| Leaf spot was a serious leaf disease of alfalfa in Gansu Province in recent years. It caused seriously reduction in alfalfa yields. during the master work, the research about this disease has been done, the results showed as follow:1.The major symptoms of the disease were described in detail.It appears at first in the stem, leaf, and typical symptoms are small, dark brown spots, circular or irregular shape, a size of 0.1-0.3 mm in diameter, surface smooth, without bumps, starting from the leaf, plant disease can be more serious illness caused by leaf spot healing, each material, meanwhile, also stem analysis. Usually late June first, October peak around for disease. When serious illness onset of leaf area of variegated 60%, make the plant photosynthesis, quality and nutritional decrease, bring huge economic losses.The pathogenic strain was isolated from different diseased sites and parts of alfalfa.Conidiophores are straight, or little curved, the list living or the culiving,light brown, septate.Conidia is clavate,ovate or shuttle,primarily light-colour, gradually yellow-brown, fuscous-brown, 16.1-33.6×7.4-13.2μm with 2-5 transverse and 0-3 longitudinal septa and its beak has different size. The short beak is 2.6-4.7×2.1-3.8μm,The long can reach 1/4 spore turned into a spore, the size of 8.3-31.2×3.9-6.1μm, no septate or a septate. In view of the morphological characteristic,the morphology is consistence,so it is confirmed the same species.The pathogen is identified as Alternaria alternata (Fr.:Fr) Keissler.2. The biological test showed that Alternaria alternata can utilize sucrose, maltose, glycin Well. The optimum temperature, pH value and relative humidity (RH) of the pathogen are 25℃, pH 6, RH above 98% respectively. Under such conditions the pathogens grow quickly, the aerial mycelium of colony is abundant, the conidia germinate well.3. The more effective fungicides to control it were chosed,The result indicated that 10% score has the highest control effect(EC50=91.9276ug/ml), 15%Chlorothalonil and 50% Iprodione has better control effect than any other fungicides. 58% Metalaxyl almost can not inhibited development of Alternaria alternata.4. In this study, tolerance of different alfalfa varieties to spot disease were evaluated, the results showed that the different alfalfa varieties had different resistance to spot diaease and varied obviously in 17 alfalfa varieties. Gannong No.3, Xingjiang Daye and Sandeli were highly susceptible varieties. LongDong, Brown Spots Alfalfa, Alfaking and Algonquin were moderate resistant varieties. Defu was resistant variety, the others were medium susceptible varieties, and without high resistant variety in 17 alfalfa varieties. Chlorophyll content in alfalfa varieties were studied and the results showed chlorophyll content were varied obviously, there were not significantly different between alfalfa spot diaease and chlorophyll content, while the chlorophyll content in infected leaves decreased somewhat as compared with the normal leaves, the chlorophyll relative content measurement indicated that high susceptible varieties and susceptible varieties were higher than resistant varieties.
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