The question of metalaxyl resistance on late blight fungus in Estonia

Metalaxyl containing fungicides are commonly used by Estonian potato growers because of their efficiency in control of potato late blight. Metalaxyl-resistant isolates have become an important part of the Estonian population of Phytophtora infestans. The aim of the current study was to measure the frequency of metalaxyl resistance among the Estonian population of Phytophtora infestans and to test the efficacy of metalaxyl-based fungicides in controlling potato late blight. 138 isolates of Phytophtora infestans were collected from conventional, experimental and small scale growers' fields in different parts of Estonia in 2003- 2005 and were assessed for resistance to metalaxyl. Metalaxyl-resistant, intermediate and sensitive isolates were recorded at frequencies of 37.1, 45.4 and 15.1%, respectively. There were differences in frequency of metalaxyl resistance between different locations, years and different types of potato cultivations. Metalaxyl-resistant isolates were predominant (66.7%) on conventional potato fields in 2004 and 2005. Metalaxyl- resistant strains were not detected among isolates collected from the field where fungicide containing metalaxyl was not used. The increased frequency of metalaxyl-resistant isolates was detected in 2003-2005. Trials of late blight control with fungicides containing metalaxyl were performed in the same period. Metalaxyl showed lowered efficacy only in conditions of extreme late blight pressure in 2004. Therefore metalaxyl could be effectively used for control of potato late blight a maximum of two times during the growing period.