Studies Of Yellow Stunt And Root Rot Of Standing Milk-vetch (Astragalus Adsurgens Pall.)

Standing milk-vetch (Astragalus adsurgens Pall.) is an important legume in northern China, not only as a palatable fodder, but also as an ecological plant for windbreak, sand-fixation, watershed management, and soil improvement. Very short life can be used of the pasture is the main limiting factor in practice. Fungal disease is the one of the important contributors. The object of this thesis is various age standing milk-vetch pastures grown in Huan County. A series of studies were carried out in the fields, laboratory and greenhouse. The main results are as follows:1. A new species (Embellisia astragali Y. Z. Li & Z. B. Nan nov. sp.) was discovered and formally described. Host range, biological characteristics and physiological characteristics of this fungus were further studied. E. astragali grew very slowly (0.32-0.68mm/d) on PDA, PCA, V-8 and wheat hay decoction agar (WHDA). No other host plant except standing milk-vetch were found in Huan County, but Astragalus sinicus can be infected in artifical inoculation. It can grew on 10 kinds carbohydrates substrates, and 5 of 9 kinds nitrogen substrates. WHDA, 25℃and dark are the optimal conditions for colony growth and sporulation. The five inoculations with various age plants caused 17.5% to 31.6% plants death. Potted plants sown in disinfected soil with seeds borne E. astragali at a frequecy of 0.4%, 75.6% and 92.5% plants became infected, and 13.4% and 71.2% plants dying in second and third year, respectively. Compared with control, disease incidence in the Lanzhou pastures where diseased plants were transplanted from Huan County into which before sowing, increased by 99.7% and 88.0%, mortality increased by 93.0% and 89.0%, and plant density decreased by 36% and 57% in second and third year, respectively.2. One new disease (yellow stunt and root rot of standing milk-vetch, YSRR) was firstly reported. Several aspects, including overwintering, dispersal, geographical distribution, sporulation, germination of E. astragali, and symptoms, initial infection, late infection, infection course, disease severity and environments of the disease, were studied. E. astragali overwintered in seeds by mean of hypha, on/in debris by means of conidia/hypha, and within stem base, crowns, roots of diseased plants by means of hypha. Seed-borne E. astragali is the initial inoculum for new areas. Conidia produced from dead diseased plants, and dispersed by air and rain splashing in autumn and next spring. Typical symptoms are leaf lesions, leaf yellowing, shoot stunting, stem discoloration, crown rot and root rot. Frequencies of E. astragali in petioles and stems were significantly (P<0.05) higher than in leaf blades and roots. Initial symptoms expressed in re-growing stage. Disease incidences in June were the highest (up to 98.5%). Higher soil water and more dense plants would reinforce the damage.3. In total, 35 species of 29 genara plant-invading fungi and seed-borne fungi were isolated or detected. From 5 tissues of 120 diseaed plants and 120 healthy plants, 28 species of 22 genera were isolated. The fungi most frequently isolated from aerial tissues were E. astragli, Alternaria alternate and Alternaria sp., from roots were Fusarium solani, F. oxysporum, F. chlamydosporum and E. astragali. Frequencies of E. astragali isolated from seeds of Gansu, Inner Mongolia, Ningxia and Shaanxi were 0.2—44.6%. The results of pathogenicity tests of 11 fungi most frequently isolated indicate E. astragali was strongly virulent, F. oxysporum, F. chlamydosporum, F. avenaceum, F. solani, F. semitectum, F. verticilloides (Syn. F. moniliforme), Conostachys rosea (Syn. G. roseum) and C. herbarum were also virulent but different, and A. alternate and Alternaria sp. were non-virulent. All of the 9 virulent fungi induced root rot of standing milk-vetch. In Huan County, mortalities significantly (P<0.01) increased, density and forage yield significantly (P<0.01) decreased with the ages increased. These results suggested the decline of standing milk-vetch pastures was root and crown rot syndrome, the most important contributor is E. astragali, followed by Fusarium spp..