| Box elder(Acer negundo)is a rare colored-leaf maple tree with high ornamental value,which is widely grown in North China and Yangtze River,and plays an important role in urban greening and ecological environment.However,due to the continuous expansion of planting scale,poor management,and lax detection of imported seedlings,dieback occurred in the planting area of Acer negundo in Taian City,Shandong Province,which not only destroyed the local ecological environment,but also brought economic losses to local farmers and related enterprises.In this paper,we collected local diseased shoots and leaves of box elder for tissue isolation,used Koch’s postulates as a guide to find out the pathogenicity and identify them,conducted biological characterization,tested the inhibitory effect of 9 agents for the pathogen,providing theoretical support for the identification and control of the disease in production practice.The main results are as follows:1.A field survey was conducted on the incidence sites,and the results showed that the compound maple wilt began to occur from the beginning of May 2019,and the pathogenic fungus invaded the plant through the leaves,after which the pathogenic fungus passed through the leaf veins and invaded the bast or xylem of the branches causing lesions in the transport tissues,which affected water transport,thus causing the trees to exhibit symptoms of wilt.In July and August,with the rise in temperature,the disease broke out on a large scale,reaching its peak in September November,the pathogenic bacteria on dead branches or leaves produce spores for overwintering and reinfestation the following year.The diseased trees show partial blackening at the shoots,withering on severely affected branches,and leaves above the diseased areas of shoots show symptoms of leaf blight.2.35 strains of fungal strains were isolated from the diseased areas of branches and leaves of box elder,and the isolation frequency of SDFG3 was the highest,reaching 71.42%,with a total of 25 strains.After 35 strains of 4 kinds of strains were inoculated on the branches and leaves of healthy box elder cuttings,only SDFG3 strains could cause symptoms like those in the field.According to Koch’s postulates,it was confirmed to be the pathogen of box elder dieback.3.According to the morphological characteristics of the pathogens,including colony morphology and conidia morphology and characteristics,it is classified as Exserohilum.Seven gene fragments of the pathogens--cam,gapdh,ITS,rpb2,tef1,tub2 and his were connected in series and the phylogenetic tree was constructed by maximum likelihood and Bayesian method.100% BP(bootstrap proportions)and 0.61 BPP(Bayesian posterior probability)were obtained by clustering 25 SDFG3 strains and reference strains Exserohilum rostratum,which were further confirmed as E.rostratum.E.rostratum is a newly discovered pathogen of box elder,and it is also the first report on Acer.4.In the study of biological characteristics,the pathogen of dieback of box elder could grow well on PDA,CZ and CMC,and the optimum medium for growth was PDA,which could grow in the temperature range of 5 ~ 40,and the optimum growth temperature was 30 ℃.The pathogen could grow in the range of p H 5 ~ 11,and p H=8 was the most suitable for the growth of the pathogens.5.At low concentrations,difenoconazole and prochloraz showed strong inhibition of pathogenic filament growth with EC50 of 0.1017 μg/m L and 0.1142 μg/m L,respectively,followed by propiconazole and iprodione with EC50 of 0.2925 μg/m L and 0.458 μg/m L,respectively.Eugenol,an environ mentally friendly plant-derived fungicide,also performed well with EC50 of 3.7778 μg/mL. |
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