Evaluation Of Resistance Of Standing Milkvetch (Astragalus Adsurgens) Varieties To Embellisia Astragali

Standing milkvetch(Astragalus adsurgens) is an important forage legume. It has the triats of drought tolerance, high herbage dry matter yield and wide adaptability, which enable the crop plays important roles in fixing sand,dune,controling soil and water erosion,improving soil fertility,and development of livestock production. Fungal diseases are one of the limiting factors for standing milkvetch production and persistence. Yellow dawarf root rot disease caused by Embellisia astragali is a disease reported recently. The disease is responsible for the stand decline and poor persistence,low productivity of the standing milkvetch in northern China where the crop is growing. Use of disease resistant cultivar is the most effective method to control the disease. However, there is no information available on resistance to Embellisia astragali among the cultivars used in practice. Therefore, a series of experiments were carried out in laboratory, greenhouse and field for evaluating the resistance of standing milkvetch cultivars to Embellisia astragali. The main results are as the follows:1. In laboratory conditions, germination in 3d of all inoculated varieties were 40%-71%, which were 13.1%-177.8% higher than control. The highest germination variety was Neimeng and the lowest germination variety was Zhongsha No.1. Shoot length of all varieties was reduced by E. astragali inoculation, and six varieties, Erdos, Guyuan, Henan, Liaoning, Neimeng and Ningxia, showed significant (P<0.05) and the decreased rate was more than 20%. Varieties of Neimeng (2.57cm) and Ningxia (2.48cm) had a significantly greater (P<0.05)mycelium distribution range compared with other varieties. In contrast, varieties of Shanxi (1.42cm) and Zhongsha No.1 (1.37cm) had a significantly small-scale (P<0.05) mycelium distribution compared with other varieties. Incidence (90.60%,88.76%), mortality (36.60%,34.11%) and DSI (78.00,74.00) of Neimeng and Ningxia were significantly higher than other varieties. Conversely, Shanxi and Zhongsha No.1 had the lowest Incidence, mortality and DSI. In greenhouse and field conditions, emergence, plant height, root length, plant dry weight, incident, mortality and DSI had the same trend with laboratory results. Meanwhile, inoculation reduced the leaf number, leaf thickness, leaf area, stem diameter and all photosynthetic characters, but increased the stem number.2. Results from laboratory, greenhouse and field experiments showed very similar trends. Every disease parameter was significantly (P<0.01) correlated with all other disease parameters even if in different experiments (correlation coefficient ranging from 0.85 to 0.99). Multiple linear regression analysis showed DSI of different varieties in the laboratory and greenhouse experiments was significantly (P<0.05) correlated with that of the field experiment. The dualistic linear regression equation between the DSI of the field experiment (y) and that of the laboratory experiment (x1) and the greenhouse experiment (X2) was: y=8.28+0.18x1+0.15x2 (R2=0.83). Emergence and other 10 parameters were used to evaluated the resistance of all varieties to Embellisia astragali with grey relate degree analysis, principal component analysis and other three mathematical models. According to the research reported here, the most resistant varieties of those tested are Shanxi and Zhongsha No.1, the moderately resistant varieties are Putong and Zahua, the moderately susceptible varieties are Erdos, Guyuan, Henan and Liaoning, and the most susceptible varieties are Neimeng and Ningxia.3. Plant defence mechanisms are usually complex and composed of multiple layers of defence that are effective against diverse array of pathogens. There were positive correlations between disease resistance and the epidermis cell density, palisade tissue layers, palisade tissue cell density, corneum thickness. But there were negative correlations between disease resistance and stoma density. There were also positive correlations between disease resistance and the activities of all the enzymes such as POD,SOD,PPO,PAL,β-1,3-glucanse and chitinase. But there were negative correlations between disease resistance and MDA activity. After inoculation, the levels of endogenous phytohormones in both resistant and susceptible varieties were increased, especially in susceptible variety Neimeng. Disorders of endogenous phytohormones after inoculation would lead to a series of symptoms even malformations. In resistant varieties, the length of germ tube, penetration density and penetration were significantly (P<0.01) lower than that in susceptible varieties. As colonization of the cortex, hyphae in resistant varieties were much less than that in susceptible varieties in the same time. More over, hyphae spread much.slower in resistant varieties than in susceptible varieties. Papillae were often found at sites of penetration, but the percentages of papillae in resistant varieties were much higher than that in susceptible varieties. Intercellular hyphae penetrated cortical cell and formed haustoria in it. In resistant varieties haustoria was covered by cell wall appositions, and some haustoria were malformed.