| Alfalfa(Medicago sativa) is the most important and the most widely cultivated leguminous forage in the world, which is also one of the highest economic value leguminous forages in our country. But in recent years, there are three typical alfalfa diseases happened in many place in Shaanxi province. It caused the alfalfa leaf chlorosis, the plants withered, seriously effected the production and the economic of alfalfa. In order to find out the disease types, occurrence regularity and its popular trend, this study had carried on a long time fixed-point investigation for four natural grassland sites in Shaanxi province, preliminary known about the alfalfa diseases distribution and its harm, a series of pathogen isolation and identification and biological characteristics test did in our laboratory, The final purpose was provide more reliable theoretical information for disease control of alfalfa production.The results showed as follows in this paper:1. Three typical alfalfa disease’s incidence of Shaanxi province statisticsed in 2013: the peak of alfalfa black spot was in June, incidence rate was up to 51.96%; the peak of alfalfa rust was in early September, incidence rate was 40.48%; Alfalfa Phoma leaf spot happended seriously in rainy may or damp August, incidence rate was 21.88% and 17.81%, respectively.2. Recent years, the alfalfa Phoma leaf spot happened frequently in Shaanxi province, the symptoms on the blade tip concentrated dark brown spots and expanded like a "wheel"; the colony morphological characteristics were typical striate, white and beige sori in the central, the color by dark brown became shallow gradually from inside to outside, the outermost was light yellow; The morphological characteristics of pathogens was pycnidium spherical; Conidium colorless transparent, cylindrical, single cell without septate, its size was 7 x 1.25μm. Through the rDNA-ITS sequence’s phylogenetic analysis and exercised leaf pathogenicity determination showed that the pathogen of alfalfa Phoma leaf spot was Phoma medicaginis var. Medicaginis.3. First reported the alfalfa black spot was a new serious disease in Shaanxi province, the typical symptoms were many pinpoint and dark brown spots onset of leaf, the disease spots enlarged circular or irregular shape, its diameter size was commonly 0.1-0.6 mm, when serious disease the spots concentrated distribution on blade, the neighboring spots converge into an irregular big spot, around the spots faded green and became yellow.4. The colony of alfalfa black spot was olive green, then dark brown, less aerial hyphae; Conidiophores were beige, septate; Conidium was clavate or ovate, light brown, with 1-4 transeverse and 0-2 longitudinal. The analysis of rDNA- ITS sequence just identificated the genus of Alternaria, it can’t judge the pathogen wich strain it was. But the phylogenetic analysis of OPA2-1 sequence shows that the pathogen of alfalfa black spot was Alternaria alternata.5. The optimum growth temperature of Phoma medicaginis var. Medicaginis was 25 ℃, the optimum p H value was 7, the polysaccharides starch was the the pathogen optimal carbon sources, organic nitrogen beef extract was the optimum nitrogen source. The optimum growth temperature of Alternaria alternate was 28 ℃, the optimum pH value is 6, the optimal carbon source was also starch, the beef extract and urea, potassium nitrate all were the good nitrogen source.The results from molecular phylogenetic analysis and excised leaf pathogenic determination showed that the pathogen of alfalfa Phoma leaf spot was Phoma medicaginis var. Medicaginis and the pathogen of alfalfa black spot was Alternaria alternate which were happened in Shaanxi province. |
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