| Phytophthora blight, caused by the oomycete pathogen, Phytophthora capsici L., is becoming a serious ongoing agricultural problem. At home and abroad, although many measures have been carried out to control this disease, so far, satisfactory results have not been achieved since available methods with low control effect or cost too much. Practice has proved that flooding, with less cost, works well for soil-borne disease control although the control effect is sometimes unstable and short-effect. Based on this fact, we studied the effect and mechanisms of flooding on disease control of phytophthora blight in chilli pepper. And also we did some research in other measures to strengthen flooding effect and improve control effect of Phytophthora blight. The results are as following.We studied Effects of flooding on physical, chemical and biological properties of soil and on disease control of Phytophthora blight of chilli pepper. The results showed that pH value and phenolic acid content of soil were increased slightly while the organic acid concentration was increased significantly after flooding. However, EC value, Eh value and ammonium nitrogen content decreased significantly. After transplantation of chilli pepper, different parameters of rhizosphere soil of chilli pepper showed different change trend. Organic acid and phenolic acid content decreased rapidly and ammonium nitrogen content and Eh value increased gradually, but EC and pH value were stable. As to biological character of soil, flooding reduced activity of dehydrogenase and urease, so did the number of fungi and actinomycetes. The soil inoculated with zoospores of P. capsici at density of1000zoospores per gram dry soil were respectively kept wet and waterlogged for35days. The density of P.capsis in control group is5-6times of flooding group when the soil treatments were finished. Our study also showed that flooding for20days and35days respectively received100%control effect against pepper blight in two soils collected from two greenhouse, and40-100%control effect was obtained in two soils inoculated with P.capsic. We speculated that the decrease of soil Eh value, the increase of organic acid content and phenolic acid content, the change of soil microbial community structure were the main reasons for the reduction of the number of P. capsici in soil. In addition, we speculated that flooding solely was not suitable for soil where soil-borne disease was serious and other measures for soil treatments should be integrated to achieve satisfying effect in disease control.We studied the impact of a series of strengthening flooding measures on soil bio-physicochemical properties and the incidence of pepper blight by adding organic substance. The results showed that organic acid, phenolic acid content of soil were significantly increased and Eh value were significantly decreased in treatments of flooding with rapeseed meal and straw. Eh value of treatments of flooding and flooding with biogas slurry were also significantly decreased. But there were no significantly changes in organic acid and phenolic acid content. Organic matter content of flooding with rapeseed meal or flooding with straw were higher than treatment of flooding with biogas slurry. After transplantation of chilli pepper, polyphenoloxidase and cellulase activities of soil of control group, flooding and flooding with biogas slurry were lower than soil of flooding with rapeseed meal or with straw. However, the urease activity of soil was completely opposite to the two enzyme above. Compared with regular flooding, strengthening flooding with organic substance significantly reduced the number of fungi and actinomycetes. While flooding with rapeseed meal or with straw significantly increased the number of bacteria. RT-PCR analysis results showed that regular flooding decreased the amount of P.capsic in soil. And the amount of P.capsic in treatment flooded with rapeseed meal or with straw were the least. In addition, We found that flooding with biogas slurry increased the density of P.capsic..while no P.capsic was detected in soil flooded with straw. But the number of P.capsic in treatments of flooding, flooding with rapeseed meal and flooding with biogas slurry were5.31,2.74and1.73times as before. The disease incidence was significantly associated with the quantity of P.capsic. The incidence rate of control group was100%, while it was respectively16.67%,26.67%,3.33%and0in treatments of flooding, flooding with biogas slurry, flooding with rapeseed meal and flooding with straw. Thus, flooding with straw achieved100%control effect with no P.capsic was detected in soil. So we made preliminary conclusion that flooding with straw was suitable for disease control of Phytophthora blight of pepper. Although flooding with rapeseed meal achieved control effect of96.67%, the number of P.capsic in soil increased and it is risky to control Phytophthora blight by this measure. The control effect of flooding with biogas slurry was lower than flooding. So it may not be suitable to control Phytophthora blight. The high organic acid, phenolic acid content and low Eh value in soil during the flooding stage may be the main reasons that flooding with rapeseed meal and flooding with straw achieved significantly satisfying effect in disease control.On the other hand, we improved flooding effect on disease control by regulation of microbe community, namely adding Trichoderma biocontrol agent to soil after flooding.22Trichoderma strains were isolated from rhizosphere soils obtained from chilli pepper production greenhouse. Three antagonistic Trichoderma strains were selected as potential agents for their strong antagomism against P.capsic and were identified as Trichoderma harzianum by18S DNA sequencing procedure. Subsequently, we studied effects of flooding, addition of Trichoderma harzianum and flooding followed by addition of T. harzianum on the quantity of P.capsic, microorganism community structure and disease incidence. The results showed that addition of Trichoderma agents to control soil or flooded soil maintained the number of Trichodermain in soil high in growing period of chilli pepper. Effect of flooding on soil microbial flora was weak. While adding biocontrol Trichoderma agent alone or after flooding could significantly improve the soil microbial diversity. In addition, flooding for20days, adding biocontrol agent alone and flooding followed by biocontrol agent addition improved the content of chlorophyll of chilli pepper. RT-PCR results showed that flooding for20days or13days and adding biocontrol agent reduced the population of P.capsic in soil. But the elimination were not enough since there were still incidence case. However, flooding followed by biocontrol agent addition could completely eliminate P.capsic in soil and no incidence case was observed. Therefore, we made conclusion that flooding with the combination of biocontrol agent could improve disease control effect of peper caused by P.capsic. In addition,we got that flooding for13days followed the addition of biocontrol agent could achieve significantly control effect in disease control of Phytophthora blight in chilli pepper. |
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