Identification Of The Pathogen Causing Green Orange Fruit Rot And Screening Of Effective Agents

As a well-known tropical fruit in China,green orange has been loved by more and more consumers in recent years because of its unique taste and rich nutritional value.However,because the green orange is easily infected by a variety of pathogens in the postharvest stage,resulting in serious fruit rot,it has caused serious economic losses to the local green orange industry,and there is an urgent need to develop new control strategies to control the disease.In this study,a new postharvest disease was identified through tissue isolation and purification,pathogenicity determination and pathogen identification of diseased green orange from Qiongzhong,Hainan,and its biological characteristics and control agents were screened and applied.The main results are as follows:1.A pathogenic fungi was isolated from the rotten fruit collected from Qiongzhong green orange planting base in Hainan.Verified by Koch’s law,morphological and molecular biology identification,it was determined that the pathogenic fungi could cause green orange fruit rot,which was Lasiodiplodia pseudotheobromae.This is the first report of the pathogen on green orange fruit.The results of pathogenicity test showed that L.pseudotheobromae could infect other citrus,such as Citrus sinensis Osbeck,murcott,navel orange,shatang mandarin and Orah mandarin.At the initial stage when L.pseudotheobromae invaded the fruit,the peel appeared round brown disease spot,which expanded rapidly,then became soft and rotten,and the pericarp was covered with sparse white hyphae.After 3-4 days,the disease spot spread to the whole fruit,showing a dark brown water stain,and the inner flesh of the fruit rotted.The optimum growth conditions of L.pseudotheobromae were 28℃,p H 5.0 and 24 h light.2.The in vitro toxicities of 14 kinds of fungicides and 4 kinds of plant essential oils to L.pseudotheobromae were determined.It was found that the toxicity of fluazinam,carbendazim,prochloraz,difenoconazole,iprodione and tebuconazole were strong,their EC₅₀ were 0.009,0.058,0.103,0.687,0.762,0.789 mg/L,respectively;while those of plant essential oils carvanol,thymol,eugenol and d-limonene were poor,and their EC₅₀ were49.961,55.115,246.591 and 402.055 mg/L,respectively.The synergistic effects of prochloraz and fluazinam,iprodione and fluazinam on L.pseudotheobromae were measured.It was found that when the volume ratio of prochloraz and fluazinam was 8:1,and the volume ratio of iprodione and fluazinam was 6:1,there was a better synergistic effect,with the synergy ratio（SR）of 1.691 and 1.590,respectively.The results of postharvest control test of fruit showed that the protective and curative activity of thiophanate-methyl on the disease at 200 mg/L were 93.12%and 85.42%,respectively.The synergistic combination prochloraz and fluazinam had a strong synergistic effect on L.pseudotheobromae when the volume ratio was 8:1,and the protective and curative activity at 200 mg/L were 91.67%and 83.87%,respectively.