Ecology and Control of Phytophthora melonis in Drained Paddy Field

Abstract

胡瓜疫病菌(Phytophthora melonis)之游走子囊在病組織上不具脫落性，乾風無法將之吹落。但是它的游走子及游走子囊可隨灌溉水及飛濺的雨水傳播。土壤濕度強烈影響病原菌的存活，病原菌在淹水的土壤中僅能存活4星期，但在含本量6%及20%，稍可通氣之濕土裡，也僅能存活5~20星期。田間試驗顯示，在經過兩期水稻輪作的病圃裡，胡瓜疫病菌不能存活。而引起胡瓜疫病之病原菌初次感染源，並非來自土壤或種子，而是來自隣田的灌溉水。在田間，殺菌劑(Ridomil. MZ)和土壤添加物（雞糞和S-H混合物）配合使用對胡瓜疫病之防治效果最顯著。該二種土壤添加物可提升土壤的pH值，但土壤pH值的變化與病害防治無關。雞糞和S-H混台物僅有促進植株生長勢的效果。在試驗中Ridomil MZ殺菌劑才是抑病的主要因素。 Dry air current failed to remove the sporangia of Phytophthora melonis from the disease lesions. However, the fungus was easily dispersed by its zoospores and sporangia carried in irrigation water or liberated to the air by wind-blown rainsplashes. The soil moisture strongly influenced the survival of P. melonis. The fungus persisted only for 4 wks in flooded soil, while in soil with moisture content at 6 % or 20%, a condition that permitted soil aeration, the fungus could be detected from the infested soil for a period of 5~20 wks. Field experiments demonstrated that P. melonis did not survive, nor did it induce cucumber blight in a disease nursery after two successive crops of rice plants. The primary inoculum of P. melonis to cause cucumber blight in the drained paddy fields did not come from the paddy soil nor from cucumber seeds. It was mainly from irrigation water from neighbor fields. Cucumber blight was best controlled by using combinations of fungicide Ridomil MZ and soil amendments, i. e. chicken manure or S-H mixture. The soil amendments raised soil pH to alkaline range for 7 days, then it fell to the acid range again. The changing pattern of soil pH had no correspondence to the inhibition of cucumber blight. Both chicken manure and S-H mixture had nutrient effects to result in improved plant growth. However, Ridomil MZ was the main factor that inhibited disease development in our experiments.