Investigation Of Sweet Potato Fungal Diseases And Pathogenic Identification Of Three Fungal Diseases In Storge Period

Sweet potato is one of the important food crops in Guangxi Province.The fungal diseases of sweet potato not only affect the growth and development of sweet potato,but also lead to the decrease of yield,the quality of sweet potato and the economic benefits.Investigations on sweet potato fungal diseases were carried out from sweet potato planting area in Guangxi Yulin City and Nanning City,and from farmers market of Guangxi Chongzuo City and Nanning City and pathogenic identification of the fungal diseases and relevant research works were conducted.The occurrence of sweet potato fungal disease in the investigation area,the species of sweet potato fungal diseases and its occurrence characteristics were be sure basically.The field investigation found that diseases causing serious damage in the field was mainly suspected sweet potato scab disease.The gray white or yellow round or oblong scab spots were found on stems,petioles,back of blade and veins of plants infected with sweet potato scab,infected leaves and petioles were incurve and geniculate.The buds and leaves conld not grow up,the tender shoots were shrinking and the vines could not elongate normally.The investigation results shown that most of the sweet potato cultivars had no scab symptoms at sweet potato planting area in Renhou Town,Yulin City of Guangxi,only few sweet potato cultivars were infected by scab disease.The diseased plants shown seriously symptoms.Among the five sweet potato cultivars cultivated in the experimental field of Guangxi University,only two varieties of sweet potato were infected by suspected sweet potato scab disease,the incidence rate of both varieties was less than 10%.Disease was more serious in wann and humid climate,it is not benefit to the occurrence of suspected sweet potato scab disease if the temperature is too high or too low.whether ridging or not has little effect on the occurrence of suspected sweet potato scab disease.The survey in parts of the farmers markets of Chongzuo City and Narming City in Guangxi Province,and in sweet potato planting area in Guangxi Yulin City in Guangxi Province shown that three kinds of sweet potato fungal diseases,including sweet potato soft rot,sweet potato root rot and sweet potato dry rot.The disease that mainly occurred in stored sweet potato is soft rot disease.Most of sweet potato soft rot disease observed at late storage phase.Lack of ventilation able to increase flesh decay and disease spread rapidly.Infected sweet potato by soft rot,easily infected by other pathogen and decaying process become faster.Sweet potato root rot disease usually occurs in the early stages of storage,effect to other sweet potato not as big as soft rot disease.Sweet potato dry rot disease was observed in Yulin.In sweet potato storage process,favorable ventilation combined with application of plant ash able to control soft rot disease and play important role in extend storage time of sweet potato.The sweet potato tubers infected sweet potato soft rot were softening with brown watery and alcohol flavor,then rotten completely with foul smell.The white cotton-like mycelia appeared on lesion surface.Black sporangia produced on the top of the mycelia.The pathogenic rDNA-ITS gene was amplified by PCR.The sequencing results shown that the gene's fragment was 598bp,with 99%to Rhizopus stolonifer(accession number:HM212636.1).The sweet potato tubers infected sweet potato root rot were dark brown lesion on outer surface,tuber crack can be seen in severe infection,sweet potato flesh became black.Isolation and purification of sweet potato pathogenic fungi showed that the pathogen has thinner mycelium,orange colony color and produced 2 types of conidia.Large conidium has sickled-shape while small conidium has round to oval shape.The 515 bp rDNA-ITS fragment analysis showed that 99%of sequence similar to to Fusarium oxysporum(accession number:KY078213.1).The sweet potato tubers infected sweet potato dry rot were irregular shrink at infected surface,sponge-like and dry-rot flesh,when the condition worsen,the outer surface of sweet potato formed dark brown hollow lesion with clear-edged.Pathogen colonies were petal-like,there are small black pycnidiums at pathogen upper surface,a conidium has oval shape to spindle shape with two oil globule while(3 conidium has linear shape without oil globule.The length of the rDNA-ITS fragment isolated is 541 bp and the similarity to the Diaporthe batatas(accession number:KP995194.1)was 100%.The Diaporthe batatas was teleomorph of pathogen,but the morphological characteristics of pathogens shown that the pathogen was Phomopsis which was anamorph of pathogen.So the pathogen of sweet potato dry rot was identified as Phomopsis batatas(Ellis&Halst.).