Studies On Regularity Of Infection And Control Techniques Of Sour Rot In Citrus Fruits

In this paper, we studied infection path and period of Geotrichum citri-aurantii and morphological characters, pathogenicity, part of biological characteristics of different isolates of Geotrichum spp which were isolated from citrus fruit surface and in rhizosphere soil. Worthwhile, we researched the inhibition effects and mechanisms of different antagonists, plant extracts and low dose fungicides against citrus postharvest diseases, especially sour rot. Main conclusions were drawn as follows:1 The results about infection path showed that Geotrichum spp could not be found on foliage, bract, blossom, and stalk of citrus. We isolated several Geotrichum spps on the surface of citrus which was the most major and important carrier. A G. citri-aurantii was isolated from the rhizosphere soil.2 We isolated no Geotrichum spps from citrus surface at different growth phase. The isolation results showed that Geotrichum spps may infect citrus fruits and make them rot by wounds in transport and storge.3 Seven typical pathogenic strains includingG₁, G₂, G₄, G₁0, G₂7, G₅0, T₁0, which were isolated from the surface of citrus fruits in different areas, were identified as Geotrichum spp by morphological characters and internal transcribed spacer (ITS) analysis.4 The incidence and development of development of disease were determined by pathogen, host, environment and interaction of each other. Those factors (including pathogen capacity, the way and period of infection, temperature and humidity of environment and so on) influenced disease development. It showed that sour rot incidence and severity both increased when temperature increased during citrus storage. Pathogenicity of G. citri-aurantii (G50) was the best, the incidence disease could be whole only for 5-7d while in suitable conditions. Sequentially was G. citri-aurantii (G2,G₄,G₁0,G₂7,T₁0). G. citri-aurantii (Gi) was the worst. The sour rot of inoculated citrus fruits was only 20%after 4 days at 26℃, then the number of sour rot would not change while the diameter increased a bit. Wound, degree of maturity, and inoculation quantity of spores would increase succession of infection, and it is the key factors for the incidence and development of disease.5 The results indicate that there were not significant difference in growth characteristics (including growth rate, color of colony and thickness of colony) in PDA media. The growth temperature range from 10℃to 70℃the hyphal could survived, and the suitable temperature for mycelia growth and spores germination was 15℃to 30℃. In the deadly temperature test, the result showed that the strains were died from 80℃to 85℃.6 Thyme oil had strong fumigant toxicity followed by eucalyptus oil to all three postharvest pathogens in citrus fruits. Nutmeg oil, sage oil and curcuma wenyujin presented weak fumigant toxicity to Penicillium digitatum and Penicillium italicum while they exhibited a moderate action to G. citri-aurantii (G50). Thyme oil exhibited strong contact antifungal activities against the tested fungi. Curcuma wenyujin exhibited a moderate contact action while nutmeg oil, sage oil and eucalyptus oil present no contact antifungal activity to G. citri-aurantii (G50).7 Thyme oil showed a significant inhibitory effect against all three pathogens in citrus fruits. It reduced the percentage of decayed citrus caused by fungal pathogens. The antifungal effect of thyme oil against G. citri-aurantii (G50) was enhanced significantly by combining with Na₂CO₃ in vitro and in vivo. In vivo, sour rot could be completely inhibited by 1600μl/L thyme oil with 0.5%Na₂CO₃.8 Antagonists Cryptococcus laurentii, Rhodosporidium paludigenum, and Rhodotorula glutinis at 1x1O⁶ CFU/mL could effectively control sour rot disease in citrus fruits at 26℃. They also could control green and blue mold diseases.In addition, the antifungal effect of R. paludigenum against G. citri-aurantii (G50) was enhanced significantly by combining with low doses fungicide.9 Rapid colonizations of all three yeast antagonists in wounds were observed at 26℃. The population increased more than 20 folds after 28 h inoculation compared to the start point. Efficacy of yeasts against sour rot disease was maintained when applied simultaneously with or before G. citri-aurantii, but when applied after spores, biocontrol efficacy was significantly reduced.10 On pilot-scale experiment tests, R. paludigenum and thyme oil had great biocontrol activity against sour rot of citrus fruits,1×10⁸ CFU/mL R. paludigenum in combination with low doses fungicide (50μl/L) or 400μl/L thyme oil in combination with 1%Na₂CO₃ showed great biocontrol activity to sour rot in citrus fruits, which showed potential of biocontrol agents to control the postharvest decay of citrus fruits.