| Acacia melanoxylon which belongs to sub-family of Acacia Mill.is known as a medium and short rotation of precious tree species.It has now become the preferred species of Acacia in many provinces in C hina.In recent years,there has been a new disease in A.melanoxylon cultivation base.The disease caused the branches or plant to death,resulting in serious economic losses.Therefore,in this study,we investigated the epidemic area.After the collection of ill branches and trunks,the pathogens were identified on the basis of pathogenicity testing,as well as morphological characters,molecular identification and pathogenicity experiment.The disease resistance of different clones was preilimarily tested.This study presents useful information for control measures to cope with diseases of A.melanoxylon.The main results were as follow:1.This study found a white rot disease of A.melanoxylon which is a new fungal disease.The disease mainly endangerd four to six trees during spring and the branches were about six to ten in diameter.The main symptoms of the disease spots shape for the spindle.The disease spots was sunken than healthy tissue.The boundary between ill and healthy tissue was very clear and there has a brown or black line after cutting the bark.The phloem was brown or black and the xylem was dark brown or black through the transverse and longitudinal sections of the ill tissue.The bark of the trees which has been growing for years was cracked and the white rot was visible when scraped the bark.The disease resulted in half of the dry branches and the trees would be die in severe cases.2.Koch’s postulates were fulfilled and a species of fungus was found to be the pathogenic agent of A.melanoxylon dieback.Using the target strain inoculated No.17 clones of semi-annual and biennial plants,the symptoms of the disease are consistent with the natural symptoms of the forest,furthermore,the target strain was confirmed.The disease spot at the beginning was brown or black,shape for the spindle and sunken in the middle.At the boundary between ill and healthy tissue was very clear and has a black line after cut the bark.The disease spots around the stem lateral expansion of 3/4 circles.In the later stage of a disease,the branches with stains out of color and become dead and plants growed green buds near the roo t.Finally the tree might die.3.The pathogen was identified by morphology and molecular biology technology.The single colony on potato dextrose agar plates were nearly round,white,radiate and texture sparse cottony,then developing denser and felt in the plate.The colony became red in guaiacol solution after 8 minutes later.The hyphae bearing clamp connections,branches often arising froma clamp,the walls hyaline and smooth,3.5 ~4.2μm in diameter at the beginning and then became 2.6~3.4μm in diameter.Basidia clavate,containing 0 to 3 spheroids,mostely containing 1,5.6~9.4 ×2.5~3.5μm,the walls hyaline,thin.The fungis DNA was amplified by PCR with the universal primer of fungal r DNA ITS1/ITS4,ITS5/ITS4 and ITS1-F/ITS4-B.The alignment results indicated that the sequences are 558 bp,577bpand726bp.The sequencing result were logged in GenBank to BLAST and the result showed the identity up to 99%、99%and 98% with reference sequences and the GenBank accession numbers are MF099657、MF135489、MF135490.Based on morphology and molecular biology,the fungi is identified as Pseudolagarobasidium acaciicola.The genus belongs to Phanerochaetaceae,Polyporales,Agaricomycetes,Basidiomycota,Pseudolagarobasidium J.C.Jang & T.C hen.P.acaciicola is a new species in China and is also the first report as the pathogen of A.melanoxylon dieback disease in the world.4.The resistance of different clones of plants to P.acaciicola was evaluated.The results showed that there was a significant difference among the disease resistance levels of No.6,17,20 and 47 clones.It was suggested that No.6 clone was resistant,No.20 clone was low resistance,No.47 clone was low susceptible,No.17 clone was high susceptible. |
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