Isolation&Identification Of Pathogen Causing Potato Black Scurf And Pathogenesis Of Crude Toxin Produced By The Pathogen

The potato black scurf is one of the main soil borne diseases in Gansu province.The study focused on the isolation&identification of pathogen causing potato blackscurf and pathogenesis of crude toxin produced by the pathogen.The conventionalorganization separation menthods were used to isolate21potato black scurf strainsfrom the diseased potato tissues in Gansu potato producing areas, then identificatedthe morphological characteristics and determinated the pathogenicity of pathogen.Meanwhile, The strong pathogenicity and major anastomosis group of AG3（strains of18） as the research material were used to study the effects of growth condition onpathogen. And through the plant morphology, structure and physiological andbiochemical aspects were studied the pathogenic mechanism of crude toxins, the mainresults were as follows:1. The pathogen was successfully isolated from the diseased tissues of the fieldpotato plant having the symptoms of black scurf in twelve county (district)of Gansuprovince, the isolates were identified as Rhizoctonia solani (R. solani) based onmorphological traits, including the isolates could occupy the whole9mm petri dishabout3days on the PSA; The fresh mycelia were colorless and then became to browncolored; sclerotia was irregular，light to dark brown，and had no differentiation inrind and interior; mycelium branched at right angles with a septum near the branchand a slight constriction at the branch base.The isolates had strong pathogenicity and can infect the stems, root and tuber atdifferent positions of the potato plant, caused the typical symptoms of black scurf. Atthe same time, R. solani was re-isolated consistently from symptomatic stems, and theidentity of the re-isolates confirmed by the morphological identification, fulfillingKoch’s postulates. So it was proved to be the R. solani in Gansu province, whichcaused by potato black scurf. R.solani by means of morphological identification andpathogenicity determination.2. Identified by mycelial fusion reaction, the result showed that twenty onestrains of R.solani isolated from different areas of Gansu province were divided into four anastomosis groups AG2-1, AG-3, AG-4and the non-fusion. There were noobvious differences of different anastomosis group in regional distribution and variety,but there were differences between the isolation frequency. The frequency of AG-3was57.14%of all the tested strains, which indicated that AG-3was the majoranastomosis group of R.solani on the potato.Different vegetative compatible groups of AGs strains were determined by theirbehaviors on PSA medium. Of these, AG-3-1was the major vegetative compatiblegroup(33.33%of total strains in AG-3anastomosis group), which indicated thatAG-3-1was the major vegetative compatible groups of R. solani.3. The strains of18as the experimental material for research, the growthcondition of the pathogen were studied under the solid and liquid medium conditions,to clear the effects of incubating conditions(including carbon&nitrogen nutrients,temperature, light and pH at. el) on colony growth and mycelium dry weight of R.solani, The results shown that:The optimum carbon sources for the pathogen colony growth on solid mediawere soluble starch and sucrose；the optimum nitrogen sources were potassium nitrateand peptone, and the best pH was6-7; in liquid media, soluble starch and Potassiumnitrate were also the optimum carbon and nitrogen sources for mycelium growth, andthe most suitable pH value was7.0, the optimum oscillation frequency was80r/min.The optimum temperature was25℃for the pathogen growth in both solid and liquidmedia. Dark under the condition of incubating is good for pathogen growth on colonygrowth and mycelium dry weight.4. Through Richard toxin-producing in nutrient solution culture, obtained thecrude toxin. The biological activity determination found that the crude toxin canproduce similar symptoms as pathogen inoculation, as the key pathogenic factor ofthe disease. Different concentrations crude toxin can cause the different severity ofthe disease, when the crude toxin was50times diluted, it still had some biologicalactivity.After the R.solani toxin treatment, shoot and root biomass was significantlyreduced, especially the part of underground biomass in crude toxin treatment after60 days, the largest decline is53.66%; series of changes occurred in root morphology,including length, surface area and tips number has a downward trend; the rootactivity of plant was decreased, The biggest drop of37.23%; the malondialdehydecontent increased significantly after crude toxin can induce the potato plant leaves androots cells, the root system is more sensitive to changes of malondialdehyde.Therefore, preliminary inference, the crude toxin produced by Rhizoctonia solani isone of the main causes of potato black scurf.