Metalaxyl sensitivity and geographical distribution of Phytophthora infestans on potato and tomato in Yunnan.

Metalaxyl sensitivity of 257 isolates of Phytophthora infestans collected from twenty-two potato and tomato production regions in Yunnan from 2003 to 2005 were tested by fungicide-amended petri plates.The result showed that resistant or intermediate resistant isolates of P.infestans were detected in eleven potato or tomato production regions.However,the sensitivity of P.infestans to metalaxyl was obviously different between isolates collected from potato and that from tomato.P.infestans populations from potato were mainly composed of sensitive isolates to metalaxyl and populations from tomato were mainly resistant and intermediate resistant.Among the 167 isolates collected from potato,the ratio of resistant,intermediate resistant and sensitive isolates was 20.4 %,7.2 % and 72.4 %,respectively.Among 90 isolates from tomato,the ratio of resistant,intermediate resistant and sensitive isolates was 56.7 %,28.9 % and 14.4 %,respectively.The frequency of resistant and intermediate resistant isolates from potato and tomato was high in middle multiple cropping zones of Yunnan.The result suggested that the efficiency of metalaxyl to control late blight disease was decreasing,even losing in some potato production regions and most tomato production regions in Yunnan due to occurrence of resistant populations of P.infestans.It was necessary to avoid the situation that sprinkled single fungicide to control late blight disease for a long time.