Studies on the inheritance of resistance to metalaxyl in Bremia lactucae and on the stability and fitness of field isolates

Abstract

One hundred and eighty‐one sexual progeny isolates from three F1, one F2, five BC1 and one BC2 generations of crosses involving two metalaxyl‐resistant and two metalaxyl‐sensitive isolates of Bremia lactucae were assessed for response to metalaxyl. In most cases, isolates were also tested for virulence phenotype and sexual compatibility type. Isolates could be classified into three phenotypes (sensitive, resistant and intermediate) on the basis of their response to fungicide. Sensitive progeny isolates were similar to the sensitive parents and did not sporulate on lettuce seedlings grown in the presence of > 1.0 μg/ml metalaxyl. Resistant progeny isolates, in common with the resistant parents, sporulated as readily on lettuce seedlings grown in the presence of 100 μg/ml metalaxyl as on untreated seedlings. Progeny isolates which expressed an intermediate phenotype not observed in field samples were also recovered. This phenotype was more variable but isolates so classified were able to sporulate on seedlings grown in the presence of 100 μg/ml metalaxyl but exhibited a latent period (time from inoculation to appearance of sporulation) which was on average twice as long as that observed on seedlings grown in the absence of fungicide. Segregation patterns suggested that response to metalaxyl is controlled at a single locus exhibiting incomplete dominance but that the expression of response in the heterozygous condition may be affected by modifier genes. There was evidence that the metalaxyl‐resistant parental field isolates were heterokaryotic (possibly carrying nuclei homozygous and heterozygous at the locus controlling metalaxyl response). After 10‐30 asexual generations in the absence of fungicide, the response to metalaxyl of resistant field isolates began to approach that of an intermediate type. This phenomenon did not occur with a resistant sexual progeny isolate (presumed homokaryotic) and is additional evidence for the heterokaryosis of field isolates. The relative fitness of a range of isolates was examined by observing changes in the proportions of spores over three asexual generations after they were initially mixed in a 1:1 ratio with mutant isolates carrying a microscopically visible genetic marker (large lipid droplets). Most field isolates were more fit than the mutant isolates used, regardless of whether they were resistant or sensitive to metalaxyl, and there was no evidence that resistant isolates carried any fitness deficit in comparison with the sensitive isolates examined. In contrast, sexual progeny isolates (particularly from BC1 generations) were often less fit than the mutant isolates used, but there was no evidence that this was associated with response to fungicide. These findings are discussed in relation to the mode of action of metalaxyl and disease control strategy.

Genetics of Metalaxyl Resistance inPhytophthora infestans

“…He concluded that the heterozygous genotype conferred the intermediate phenotype (Shattock, 1988). In analyses with other oomycetes, a similar conclusion has been derived (Bhat et al, 1993;Crute, 1987;Crute and Harrison, 1988;Goodwin and McGrath, 1995;Lucas et al, 1990). However in Phytophthora parasitica, resistance was considered to be under the control of a single dominant gene (Chabane et al, 1996;Chang and Ko, 1990).…”

Mefenoxam Insensitivity and the Sexual Stage of Phytophthora capsici in Michigan Cucurbit Fields

“…Researchers investigating a range of obligate and hemibiotrophic oomycetes suggest that tolerance to PAFs is conditioned by a single locus having a major effect exhibiting incomplete dominance subject to modification by genes having minor effects (2,5,7,12,28). Insensitivity to mefenoxam, which also conferred insensitivity to metalaxyl, recently has been reported from field populations of P. capsici on bell pepper (22).…”

Metalaxyl-M-Resistant Pythium Species in Potato Production Areas of the Pacific Northwest of the U.S.A.

“…MS isolates were determined to be more fit due to a reduction in the frequency of MR isolates when the use of metalaxyl was temporarily suspended (Dowley et al 2002). In contrast, in several cases the fitness of MR isolates of oomycete plant pathogens has been similar, or better than, MS isolates (Gent et al 2008;Café-Filho and Ristaino 2008;Crute and Harrison 1988;Gisi and Cohen 1996;Kadish and Cohen 1989;Porter et al 2007). For example, MR isolates of Phytophthora infestans (Kadish and Cohen 1989) and Phytophthora erythroseptica (Porter et al 2007) have demonstrated signs of being more fit and aggressive than MS isolates in sexual and asexual reproduction, increased growth rates, and abilities to infect and colonize host tissue.…”