginis,anthracnose(Colletotrichum sp.),leaf spot diseases caused by Leptosphaerulina briosiana,leaf spot diseases caused by Spetoria medicaginis and alfalfa root rot.The leaf spot disease caused by Phoma medicaginis,Leptosphaerulina briosiana and Spetoria medicaginis,and alfalfa root rot were the main disease in this area.2.Nomenclature and symptom of Alfalfa Paraphoma Root Rot.The incidence of root rot in the third-aged alfalfa in Chifeng was as high as 68%.The middle and upper parts of the root cortex appeared as black and rot.The stele of root turned yellowish brown,black,and rot.However,the aboveground of the root rot plants were normal.The isolation frequency of Paraphoma was 77.1%,which was the preponderant fungus.The results of seed inoculation and root dip inoculation showed that the fungus was pathogen of alfalfa.According to the morphological characteristics of the pathogen and the use of ITS,EF1-? and TUB sequences to construct a phylogenetic tree,the pathogen was identified as Paraphoma radicina,and the alfalfa root rot caused by it was a new disease in the world.The disease was named as alfalfa Paraphoma root rot(APRR).The typical symptom of APRR was that it mainly harmed the middle and upper parts of the main root,causing the root cortex to be black and rot,and the stele becomed brown and rot.The key points for the identification of APRR was that there were small,globose black spots on the root cortex,which were pycnidia of Paraphoma radicina.3.APRR was harmful to alfalfa growth and caused seed rot.After seed inoculation with P.radicina one week,seedling incidence were 84.0 %.Two months after root dip inoculation with Paraphoma radicina caused root infection in alfalfa and visible symptoms were present in 70% of inoculated plants,and the plant height,root length and biomass of the plant were significantly(P < 0.05)reduced than control,but no plant death was observed.4.Study on infection cycle of APRR found that the incidence was 60% and the disease index was 22.0 after the soil collected from the diseased field was planted with alfalfa seeds in the greenhouse for 2 months.APRR was a soil-borne disease.Study on the biological characteristics of P.radicina found that the optimum growth temperature was in the range from 25 to 30 ?,and the best growth pH was of pH 8 and 9.The optimum temperature for spore germination was 25? and pH 7.In addition,the lethal temperatures of the hyphae and conidia of P.radicina were 55 ? and 41 ?,respectively.P.radicina can grow on all carbon and nitrogen sources tested.The pathogen was extremely difficult to produce spores.Among the eight kinds of culture mediums,spores can only be produced on ARA during one week of cultivation,and can also be produced on PDA at four weeks.However,the spore production on ARA was significantly higher than PDA.5.After measuring the indicators of 30 alfalfa varieties in the greenhouse after inoculation with P.radicina,it was found that Caoyuan No.3 had the highest incidence and disease index of 90% and 62.5 respectively,while Longwei 3010,Gongnong No.3 and Supersonic incidence and disease index were significantly lower than most other varieties.Evaluation of the resistance of 30 alfalfa varieties to P.radicina root rot found that Longwei 3010,Juneng No.2 and Gongnong No.3 were high-resistance varieties,while Caoyuan No.3 and Gongnong No.1 were susceptible varieties.