A Survey Of Pathogenic Fungi On Common Gramineous Weeds And Studies On Their Herbicidal Potential

Microbial herbicides are receiving more and more attention because of theirlittle effect on environment, difficulty for weeds to develop resistance, shortperiod and low cost of R&D. In this study, isolation, identification andscreening of pathogenic fungi were performed on the basis of sample collectionof diseased gramineous weeds in the field. The secondary metabolites from twoisolates with herbicidal activity were separated, structurally identified andphytotoxically tested. The results provided the basis and information forutilization of pathogenic fungal resources on weeds and development ofmicrobial herbicides.In grass growing season of2009, field survey and sample collection ofdiseases were carried out on common gramineous weeds, such as Setaria viridis,Digitaria sanguinalis, Eleusine indica, Echinochloa crusgalli etc. Two hundredand six fungal isolates were obtained from99samples collected from ten citiesand counties of Shandong and Jiangsu provinces and Shanghai. Based onmorphological characteristics of160spore-producing isolates and sequencealignments of rDNA ITS of67isolates,21species of11genera were identified:Alternaria compacta, Bipolaris cynodontis, B. heveae, B. sacchari, B.sorokiniana, B. setariae, Cladosporium cladosporioides, Colletotrichumdematium, Colletotrichum gloeosporioides, Curvularia affini, Curvulariaintermedia, Curvularia lunata, Exserohilum rostratum, E. fusiforme, Fusariumequiseti, F. lateritium, F. tricinctum, Nigrospora oryzae, N. sphaerica, Phomaglomerata, P. herbarum. Among them, the fungi belonging to Alternaria,Bipolaris, and Curvularia were the primary on the common gramineous weeds.One hundred and fifty-six isolates, selected from the206isolates obtainedin this study and the689isolates obtained in earlier study, were tested by usinginoculation with spore suspension, detached leaves, seminal root and ste mdipping with the crude culture filtrate of the fungal isolates. The crude culturefiltrates from six isolates NY1, ZXL09013a, ZXL09025a, ZXL09035a,ZXL09066d and ZXL09078a caused obvious brown spot on leaves of Setariaviridis, and remarkable inhibitive effect on the seminal roots and stems ofSetaria viridis, Digitaria sanguinalis and Echinochloa crusgalli. The results ofoutdoor inoculation showed that the isolate NY1of B. setariae and the isolateZXL09078a of B. sorokiniana caused severe disease on Setaria viridis. Ten daysafter inoculation, leaves and stems of the grass were covered with brown spots,and some spots merged and leaves wilted. Both of the two isolates causeddisease on the tested gramineous plants in different degrees, especially severe disease on Setaria viridis and Digitaria sanguinalis, but light or no disease onthe tested non-gramineous plants.Seven compounds were obtained from the ethyl acetate extract of ricefermentation of B. setariae and determined as diisononyl phthalate,3-anhydroophiobolin A, ophiobolin I, ergosta-5,7,22-trien-3β-ol,5α,8α-epidioxyergosta-6,22-diene-3β-ol,6-methylbenzene-1,2,4-triol, and2-hydroxybenzaldehyde, respectively, by extensive spectroscopic analysis.Bioassay on punctured and intact detached leaves of Setaria viridis indicatedthat ophiobolins produced obvious brown lesions, which were similar to thatcaused by the pathogen B. setariae. It suggested that ophiobolins were theprimary phytotoxic me tabolites of the isolate NY1of B. setariae. Among them,the formation of the lesions produced by3-anhydroophiobolin A was the fastestand the concentration of agent was the lowest. Therefore the phytotoxicity of3-anhydroophiobolin A was the highest.In bioassay on punctured detached leaves of43plant species of18families,both the3-anhydroophiobolin A and the mixture of three ophiobolins including3-anhydroophiobolin A showed phytotoxic to all of the monocotyledons tested indifferent degrees, especially strong phytotoxicity on Setaria viridis, Eragrostiscilianensis, Digitaria sanguinalis, Echinochloa crusgalli, Setaria italic,Sorghum dochna and Commelina communis. The phytotoxicity of the3-anhydroophiobolin A and the mixture to the tested dicotyledons varied greatlyon different plant species. There were strong toxicity on Chenopodium album,Solanum nigrum, Euphorbia humifusa, Oxalis corniculata and Portulacaoleracea and no phytotoxicity on Helianthus annuus, Lactuca sativa, Spinaciaoleracea, Potentilla supine, Fragaria ananassa, Arachis hypogaea andPhaseolus radiates. The results also indicated that the lesions produced by themixed solutions were larger than those produced by3-anhydroophiobolin A,which suggested the synergy between different ophiobolins.