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

Abstract

During the period 1978-1992, phenylamide fungicides in co-formulation with the dithiocarbamate fungicide mancozeb were tested for the control of potato late-blight in 51 separate field experiments in England and Wales. Whilst there was a general trend over all the experiments which indicated that foliage blight was less severe where the phenylamide dithiocarbamate mixture had been used, the benefit was more marked in some than in others. Despite the detection of phenylamide resistance, at some sites the mixture gave better control of foliage blight than the dithiocarbamate alone. At one site where continuous data were available, the additional benefit of the phenylamide compound for control of foliage infection was lost after 1986 coinciding with a rise in phenylamide resistance from 31 . 7% in 1986 to 81% in 1987. Where foliar blight epidemics occurred, yield responses to fungicide programmes compared with unsprayed controls ranged from 0 to 118 . 5% with a mean response of 33 . 2%. This is equivalent to 30 . 8 and 12 . 93 t/ha respectively. Standardized yield differences were calculated to allow inter-trial and inter-year comparisons and showed no benefit from the phenylamide fungicide applied at 14-day intervals in 33 out of 38 experiments where foliar blight epidemics occurred. Cusum analysis of standardized yield differences following treatment with the phenylamide dithiocarbamate mixture and the dithiocarbamate alone showed a mean benefit from the phenylamide mixture of 2 . 23 t/ha during the period 1978 to 1986. From 1986 to 1992, the yield benefit dropped to a mean level of 0 . 68 t/ha a decrease of 69 . 5%. In experiments where blight was not recorded, fungicide treatments had no deleterious effect on yields. Over all the trials, there was no effect of fungicide treatment on the incidence of tuber blight at harvest.

Phenotypic and genotypic diversity of Phytophthora infestans populations in Scotland (1995–97)

“…In Flevoland (the Netherlands), from 4 to 56% of isolates collected between the years 1994 and 1996 were A2 (Zwankhuizen et al ., 2000). In addition to mating type, the incidence of metalaxyl resistance has been investigated in many countries (Tooley et al ., 1993; Bradshaw & Vaughan, 1996; Gisi & Cohen, 1996); such monitoring assists rational fungicide usage.…”

Eucablight – collating and analysing pathogenicity and resistance data on a European scale*

“…The emergence of phenylamide resistance and subsequent control failures (Bradshaw & Vaughan, 1996) demonstrated the threat that fungicide resistance poses and the need to monitor populations for their sensitivity to phenylamides and other active ingredients. Similarly, the tracking of virulence and aggressiveness in P. infestans populations has helped our understanding of past failures in R gene deployment.…”

Effect of metalaxyl on formation and germination of oospores of Phytophthora infestans

“…Despite the widespread occurrence of metalaxyl‐resistant isolates in the field (Gisi & Cohen, 1996), metalaxyl is still used in prepack mixtures with dithiocarbamates such as mancozeb to control late‐blight (Bradshaw & Vaughan, 1996). Other fungicides known to reduce intercellular mycelial growth, to which resistance has not been found, may be expected to reduce oospore formation by preventing hyphae of mixed mating type from interacting, but this depends on their systemicity.…”

“…Despite the continuing occurrence of phenylamide‐resistant phenotypes of P. infestans (Shattock & Day, 1996), compounds such as metalaxyl, sold in prepack mixtures with protectant compounds, e.g. mancozeb, continue to be used as a part of potato late‐blight control programmes (Bradshaw & Vaughan, 1996).…”