| Ailanthus altissima(tree-of-heaven) was first introduced to USA by a gardener in Philadelphia in 1784,and was commonly available from nurseries by 1840.The species was also brought into California mainly by the Chinese who came to California in the mid-1800s.Today it is frequently found in abandoned mining sites there.Tree-of-heaven is widely distributed in 42 states,from Maine to Florida and west to California across the United States.Tree-of-heaven is a prolific seed producer, grows rapidly,and can overrun native vegetation.Once established,it can quickly take over a site and form an impenetrable thicket.Ailanthus trees also produce toxins that prevent the establishment of other plant species.The root system is aggressive enough to cause damage to sewers and foundations.The control of tree-of-heaven by different strategies in America,including mechanical or chemical technology and classical biological control,is widely applied.However,more effective and economical strategies for tree-of-heaven control in different environments have to be developed. In this study,30 isolates were obtained from the diseased plants of tree-of-heaven in China and two pathogens were shown highly pathogenic to the trees.The two fungal pathogens were identified via their morphological characteristics and with rDNA ITS-PCR method.The major biological characteristics and host range of the two fungi were investigated;the development of the powdery mildew disease were described with the Richards function.Powdery mildew of tree-of-heaven is caused by the fungus Phyllactinia guttata.Like other powdery mildews,Erysiphales are obligate parasite and can grow only on living tissue.The initial symptom on infected plants is the appearance of small white to light grey spots on the underside of the leaf and these spots often appear first on the lower leaves.Later infections may cause yellowing or defoliation.White fungal mycelia develop on the upper surface of leaf lesions in later stages.The fungus produces conidia on conidiophores in sori on infected leaves.Conidia are produced on the singlarly or in spore chains.They are transparent,thin,single-celled with cyclically arranged microhairs on the surface arround the spores.The cleistothecia of the fungus are globose,convex on the ventral side,and plane on the dorsal side;each with a gelatinous mass of penicillate cells and acicular appendages.The mature cleistothecia are 144-312μm in diameter with nine to twelve appendages which are 4.5-22.5μm long.Usually there are 10-45 asci in each cleistothecium,most of cleistothecia contain 20-30 asci.Mature asci,54-105μm×22-039μm,contained 1-2 ascospores,a few asci contained only one ascospore measured 19-38.8μm×15-25μm.Based on the morphology and rDNA ITS-PCR sequence(GenBank accession No:GQ119340) BLAST analysis,this powdery mildew inducing fungus was identified as Phyllactinia guttata(Wallr.) L(?)v.(teleomorph) and as Ovulariopsis guttata(anamorph).The conditions for growth and infection of P.guttata were examined.Results showed that conidia germinated at temperatures of 10-30℃and the optimal temperature was 20-25℃for conidial germination.The germination rate was reduced at temperatures lower than 10℃and higher than 30℃.The relative humidity(RH) also had a little influence on conidial germination and they germinated at RH11.3%-100%;the optimal RH was 75%-85%with the highest germination rate of 62.46%.Darkness inhibited germination of conidia,and when the ungerminated conidia germinated normally when they were brought back into light conditions.Different conditions of P.guttata inoculation were studied.The result showed that conidia could infected the leaves at 10-30℃and highest infection was observed at 20℃and the infection rate was about 43%.Relatively conidia could inoculate at RH30-90%;50%was the optimum RH for successful inoculation.The fungus infected the inoculated leaves most easily under 12h of light per day.A field plot experiment was conducted to study the development dynamics of tree-of-heaven powdery mildew caused by Phyllactinia guttata.The results showed that the severity of tree-of-heaven powdery mildew developed from the initial symptom production to the highest severity within two months under suitable weather conditions with the highest disease index of 75%.The epidemic development was recorded in the natural field of tree-of-heaven plants in Beijing during the time period from May to November in 2008.The oberved disease progress data were analyzed with a computer software Episimulator(?) incorporating different descriptive mathematic fuctions.Both the logistic function and Compertz function described the present disease epidemics, but the Richards function resulted in the best empirical model when the shape parameter m was 0.7289.The computed best empirical model was:x=[1+(-1.4766e-0.0172t)]3.6881。(x is the powdery mildew index at the time t).In this study,another new foliar disease on the tree-of-heaven was found in China and it was diagnosed as an anthracnose.Both the anamorph and teleomorph of the pathogen causing the anthracnose were observed and identified as Colletotrichum gloeosporioides Penz.and Glomerella cingulata Spauld.et Schrenk,respectively(GenBank accession No:GQ119341 of rDNA-ITS sequence).Inoculation with the conidial suspension under growth chamber conditions showed that the fungus was of relatively high infectivity and pathogenicity to the plant.The disease can be controlled in the tree cropping areas of China with the integrated strategies used for managing anthracnose of other crops.Also,the fungal pathogen may be a potential biocontrol agent useful in controlling the invasive weed(tree-of heaven) in the North American countries.The effects of media,carbon sources,nitrogen sources,temperatures and pH values on mycelial growth,sporulation and conidial germination of SQD-107 were tested and analyzed.The results indicated that potato sucrose agar(PSA) was found the best medium for mycelial growth and sporulation of the fungus.The mycelial grew much faster and produced significantly more conidia on PSA.When different C and N sources were tested,sucrose or glucose and NH4NO3 or NH4Cl were the best sources for the fungal mycelial growth.SQD-107 could grow normally at 10-35℃, but the best mycelial growth was recorded at 25℃.The suitable pH values were from pH3 to pH10, with the optimum being pH7-8.SQD-107 sporulated normally at 10-35℃and pH3-10,with the optimums being 25℃and pH7,respectively.The apt temperatures for conidial germination ranged from 5℃to 35℃,and the optimum was 25℃.The range of pH values for conidia germination was pH2-11,the optimum being pH6-8.Conidia of the fungus were inactivated at 60℃for 10 min.The candidate plant species for host specificity testing of the powdery mildew fungus and anthracnose fungus of tree-of-heaven were determined according to the TAG test plant list suggested by Salom and Kok in the USA.The selected plant contained 38 plant species of 37 genera within 19 families.In the test,5 plant species were not cultivated successfully.Among the successfully-cultivated 33 species,Brucea javanica was lightly susceptible to P.guttata and the other species were not infected;Robinia pseudoacacia,Arachis hypogaea and Averrhoa carambola were infected slightly by C.gloeosporioides and the other plants were not diseased.
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