| Since1999, serious broccoli disease had happened on broccoli in Linhai, Zhejiang province and threatened the development of local broccoli industry. The diseased plants appeared a series of symptoms, including the brown and necrotic peduncle, dysplasia inflorescence and rough head. Owing to the short planting history of broccoli in Zhejiang province and a lack of understanding of diseases species, we could not determine whether the disease on broccoli was downy mildew for a time. In this paper, we proved it by inducement and inoculation of the downy mildew sporangium. Furthermore, A series of studies about the optimum temperature for the formation of sporangium, resistance of broccoli cultivars to the downy mildew, the invading path and optimum period for downy mildew pathogens, the incidence and climate factors survey of the disease in field and the chemical control test had been done. The results obtained were as follows:1. Typical mildew was observed when Based on inducing results from the peduncle with brown path was kept at high-humidity and at17～18℃, we microscopic examination indicated discovered the this typical mildew on the brown patch and determined further that it was were the sporangium and sporangiophores of Peronospora parasitica (Pers.) Frowny mildew pathogens by the microscopic examination. Then we Iinoculationed these sporangia to healthy peduncles and kept the inoculated tissue after6～12day, the same symptom of grey-brown patch as that observed in field was observed. So we confirmed that this disease is the downy mildew.2. The results suggested that the pathogens invaded the small peduncles of the head firstly, and spread gradually from the apex downward and finally into the corner. We also found that the budding stage was the most susceptible period and the young buds were liable to be invaded and the wound could accelerate the infection. The best method to control this disease is to spray chemical at the budding stage which can prevent the peduncles browning at later developmental stages.3. In the present study, the field investigation was adopted in studying resistance of broccoli cultivars to the downy mildew. The results showed that the resistance of different varieties existed significant difference. The incidence of late-maturing varieties was much higher compared with the early-maturing and mid-maturing ones. For example, Wansheng, Shenglv (the late-maturing varieties), the incidences of leaves were about70～80%and3～18%, respectively. But the incidence for Youxiu (Early-maturity) and Lvxiong-90(Middle-late maturity) was almost0.4. The application of disease prevention measurement of downy mildew of broccoli should be started after plantation, especially at the stage of17-18leaves. The incidence of downy mildew could be controlled to be0if the prevention measures were well-done.5. Based on the epidemic trend analysis of downy mildew in broccoli fields, two occurrence peaks on the leaves were discovered in proper climate conditions. Since the secondary infection of downy mildew spread by the air and rain, the early control (mainly on leaf downy mildew) that inhibited the inoculium source for peduncle infection was the crucial means for the integrated control of downy mildew in peduncle.6. Climate was an important factor for infection of downy mildew in broccoli. By the inducing the formation of of sporangia and the survey of infection condition, we discovered that the downy mildew pathogen of broccoli occurred and spread easily in the high-humidity and low-temperature conditions. Statistical analyses also displayed the relationships between the incidence and temperature, rainfall and humidity. The linear collection between the temperature and incidence is y=25.985-1.483x.The typical exponential function relations between the rainfall, humidity and incidence is y=2.724exp(0.027x) and y=0.212exp(0.063x)... |
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