The occurrence, characteristics and distribution in the United Kingdom of resistance to phenylamide fungicides in Bremia lactucae (lettuce downy mildew)

Abstract

Failure to control Bremia lactucae (lettuce downy mildew) with metalaxyl in an intensive lettuce‐producing region of Lancashire at the end of 1983 was shown to be due to the occurrence of a high level of resistance to this fungicide (isolates capable of growth at < 100 μg/ml metalaxyl). During most of 1984, metalaxyl‐resistant isolates were obtained from numerous sites but all within a 20‐km radius of the initial outbreak. Thereafter, at the end of 1984 and during 1985, metalaxyl‐resistant isolates were recovered from most major lettuce‐producing regions in the UK with protected crops more affected than field crops. AH metalaxyl‐resistant isolates tested were identical in their response to fungicide, sexual compatibility type (B2) and virulence phenotype, probably representing a clone from a single origin. The resistant pathotype was virulent on resistance factors R 1‐10 and 12‐15 but lacked virulence for R 11 and 16‐18. This was also the most common virulence phenotype among sensitive isolates collected at the same time. Cross‐resistance to other phenylamide fungicides was demonstrated but isolates resistant and sensitive to phenylamide showed a similar response to the unrelated systemic fungicides propamocarb and fosetyl‐Al. An F1 sexual progeny isolate from a cross between a phenylamide‐sensitive and a phenylamide‐resistant isolate (presumed heterozygous at the locus or loci regulating response to phenylamide fungicides) exhibited an intermediate response to phenylamide fungicides. No isolates of this type were obtained from the field. At the high concentrations affecting spore germination, phenylamide fungicides exhibited lower activity against a resistant isolate compared with a sensitive isolate. The findings are discussed in relation to future control strategies, the population biology of the fungus and possible directions for lettuce breeding programmes.

Classical and molecular genetics of Bremia lactucae, cause of lettuce downy mildew

“…The segregation of self-sterile lines in secondary homothallic isolates is one example (Michelmore and Ingram 1982). Fungicide insensitivity seems to have arisen in the most common virulence phenotype, rather than involving sexual progeny (Crute et al 1987;Schettini et al 1991;Brown et al 2004). Recent changes in virulence phenotype in California seem also to be somatic Ochoa and Michelmore, unpublished).…”

“…These are currently controlled by a combination of genetic resistance, cultural practices, and chemical protection including the application of over 1.5 million pounds of insecticides and fungicides per year (US Department of Agriculture 2003). Several of these compounds are being withdrawn from agricultural use due to environmental concerns over their safety or have been rendered ineffective by changes in B. lactucae (Crute et al 1987;Schettini et al 1991;Brown et al 2004). Breeding for resistance to B. lactucae is a major activity of most lettuce improvement programmes, and there is an increasing need for information and methods to accelerate the development of new disease-resistant cultivars.…”

Is there a trade‐off between aggressiveness and overwinter survival in Phytophthora infestans?

“…This index is indicative of the epidemiological potential of an isolate. Similar aggressiveness indices have been used for this and other pathogens (Crute, Norwood & Gordon 1987; Thakur & Shelly 1993; Day & Shattock 1997; Flier & Turkensteen 1999). Aggressiveness indexes in the collection of 176 isolates ranged from 12·48 to 14·74 (data not shown); the differences in aggressiveness indices reflect up to fivefold differences in SP and up to twofold differences in LP and LS between the least and most aggressive isolates.…”

Fungicide resistance in soil‐borne Phytophthora species1

“…Resistance is considered more likely to develop in a polycyclic, aerially dispersed pathogen population than in a monocyclic soil-borne pathogen with restricted spatial spread (Dekker, 1987). Field experience has usually supported this prediction, for instance resistance to metalaxyl appeared rapidly in Phytophfhora infestans on potato (Davidse, 1981) and also in several air-borne downy mildew species (Reuveni et al, 1980;Bruck et a/., 1982;Crute et a/., 1987). The risk of resistance developing in soil-borne Phytophthora species is less well defined.…”