Studies On Methods For Identification Of Phytophthora Blight And Mechanism Of The Resistance At Seedling Stage Of Watermelon

Watermelon phytophthora blight is a phytophthora capsici in fungus, belong to Mastigomycotina,Peronosporales and Phytophthora. The phytophthora capsici Leon is the major cause phytophthora pathogen that had a large proportion in pathogen of watermelon phytophthora blight, and also seriously infected cucurbits and solanaceous vegetables, besides infected melons. The disease can progress very rapidly and symptoms can occur quickly when plants infected and very difficult to healing. This caused a large economic loss to the production of watermelon. The watermelon cultivars with different resistance were used in this study as plant materials. On the basis of inoculation method with zoospore seedling watermelon inoculation, such as inoculation density,inoculating way,seedling stages and the appropriate disease survey time after inoculation were studied to determine a way of accurate and practical in identification at seedling stage, and also make sure the standard of disease and resistance grades, as well as to explore the changes of the protective enzyme activities and hormones after inoculation.The main results were as follows:1. The three inoculating density were adopted in the study, 1×105 pfu/ml, 1×106 pfu/ml and 1×107 pfu/ml, the reaction of plants after inoculation was analysed and found that the comparatively suitable inoculating density was 1×106 pfu/ml.2. There are 3 inoculating ways, injecting, spraying and pouring respectively. Among them, pouring is comparatively like in the field, also is the most suitable; injecting causes the disease quickly and seriously; spraying needs the long time made the materials infected and irregularly spray.3. It is found that the best seedling stage is 2 euphyllas comparing 3 different seedling stage with pouring.4. The between suitable disease survey are day 12 and day 15 after inoculation by analysed the plant reaction. Because in this period the disease index is comparatively steady.5. On the basis of the above research, a test was carried on to identify the disease resistance of 27 samples, selection resistant cultivars of 11 samples, mid-resistant cultivars of 6 samples, susceptible cultivars of 9 samples and high-susceptible cultivars of 1 samples.6. Through analysis of the protective enzyme activities of different watermelon cultivars with different resistance found that the differences of the activities of polyphenoloxidase (PPO), peroxidase (POD) were indistinctive between resistant cultivars and susceptible cultivars in the healthy (before inoculation), but the difference of the phenylalanine ammonia-lyase (PAL) activity was distinctive, and the activities of resistant cultivars were higher than those of the susceptible cultivars. After inoculation, the enzyme activity was significantly higher in watermelon cultivars with different resistance, and the activities of resistant cultivars were higher than those of the susceptible cultivars all the time. So the relative PPO, POD, PAL activity and watermelon phytophthora blight were possibly correlated with disease resistance of the tested watermelon cultivars.7. Through analysis of the hormones in resistant cultivar 09-1-1,mid-resistant cultivar 148 and susceptible cultivar GS-37 before and after inoculation found that the content of ZR,IAA and GA in the resistant cultivars were higher than those of the susceptible cultivar after inoculation,but the content of JA,ABA in the resistant cultivars were lower than those of the susceptible cultivar. So hormones have a vital function in the aspect of disease resistance of the tested watermelon cultivars.