Migration and Selection Enforced Multiple Phenotypic and Genotypic Changes in the Population of Phytophthora infestans in Israel during the Last 36-Year Period

Late blight caused by the oomycete pathogen Phytophthora infestans is a devastating disease of potato and tomato worldwide, including Israel. The population structure of this pathogen was monitored in potato and tomato fields in Israel during a 36-year period of 1983-2019. Isolates of the pathogen were tested for sensitivity to phenylamide fungicides, mating type, race structure, and genotype. The phenotypic and genotypic structure of the population from potato have changed greatly from one year to another, from one season to the next, within a season and within a single field. Major changes also occurred in the population collected from tomato crops. The mechanisms driving these multiple changes and the heterogeneous nature of the population in Israel are shown to derive from multiple migration events of the pathogen via seed tubers from Europe and from fitness-driven selection processes.

Suppression of Pythium and Phytophthora Damping-Off of Rooibos by Compost and a Combination of Compost and Nonpathogenic Pythium Taxa

Pathogenic oomycetes, including Phytophthora cinnamomi and several Pythium spp. (Pythium irregulare, P. mamillatum, P. myriotylum, and P. pyrilobum), cause serious damping-off problems in rooibos (Aspalathus linearis) nurseries. The management of these pathogens in organic nurseries is problematic, because phenylamide fungicides may not be used. Compost, or compost in combination with Pythium taxa that are nonpathogenic to rooibos (P. acanthicum, P. cederbergense, and Pythium RB II), were investigated as alternative management options. Compost was able to suppress damping-off caused by several oomycete isolates but there was within- and between-species variation among the 30 evaluated isolates. This phenomenon was observed using two compost batches (A and B) sourced from independent suppliers. Compost B significantly reduced damping-off caused by 60% of the isolates, whereas compost A controlled only 37% of the isolates. The pathogens that were more readily controlled by both composts included P. mamillatum and P. pyrilobum, whereas the composts were ineffective at suppressing damping-off caused by >62% of P. irregulare and >50% of P. myriotylum isolates. Based on the evaluation of one Phytophthora cinnamomi isolate, this pathogen may also be controlled by compost. Neither of the composts as a stand-alone treatment could suppress damping-off caused by a combination of pathogenic species (P. cinnamomi, Pythium irregulare, P. mamillatum, P. myriotylum, and P. pyrilobum). However, damping-off was significantly reduced when nonpathogenic Pythium taxa (P. acanthicum, P. cederbergense, and Pythium RB II) were combined with the composts. Similarly, damping-off caused by a P. irregulare isolate that was not suppressed by either of the composts alone was significantly suppressed when the two composts were inoculated with the nonpathogenic Pythium taxa.

Persistence of Phenylamide Insensitivity in Pseudoperonospora humuli

Downy mildew, caused by Pseudoperonospora humuli, is an important disease of hop in most production regions in the northern hemisphere. Insensitivity to phenylamide fungicides was detected in isolates of P. humuli in production regions in Oregon and Idaho in 1992, and these fungicides since have been used on a limited basis. In this study, the prevalence of phenylamide insensitivity among isolates of P. humuli collected from 2005 to 2007 in the northwestern United States was quantified using a leaf disk assay with a discriminating dose (25 μg/ml) of either metalaxyl or mefenoxam with inoculum derived from 201 systemically infected diseased shoots (basal spikes) collected from 6, 10, and 11 hop yards in Idaho, Washington, and Oregon, respectively. A subset of 47 basal spike isolates and 42 monosporic isolates collected from two yards in Idaho and nine yards in Oregon during 2006 and 2007 were assayed using a dilution series of metalaxyl to determine the effective dose that inhibited 50% of the incidence of sporulation (ED50). Insensitivity to mefenoxam was detected in 31 of 74 (41.9%) basal spikes collected from 13 hop yards. Insensitivity to the related compound metalaxyl was detected in 52 of 80 (65%) spikes collected from nine hop yards, including four hop yards in Washington. Log ED50 values ranged from –2.25 to 2.67 for basal spike isolates and –2.27 to 2.98 for monosporic isolates and had a similar distribution. Log ED50 values for monosporic isolates and entire basal spike isolates were significantly associated. However, the slope of the regression line was less than 1, indicating that the log ED50 values obtained from entire basal spike isolates were greater than the corresponding log ED50 values obtained from a monosporic isolate obtained from that spike. This research suggests that insensitivity to phenylamide fungicides is a stable phenotype in P. humuli. Management of downy mildew should rely on measures other than these fungicides in yards or regions where phenylamide insensitivity is prevalent, which is now known to include certain hop yards in Idaho, Oregon, and Washington.

Multiple Loci Determining Insensitivity to Phenylamide Fungicides in Phytophthora infestans

The diversity of mechanisms causing insensitivity to phenylamide fungicides in Phytophthora infestans was addressed by comparative genetic analyses of isolates from North America, Europe, and Mexico. Both semidominant major loci (MEX loci) and genes of minor effect were previously shown to determine insensitivity based on studies of isolates from Europe and Mexico. In this investigation, genetic analyses of three highly insensitive isolates from the United States and Canada revealed a similar pattern involving major and minor loci. However, MEX alleles in two Canadian isolates conferred higher levels of insensitivity than those examined previously, particularly in a heterozygous state. This suggested that not all MEX alleles in P. infestans were functionally equivalent. The chromosomal locations of the major insensitivity loci were also shown to vary in different isolates based on linkage analyses performed with the aid of DNA markers. The major determinant of insensitivity in the North American, Dutch, and Mexican isolates mapped to the same locus, which was named MEX1. In a British isolate, a different locus, dubbed MEX2, was implicated that mapped to the same linkage group as MEX1 but to a distinct site.