Nine years of practical experience with phenylamide resistance in Phytophthora infestans in the Netherlands

Abstract

Resistance to phenylamide fungicides in the Phytophthora infestans population in the Netherlands decreased from a level of 77~ of the samples containing resistant strains in 1980 to 0% in 1986 with decreasing use of metalaxyl, the only phenylamide fungicide registered for use against late blight in this country. Use of metalaxyl decreased because the fungicide alone and combinations of metalaxyl with preventive late blight fungicides were not commercially available from 1981 through the first part of the potato growing season of 1984. When metalaxyl was made available again in August 1984 in a mixture with mancozeb and from 1985 in a mixture with almost full rates of maneb and fentinacetate its use remained limited because growers were advised to apply the combination only two times per season in critical situations only to control the severe late blight epidemics of 1984 and 1985. Moreover application on seed potato crops was not allowed. In 1986 the combination was hardly used because the mild epidemic was easily controlled by preventive compounds only.In 1987 and 1988 phenylamide resistance increased to 16 and 42 %, respectively, inherent to a general use of the combination in both years in most potato growing regions of up to two applications to combat a very severe epidemic.In lots of seed potatoes harvested in 1988 the percentage of blighted tubers yielding resistant isolates significantly differed from 13% for regions where seed potatoes are predominantly grown for table and processing crops to 41% for a region where predominantly starch potatoes are grown, both as seed and production crops. The more intensive use of metalaxyl combinations in the latter area through the years as compared to the other areas is assumed to be responsible for the higher incidence of resistance.In most years application of the combination induced a significant shift in the population to resistance as is indicated by the higher frequency at which resistant isolates were present in leaf samples taken from treated fields or tuber samples from the harvested crop.The strategy adopted by Dutch potato growers to combat phenylamide resistance in late blight, involving limited use of metalaxyl mixtures to a maximum of two times in critical situations only, has kept up to now resistance in the population at a level at which metalaxyl still significantly contributes to the efficacy of the combination in controlling the disease. Nevertheless the observed increase in resistance in the population of 1988 after two consecutive late blight years indicates that this may change if severe epidemics occur each year.

Population genetic structure of Plasmopara viticola after 125 years of colonization in European vineyards

“…Thus, Ph. infestans could evolve only asexually during the 150 years since its introduction into Europe (Goodwin et al ., 1994) but still managed rapidly to evolve virulence against new resistance genes introduced into potato and tomato, and evolved resistance to fungicides (Davidse et al ., 1989). In 1984, the A2 mating type as well as more A1 mating type genotypes of Ph.…”

The effect of phenylamide fungicides on the control of potato late‐blight (Phytophthora infestans) in England and Wales from 1978 to 1992

“…The resistance strategy for phenylamide fungicides adopted in the UK and Northern Ireland has been described by Urech & Staub (1985). In Holland, a different strategy has been adopted where the protectant materials are used early in the season, and the use of phenylamides is restricted to a maximum of two sprays within two weeks during critical conditions (presence of disease in the locality, conducive weather conditions) (Davidse et al, 1989). In Holland and the UK (Table 6), phenylamide resistance was detected in 42% and 81% respectively of samples tested in 1988.…”

“…In the Republic of Ireland, a monitoring programme to determine the levels of metalaxyl resistance in the P. infestans population showed that there was a decrease from 75% of all isolates tested in 1981 to 6% in 1983 (Dowley & O'Sullivan, 1985). In the Netherlands, resistance to metalaxyl also subsequently declined from 100% resistant isolates to 0% in 1986 (Davidse et al, 1989). In the mid-1980s, metalaxyl was reintroduced into both countries as a proprietary mixture with other dithiocarbamate fungicides.…”

“…As a result of the development of phenylamide-resistance in the Netherlands, Dutch growers adopted a strategy of limiting the use of these materials to a maximum of two consecutive applications at the onset of severe blight pressure when contact fungicides could no longer prevent the full scale development of an epidemic (Davidse et al, 1989). In the Republic of Ireland from 1985 and the UK from 1987, a reformulated proprietary mixture of metalaxyl +mancozeb was introduced which contained a larger proportion of the mancozeb component (Fubol 75 WP, Ciba-Geigy Agrochemicals).…”

Competitive Parasitic Fitness of Mefenoxam-Sensitive and -Resistant Isolates of Phytophthora erythroseptica under Fungicide Selection Pressure

“…Unfortunately, resistance to metalaxyl has been reported in many Phytophthora spp. Most notably, resistance was first reported in P. infestans on potato in the Netheriands (10). Additionally, metalaxyl-resistant isolates have been reported in P. cactorum (43), P. citricola, P. capsici (4), P. parasitica (14), and P. nicotianae (40), as well as in other oomycetes such as Peronospora tabacina, Bremia lactucae (29), and Pythium ultimum (39).…”