In 2006, 2007, and 2008, we sampled 257 isolates of Phytophthora capsici from vegetables at 22 sites in four regions of New York, to determine variation in mefenoxam resistance and population genetic structure. Isolates were assayed for mefenoxam resistance and genotyped for mating type and five microsatellite loci. We found mefenoxam-resistant isolates at a high frequency in the Capital District and Long Island, but none were found in western New York or central New York. Both A1 and A2 mating types were found at 12 of the 22 sites, and we detected 126 distinct multilocus genotypes, only nine of which were found at more than one site. Significant differentiation (FST) was found in more than 98% of the pairwise comparisons between sites; approximately 24 and 16% of the variation in the population was attributed to differences among regions and sites, respectively. These results indicate that P. capsici in New York is highly diverse, but gene flow among regions and fields is restricted. Therefore, each field needs to be considered an independent population, and efforts to prevent movement of inoculum among fields need to be further emphasized to prevent the spread of this pathogen.

Populations of the vegetable pathogen Phytophthora capsici are often highly diverse, with limited gene flow between fields. To investigate the structure of a newly established, experimental population, an uninfested research field was inoculated with two single-zoospore isolates of P. capsici in September 2008. From 2009 through 2012, ≈50 isolates of P. capsici were collected from the field each year and genotyped using five microsatellite loci. The same two isolates were also crossed in the lab. High levels of diversity were detected in the research field, with 26 to 37 unique multilocus genotypes detected each year. Through 2012, genotypic diversity did not decline and no evidence of genetic drift was observed. However, during the 2011 and 2012 growing seasons, four new alleles not present in either parental isolate were observed in the field. Selfing (but not apomixis) was observed at low frequency among in vitro progeny. In addition, evidence for loss of heterozygosity was observed in half of the in vitro progeny. These results suggest that recombination, mutation, and loss of heterozygosity can affect the genetic structure observed in P. capsici populations.

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A study of 386 white clover (Trifolium repens L.) mapping population F1 progeny was conducted to quantify the type and magnitude of genotypic variation for a range of root morphology traits. Clones of each of the 386 white clover progeny were grown in sand. There were significant (P < 0.05) genotypic variance components and repeatability estimates for all the root traits examined. Progeny genotypes with high expression of key traits, including number of root tips and number of root forks were identified. These types may improve phosphate uptake as their highly branched roots will explore a large volume of soil per unit root weight. A strong positive phenotypic and genotypic correlation between several root traits was identified. This suggests an opportunity for indirect selection. For example, selection for high root fork number, a trait that is relatively less complicated to measure, should result in the concurrent increase in expression of the following root traits: surface area, number of tips, volume, and dry weight. Comparison of results from the sand‐based trial with an earlier trial using hydroponic conditions, with clones of the same 386 progeny, showed similar correlations exist among the root traits in both systems. The progeny genotype‐by‐trait Best Linear Unbiased Predictor matrix generated from the sand study is currently being used for the identification of root trait quantitative trait loci.

White clover breeding lines developed as high and low P-responsive, in glasshouse selection trials, were compared, over three years, with 17 other breeding lines and cultivars for growth in four soil-fertility treatments under two defoliation treatments on a hill-country farm. Differences between P-response groups were transitory and biologically insignificant. Selection for differences in response to P in a controlled environment was not successful in identifying white clover germplasm adapted to low P hill-country soils. White clover breeding lines and cultivars that achieved high growth over the 3-year duration of the trial had New Zealand parentage and were all medium to small leaf types.