Downy Mildew (Pseudoperonospora Cubensis) In Cucumber Is Classified Based On The Hypersensitive Response

In order to claasify resistance in cucumber against P.cubensis by a novel method, eight cucumber cultivars were used as materials in the experiment. The disease index, accumulation of hydrogen peroxide, callose and lignin were investigated, and pathogen infection processes were observed at two-true leave stage of cucumber seedlings inoculated with P. cubensis. The main results were as follows:1. The resistance evaluation of different cucumber cultivars to P.cubensis. There were two different lesions formed in the study, one was necrotic lesion appeared in cultivar 2; the other was diffuse lesion formed in cultivar 1, 4, 5, 6 and 8; howvevr, the two type lesions were both formed in cultivar 3 and 7. Moreover, the resistance were divided into three different levels depending on the disease index, cultivar 2, 3, 7, 1, 4, 5, 6 and 8 were the resistant, the medium resistant and the susceptible, respectively, suggesting that necrotic lesion was appeared in the resistant cultivar, diffuse lesion was formed in the susceptible cultivar and the two type lesions happened to appeare in the medium resistant cultivars.2. The stages of infection to P.cubensis. Based on microscope observation, six processes were classified on the development of pathogen infection, such as A, B, CD, EF, G and H, germ tube was formed(A); haustoriums were appeared and hyphae has been grown(B); hyphae was elongated further from haustoriums(CD); synnemas were formed and new haustoriums were appeared(EF); a large number of spores were off from there stems(G); mycelium were covered the intire leaf and there were many synnemas were formed(H), in which A, B, CD was primary infection, EF was secondary infection, G, H was multiple infection. Intrestingly, the infection of pathogen stopped at the primary infection stage in resistant cultivar 2, or further developed to occure the secondary infection in the cultivar1, 4, 5, 6 and 8, suggesting that the necrotic lesion hinder the infection of pathogen.3. The amount and style of H₂O₂ accumulation in cucumber leaves inoculated with P. cubensis. After 12 h post inoculation, the content of H₂O₂ was higher significantly in resistant cultivar 2 than in susceptible cultivar 1, a high amount of H₂O₂ accumulated in the resistant cultivar 3 and 7. The rate of H₂O₂ accumulation surrounding cell wall in the resistant cultivar was higher than in the susceptible cultivar, suggesting that the accumulation of H₂O₂ in resistant cultivars lead to hypersensitive response. The correlation analysis showed that there was a significantly negative correlation between H₂O₂ and disease index, therefore, the accumulation of H₂O₂ may be used to classify the resistance to P. cubensis in cucumber.4. The amount and style of callose accumulation in cucumber leaves inoculated with P. cubensis. The accumulation of callose rapidly started at 24 h post inoculation (hpi). The content of callose was higher significantly in resistant cultivar 2 than in susceptible cultivar 1. The rate of callose accumulation surrounding cell wall in the resistant cultivar was higher than in the susceptible cultivar, suggesting that the accumulation of callose in resistant cultivars lead to hypersensitive response. The correlation analysis showed that there was the most significantly negative correlation between callose and disease index, therefore, the accumulation of callose may be used to classify the resistance to P. cubensis in cucumber.5. The amount and style of lignin accumulation in cucumber leaves inoculated with P. cubensis. The accumulation of lignin started rapidly at 24 hpi. The content of lignin was higher significantly in resistant cultivar 2 than in susceptible cultivar 1. The rate of lignin accumulation surrounding cell wall in the resistant cultivar was higher than in the susceptible cultivar, suggesting that the accumulation of lignin in resistant cultivars lead to hypersensitive response. The correlation analysis showed that there was the most significantly negative correlation between callose and disease index, therefore, the accumulation of lignin may be used to classify the resistance to P. cubensis in cucumber.In a word, three types of cucumber resistance to P. cubensis were classified in the study, and the hypersensitive response and the process of pathogen infection may be as indicators to classify the resistance to P. cubensis in cucumber.