| To ascertain the pathogens causing GuangXi District melons blight,Phytophthora isolates were recovered from the diseased melons plants and identified based on their pathogenicity and morphological characters as well as rDNA-ITS analysis in Guangxi in this dissertation.129 isolates of pathogens causing cucurbits blight were isolated and purified from five cucurbit crops phytophthora blight in main cucurbit crops growing areas from 6 different regions of Guangxi province.Meanwhile,the resistance and baseline sensitivity of the pathogen populations causing cucurbits blight to dimethomorph were tested.At last,through drug treatment and ultraviolet light sensitive strain induced resistant mutants,,the pathogenicity and growth of the mutants in different conditions were studied also.The main study results of this dissertation are summarized as follows.1.Phytophthora blight is one of the most serious threats to production of pumpkin.It usually damages the stems and fruits and reduces the quality and yield of pumpkin.In this study,the causal organism of Phytophthora blight of pumpkin was isolated and cultivated.Its pathogenicity was also tested.Base on its morphological characteristics and ITS DNA sequences analysis,this pathogen was identified as Phytophthora capsici Leonian.2.The sensitivities of 129 isolates of Phytophthora spp isolated from cucurbit crop blight to dimethomorph were tested using mycelial growth rate method in vitro.The results showed that the sensitive and the moderately resistant accounted for 7.8%and 92.2%strains,of all strains,the antibacterial plant strains for advantage and the resistant strains is not detected in this study.There were significant differences in the resistance frequency and the resistance level of the phytophotypes of different sources.3.129 plants melons epidemic strains on the distribution of the sensitivity of dimethomorph is unimodal curve,no remarkable decreasing in sensitivity of Phytophthora spp populations was detected,and the sensitivities of the pathogen displayed nearly normal distribution.Its inhibition concentration in the EC50 values range from 0.0813 ug/mL to 1.3400 ug/mL.Hence their mean EC50 values(0.2984±0.1846)?g/mL could be used as the baseline sensitivities of Phytophthora spp to dimethomorph in Guangxi province.4.Three resistant mutants(resistant factor>2.0)were got after 20 generations subcultured on dimethomorph-amended medium,but failed to generate resistant mutants by UV-irradiation.The results showed that there were significantly different between the mutants and their parental strains in pathogenicity,e.g.two of these mutants DRGL-4 and DRYL-154 almost lost their pathogenicity on their host.However,the growth rates of most resistant mutants were greater than that of their parental strains on different pH values and different temperatures.Based on these results,the development of field resistance to dimethomorph in populations of P.melonis is unlikely under the currently employed resistance management strategies,which indicated that the dimethomorph using for melons diseases prevention and controling effectively.5.The results show that the incidence of cucurbit blight has direct relationship with rainfall and temperature through the experimental of two years,when the rainfall is abundant,the temperature in the 28?32 ?,often is the high incidence of the disease.The two fungicides 50%dimethomorph and 50%flumorph showed highest control efficacies against cucurbit blight in all five tested fungicides.These measures such as shelter cultivation,plastic film mulch and straw mulch have higher control efficacies against cucurbit blight than local way of conventional cultivation in field control. |
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