Study On The Resistance To Powdery Mildew In Different Cucumber Cultivars

Preliminary identification on the resistance of 130 cucumber cultivars(cross combination) to powdery mildew was conducted in this research. Among them, 54 cucumber cultivars were identificated in open-field, which included 6 high-susceptible, 14 mid-susceptible, 5 mid-resistant and 29 high-resistant cultivars without disease occurrence. In greenhouse, 55 cucumber cultivars were identificated, which included 10 high-susceptible, 12 mid-susceptible, 5 mid-resistant and 28 high-resistant cultivars without disease occurrence. Ten cucumber cultivars were chosen and identificated on resistance to powdery mildew both in open-field and greenhouse at the same period time. The results demonstrated that the degree of disease occurrence in greenhouse was more serious than open-field. Different cultivars showed different resistance to powdery mildew when they were cultivated in open-field and greenhouse. The cultivars GY2, GY4 showed better resistance both in two conditions. 33 cultivars were inoculated in vitro at cotyledon stage, the disease damage was more serious than in the open-field along with the better condition, 3 cultivars were high-susceptible, 12 were mid-susceptible, 15 were mid-resistant and 3 uninfected were high-resistant cultivars.Eight cultivars which had differences on the resistance to powdery mildew were chosen to study the tissue structure, physiological and biochemical characters with different resistant cultivars after being inoculated with Sphaerotheca fuliginea.The microstructure of leaves by paraffin section showed that the tissue structure of high-resistant cultivars was more compact; the cell wall was thicker; the palisade tissue arranged more formal and close; the spongy tissue was tightly close to the palisade tissue and could be seen more clearly. The microstructure of high-susceptible cultivar"PingwangRugua", which was most serious infected, appeared a great deal of interspace among cells and was not easy to see a whole cell clearly, and the cell wall was thinner. The cell wall of the mid-resistant cultivars was thinner than the high-resistant cultivars. And the palisade tissue of the mid-resistant wasn't more close than the high-resistant, much more better than the high-susceptible cultivars such as"PingwangRugua". The palisade tissue of the rest cultivars could be clearly seen, but the interspace was still larger. It may be one of the main reasons of the cucumber resistance to powdery mildew by formal and close cell arrangement, and thicker cell wall in leaf tissue. Some kinds of enzyme activities related to the resistance were investigated before and after inoculation at different time on cultivars with different resistance. The PAL activity of the resistant cultivars appeared obvious rise after inoculation. The PAL activity of the high-resistant cultivar"GY14A"appeared an obvious activity peak one day after inoculation. They had obvious difference from other cultivars and were 1.69 times more than that of the susceptible cultivar"DE843". The PAL activity of the mid-resistant cultivar"JinYou No.30"peaked three days after inoculation and was 1.75 times more than that of the susceptible cultivars. The POD activity of the high-resistant cultivars displayed the maximum rise and were 1.22 times more than that of the susceptible cultivars. The POD activity of the susceptible cultivars peaked three days after inoculation. The PPO activity of the resistant cultivars was higher at the early inoculation stage(3 days after inoculation) and then had no obvious relationship with resistance. The SOD activity of the resistant cultivars appeared an evident rise and maintained at higher level, then declined rapidly five days after inoculation. The SOD activity of the most susceptible cultivars at first declined and then obviously rose three days after inoculation.There is no relationship between the cultivar resistance and peroxidase isozyme profiles linked to the shade of gel bands colour before inoculation by comparion of the change of the peroxidase isozyme profiles before and after inoculation. After inoculation, the obvious change was found that the primary peroxidase isozyme profiles of the high-resistant cultivars were darker than any other cultivars which indicated that the peroxidase activity was increased. Apart from that all of the cultivars enhanced an additional gel band, and the band of the high-resistant was darker followed by the mid-resistant and the susceptible cultivars.In addition, the soluble sugar content in the resistant cultivars was lower than that of the susceptible, which indicated that high-sugar was good for the occurrence of powdery mildew. The reducing sugar content of different resistant cultivars didn't show obvious difference. The soluble protein content of the high-resistant cultivars appeared an obvious peak value one day after inoculation. The mid-resistant and mid-susceptible cultivars were not increased in soluble protein content. The leaf lignin content of the resistant cultivars was higher than that of the susceptible cultivars. After inoculation, the hydrogen peroxide content was increased in all cultivars, but accumulated quickly in the susceptible cultivars, and reached the peak value three days after inoculation. The chlorophyll content of the high-resistant cultivars was higher, followed by the mid-resistant and the susceptible cultivars especially the high-susceptible cultivars.