Study On Angelica Leaf Spot Caused By Septoria Dearnessii

Septoria dearnessii Ellis et Everhart, the causal agent of Angelica leaf spot is the most destructive foliar disease of Radix Angelica Dahurica. The biological characteristics of the pathogen was studied first and systematically. The results are as follows:1. The biological characteristics of the Septoria dearnessiiThe temperature rage for mycelium growth and sporulation was 10 to 35℃, the optimum temperature at 25℃, the most suitable pH value was 5-7, the pathogen could grow and sporulate well on PDA, the optimum carbon sources was sucrose and the most suitable nitrogen sources was ammonium nitrogen. The light treatment hadn't remarkable effect on the mycelium growth and sprulation. Conidia were able to germinate in sterile deionized water but at a low percentage, sugar and host juice stimulated conidial germination of the pathogen. Conidia germinated at. 10 to 35℃, with optimum temperature between 25 and 28℃. The lowestl relative humidity of germination was 93%, and the maximum rate of spore germination was in water. The optimum pH value for sporulation was pH5-7. In suitable condition, germination of conidia started within 8 hours and the percentage of germination reached above 90% at 24 hour. The light hadn't substantial effect on spore germination. The lethal temperature of conidia was 50℃(10min).2. The infection process of Septoria dearnessiiInfection process to Angelica dahurica by Septoria dearnessii was studied. Results showed that conidia germinated at the leaves surface, then formed the branched hypha, hypha was the infection structure of S. dearnessii. The pathogen was able to penetrate host through two paths such as stomata and epidermis after inoculation. The direct penetration into epidermis was considered as a major way. The appressorium was not formed before the penetration of the pathogen. After the mycelium penetrated leaf cells, they could continue through the various cells. The cells that were infected by the pathogen could became empty, ultimately lead to the cells death.3. The infection conditions of Septoria dearnessiiThe infection of Angelica dahurica by Septoria dearnessii could be occured at 10 to 30℃, the incubation period was the shortest(12d) and the optimum temperature of infection was 25 to 30℃. The wetness period rage for infecting leaf of Angelica dahurica by Septoria dearnessii was 12-48 hour, the optimum wetness period was 24-48 hour. The Septoria dearnessii infected the basal leaves firstly, the spreaded to the upper leaves. Old leaves had more conducive to the infection.4. The infection sources and diesease dynamics of Angelica leaf spot in fieldUnder Suining in Sichuan ecological conditions, the infection sources of Angelica leaf spot were the mycelium and conidia in pycnidia, the pathogen which on the seed plants and the disease leaves in the field infected Angelica dahurica seedlings directly without overwinter. The primary diseased plants could be observed in November, and the disease began to develop with warming. From November to February the following year, the disease developed slowly because of the temperature was low. From February to April, the development of the disease accelerated gradually with the temperature rise. The diseased blossom from April to July, and the index of disease up to the highest. The disease degree began to slow down after July, with the cessation of Angelica dahurica growth and the development of the disease stopped. While the pathogen encountered rain, the conidia were released, then transmitted with rain. The conidia of the pathogen continued to develop diseased leaves and as reinfection sources. The expansion of the disease from the lower leaves to the upper leaves and nearby plants.5. The fungicides screening in laboratoryThe five fungicides were tested in laboratory to determine the effect on the mycelical growth and germination of conidia. The results showed that carbendazim inhibited the mycelial growth, but did not inhibit the germination of conidia. Thiophanate-methyl, mancozeb and chlorothalonil were good fungicides on the inhibition of the mycelial growth and the germination of conidia. Triadimefon almost had no inhibition on the mycelial growth, while it was good to inhibit the germination of conidia.