| It is the first report about the largely occurrence of marigold leaf spot caused by Alternaria sp. in the mid-western region of Gansu Province in China. From the investigation, Marigold leaf spot generally occurred in Huangyang Town, Gulang County, Wuwei, Jintai County, Baiyin, Gansu Province. In common, disease incidence is 23%～95%, disease index is 5.75～48.5 in the mid-western region of Gansu Province. The disease severity has more discrepancy in the different regions, In Jintai County, the occurrence of marigold leaf spot is more severity than the other regions, Second to Huangyang Town, thirdly Gulang County.The major symptoms of the disease were described in detail. It initinally appears as circular, brown lesions with gray-white or yellow-white in center on leaves, stems and petals. Severely these lesions gradually turn dark brown or black, connect each other and eventually lead to death of very diseased sites.The pathogenetic strain was isolated from different diseased sites and parts of marigold. Conidiophores are straight, or little curved, the list living or the cu living, light brown, septate. Conidia is clavate, ovate or shuttle, primarily light-colour, gradually yellow-brown, fuscous-brown, 18.26～24.90×8.81～11.61μm with 2～4 transverse and 0～3 longitudinal septa and its beak is 2.74～4.5×2.4～4.16μm long with no septate. In view of the morphological characteristic, although the colour of different isolated strain has little discrepancy, the main morphology is consistence, so it is confirmed the same species. The pathogen is identified as Alternaria sp.The biological test showed that Alternaria sp. can utilize sucrose, maltose very well, utilize starch, glucose, while lactose poorly. In addition, the pathogens grew quickly and formed more spores on glycine and potassium nitrate than other nitrogen source. The optimal condition is at 25℃and pH6.04. The ample of light can accelerate the production of spores. In several culture medium, on 5%ML, the mycelia are wooly and aerial mycelium are abundant. The capacity of producing spores is highest on PSA. on Czapek, 0.1%DM poorly. At 25℃is the best optimum of the conidia germination. Conidia can easily germinate in water. Contrary to germination rate in water, even RH100%, germination rate is only 25%. Blew RH90%, conidia almost can not germinate.The seed-borne fungi diversity and isolation rate of several marigold species were tested. The result showed that the fungi on the surface of seeds were significantly different from each other, Alternaria sp., Fusarium sp. were main seed-borne pathogens. The surface of import coating-seeds had no pathogens, it showed that these kind of seeds were disinfected very well. The fungi-carried seed isolation rate of F1 hybrid seed, 2005 was the highest of all, the isolation rate was 40%.The testing of fungi-carried seed rate in different source showed that the pathogens were main on the surface, above 90%, the rate was different from each other. The fungi-carried of seed inside were less than the surface. Embryo and endosperm had no pathogens.The disinfection effect of marigold seed showed that 0.1%AgNO3 gave the best result of seed disinfection, it can kill any pathogens inside or outside of seed. Soaking seed with 55℃, 50℃or 50% wp750×Iprodione solution also can gave better disinfection effect, above 80%, it can validly control pathogens appearance and development. The disinfection effect of 58% wp750×Metalaxyl and 47% wp1000×Kasumin-Bordeaux were poorly.In lab, experimental result expressed that 50% Iprodione has the highest control effect(EC50=1.3249mg/ml), 75% Chlorothalonil has better control effect than any other medicament. 70% Thiophanate-Methyl and 58% Metalaxyl almost can not repress development of Alternaria sp.The experiment in the field showed that 50% Iprodione has the highest control effect, absolute-control effect is 76.54%, it can be used in production. From economic, 63% Carbendazim-Chlorothalonil and 75% Chlorothalonil are more stable medicament.
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