Study On Disease Factors Of Soybean Pod Anthracnose And Its Control

Zhejiang province is the main vegetable soybean production and refrigeration base in China. The vegetable soybean "Tai75" whose quality is good and the output is high, was introduced from Taiwan to Zhejiang in 1994. It is the only variety assigned by the foreign merchant, and there is no other variety which might substitute at present. In the summer of 2001, an unknown disease much likes mechanical damage, was observed on the pods of "Tai75", causing pods' economic value decreased seriously. The symptoms of disease, morphological and molecular identification of pathogen, and screening of effective fungicide in lab have been studied. After our project group more than 3 years of study, the pathogen which has caused the unknown disease on the pods of "Tai75", was identified to be Colletotrichum truncatum (Schwein.) Andrus & W.D.Moore. The symptom which mainly appears on the bean pod is obviously different from the symptom of soybean anthracnose as everyone knows. So we called this disease as soybean pod anthracnose. On the basis of previous results, the disease factors of soybean pod anthracnose and its control were mainly studied in this article. Main results of the study are as follows:1. Biological characteristics of the strain XST01 were studied. The results showed that illumination have no effect on the mycelial growth and conidial germination of the fungus, but the spore yield was the most under continuous illumination. Its mycelium grew between 15 and 35℃, and optimum growth was at 30℃. It grew between pH 3-12, and optimum growth was between pH 5-7. The conidia germinate between 15 and 30℃, and optimum germination was at 25℃. It germinate between pH 3-10, and optimum was 7. The lethal temperature of conidia and mycelium was at 56℃and 60℃, respectively. The conidia could not germinate when the relative humidity below 97%. The fungus could use a variety of carbohydrates and nitrogen sources, but for the mycelial growth, the optimum carbon source was sucrose, and the optimum nitrogen source was yeast. For the sporulation, the optimum carbon source was maltose, and the optimum nitrogen source was yeast.2. The main factors which affect soybean pod anthracnose were studied. The results showed that "Tai75"' was the most susceptible variety to soybean pod anthracnose among the eight tested varieties. Rainfall and temperature were the main factors which affect soybean pod anthracnose. The prediction model of soybean pod anthracnose was Y=X₁+X₁²+X₃ (Y represent disease index. X₁ represent temperature, and X3 represent rainfall). The severity of disease was positively correlated with rainfall and temperature.3. The results of field test for control showed that 70% thiophanate-methyl WP should be used as the first choice to control soybean pod anthracnose, then they were 40% pyrimethanil SC and 43 % Tabuconazole SC etc. The flowering and blooming stages of soybean were the optimal control growth stages, and it would be better to control in the intervals of rains during the flowering and blooming stages.4. The antagoistic effects and mechanism of Trichoderma Harzianum strains T6. T9, T10, T25, T34, Pythium oligandrum strain QC05, and Bacillus subtilis strains B1, B3 against the strain XST01 were preliminary studied. The results showed that all these strains have given inhibiting activity to the strain XST01. Among the eight biocontrol strains, T34 have the best inhibition effect. After that, the order of priority is as follows: T10, T25, T6, B1, B3, T9, and QC05. Dual culture and scanning electronic microscopic observation indicated that P. oligandrum strain QC05 could cause the dissolution of hypha of fungus by its secretion (<8KD), and T. Harzianum strains could parasitize and dissolve the mycelium of fungus, and B. subtilis strains had strong antagonism to the fungus.