Sensitivity to metalaxyl of Phytophthora infestans populations in potato crops in south‐west England in 1980 and 1981

Abstract

S U M M A R YA laboratory test was developed to assess the sensitivity of field populations of Phytophthora infestans to metalaxyl. Discs of potato leaf tissue were floated upon solutions of the fungicide at different concentrations and inoculated with spores. The extent of symptom development was noted after incubation under standard conditions for 5-6 days. In preliminary experiments growth of isolates of P. infestans obtained from culture collections was severely inhibited in discs treated at 2 pglml. By contrast the development of an isolate obtained from a crop in Eire in which blight control with metalaxyl had failed, and known to be markedly less sensitive in vitro, was unaffected in discs treated at 100 pglml.During the summer of 1980, 234 samples of P. infestans were obtained from 20 sites in south-west England, 10 of which had received sprays containing metalaxyl and 10 of which had not. All samples were sensitive to metalaxyl applied at 2 pg/ml. In 1981, 35 sites within the same area, 30 of which had received sprays containing either metalaxyl or ofurace (a related fungicide), were similarly surveyed. Most of the 79 samples of P. infestans examined proved sensitive and at all sites the amount of blight was small. However, at three sites, including one not treated with acylalanine fungicides, strains were found which were unaffected by 100 pglml metalaxyl in leaf disc tests. These findings are discussed in relation to the development of resistant blight in other areas and to the use of fungicide mixtures.

Characterization of changes in populations ofPhytophthora infestansin Canada using mating type and metalaxyl sensitivity markers

“…Each sample was collected from an individual field or potato storage facility. 4 Number of isolates of P. infestans obtained from individual late blight lesions found on samples in that province.…”

“…Resistance to metalaxyl in Europe developed soon after the chemical was introduced in the early 1980s (4,8,10,16). Metalaxyl resistance also occurs in isolates from the Middle East (7), Asia (36,51), Mexico (37), South America (20), and the United States (12,38).…”

Studies on metalaxyl‐resistant Phytophthora infestans in potato crops in south‐west Scotland

“…However, no resistant isolates were found in 1981 crops in south-west Scotland grown from Irish seed. Carter et al (1982) also could not account for resistant/", infestans in crops in south-west England in terms either of wind-borne sporangia or infected seed. The third, and most hkely possibility on the evidence available, is that resistant strains were present in the endemic population of P-infestans.…”

“…In Northern Ireland, where no problems were reported on the foliage, Cooke (1981) obtained resistant isolates from tubers from six of nine crops sprayed with metalaxyl + mancozeb, and from two which were sprayed with metalaxyl on its own. In England and Wales, limited monitoring of crops, mainly in south-west England (Carter et al, 1982), detected no resistance. No monitoring was done in Scotland in 1980.…”

Comparison of cuprous oxide and metalaxyl with mixtures of these fungicides for thecontrol of Phytophthora pod rot of cocoa

“…Experience with potato blight, caused by Phytophthora infestans (Mont.) de Bary, has also shown that resistant isolates which occur naturally are just as fit to survive in the absence of metalaxyl as sensitive isolates (Cooke, 1981;Carter et al, 1982). These findings emphasize the importance of avoiding resistance.…”