杜金池 張義璋 程永雄 林敬皓 李日華 阮象
Studies on the Biological Control of Panama Disease (Fusarium Wilt) of Banana
C. C. Tu, Y. C. Chang, Y. H. Cheng, C. H. Lin, J. H. Li and S. Yuang
Banana has long been an important agricultural product for export in Taiwan. Unfortunately, Panama disease has been found to attack the major commercial variety “Cavendish” and to cause a considerable economic loss since 1964.
The causal agent is identified as Fusarium oxysporum f. sp. cubense race 4. As it is a soil-born microorganism, the outbreak of the disease is closely related to soil factors such as acidity, water content, texture and microorganism of soil. Flooding, fallowing, rotation and improvement of soil texture and pH value have been tried to check the occurrence of the disease, but none is practical.
The program of biological control of Panama disease has been conducted in Taiwan since 1975 soon after the finding of flax wilt-suppressive soil in Chang-hwa, the major flax growing area of Taiwan. The flax wilt-suppressive soil was proved to be also suppressive to Panama disease of banana in both pot and field trial. Banana fields, which were severely attacked by the Panama disease in the previous year, were introduced with the wilt-suppressive soil into the Banana planting hole at the rate of 20 kg/hole. The treatment resulted in only 9% of disease and reached 100% of harvest.
The disease could also be checked by means of applying antagonistic anaerobic bacteria isolated from flax wilt-suppressive soil at the rate of 15 liter of suspension (bacterial density 107 cells/ml)/100 sq. m., or in combination with crotalaria as green manure at the rate of 140 kg/100 sq. m. Such treated plots showed only 3~12% of the disease. However, the effect of these antagonistic bacteria and flax wilt-suppressive soil on the control of Panama disease lasted for only once crop. The disease increased considerably in the consecutive year.
The application of casava residue into the sick-field for controlling Panama disease had been tested during the years from 1977 to 1980 in the field previously with 78% of the disease. Four treatments were compared: (A) application of casava residue 10,000 kg/ha, rice bran 1,000 kg/ha and some chemicals such as K2HPO4 61.2 kg/ha, (NH4)HPO4 76.6 kg/ha, MgSO47H2O 7.8 kg/ha, MgCl2 15.3 kg/ha, FeCl3 1.5 kg/ha and CaCl2 15.3 kg/ha in to the soil, followed with 21 days of fallowing, then planted with healthy seedling; (B)same as treatment A, but only with half amount of amendments added; (C)plowed and flooded, then planted with healthy seedling to serve as check plot I; and (D) without any treatment to serve as check plot II. A regular field management was given throughout the experiment. The results indicated that both treatments A and B were significantly (p=0.05) effective in checking the occurrence of the disease (Fig. 3). The diseases of treatment A and B were only 13 and 16%, respectively, in the 1st year, and both 28% in the 3rd year. In contrast, the check plot II was 58% in the 1st year, 68% in the 2nd year, and 80% in the 3rd year. Although the flooding treatment (check plot I) was 29% of the disease in the 1st year, it increased rapidly in the 2nd and 3rd years showing 56 and 72%, respectively. It was, therefore, concluded that application of casava residue in combination with rice bran and some inorganic salts gave a promising result on the control of Panama disease of banana.
The effect of casava residue, rice bran and inorganic salts on the germination of chlamydospores of F. oxysporum Lf. Sp. Cubense race 4 was also studied by using soil dilution plate method and soil smear technique. Sick-soil was prepared by adding macrospore-suspension of the microorganism onto the autoclaved silt loam. The pathogen population was thus adjusted to 85 × 103 propagules/g soil. The infested soil was then amended with casava residue, rice bran and inorganic salts at the rate of the same as those of field trial used. Flooding was then given. However, 98% reduction of viable propagules of the pathogen was found from the amended soil and 70% reduction from non-amended but flooded soil after 4 weeks of incubation at 28℃. Although a higher percentage of chlamydospore germination was detected from the amended soil, the germ tubes were rather short and always surrounded by the bacteria, resulting in the lysis of them (Fig. 5). Since the C/N ratio of casava residue and rice bran is comparatively high, it is postulated that the chlamydospore germination is thus stimulated, while the activity of some saprophytic microorganisms in the soil is also increased. Consequently, poor germling survival is resulted.