Development of fungicide resistance in fungal and oomycete pathogens is a serious problem in grape production. Quinoxyfen is a fungicide widely used against grape powdery mildew (Erysiphe necator). In 2013, E. necator isolates with reduced quinoxyfen sensitivity (designated as quinoxyfen lab resistance or QLR) were detected in Virginia. Field trials were conducted in 2014, 2015, and 2016 at the affected vineyard to determine to what extent quinoxyfen might still contribute to disease control. Powdery mildew control by quinoxyfen was good, similar to, or only slightly less, than that provided by myclobutanil and boscalid in all three years. The frequency of QLR in vines not treated with quinoxyfen declined only slowly over the three years, from 65% to 46%. Information about the mode of action of quinoxyfen is limited; previous research suggests that quinoxyfen interferes with the signal transduction process. We profiled the transcriptomes of QLR and sensitive isolates in response to quinoxyfen treatment, providing support for this hypothesis. Additional transcriptional targets of quinoxyfen were revealed to be involved in the positive regulation of the MAPK signaling cascade, pathogenesis, and sporulation activity. Grape downy mildew (Plasmopara viticola), another important grape pathogen, is commonly controlled by phosphite fungicides. A field trial and laboratory bioassays were conducted to determine whether P. viticola isolates from vineyards with suspected control failures showed reduced sensitivity against phosphite fungicides. Prophyt applied at 14-day intervals under high disease pressure provided poor downy mildew control in the field. Next-generation sequencing technologies were utilized to identify 391,930 single nucleotide polymorphisms (SNPs) and generated a draft P. viticola genome assembly at ~130 megabase (Mb). Finally, field isolates of P. viticola collected from a Virginia vineyard with suspected mandipropamid control failure were bioassayed. The EC50 values of the isolates were >240 μg.ml-1 for mandipropamid, well above the field rate. The PvCesA3 gene of two resistant isolates was sequenced revealing that these isolates had a GGC-to-AGC substitution at codon 1105, the same mutation that has been found associated with CAA resistance elsewhere. / PHD / Powdery and downy mildew are two diseases of grapes that can cause large yield losses, and are usually controlled by regular fungicide applications. Development of fungicide resistance has been a growing challenge. Quinoxyfen is a protectant fungicide commonly used against powdery mildews. Unusual grape powdery mildew isolates that grew well on quinoxyfen-treated plants in the laboratory (designated as quinoxyfen lab resistance or QLR) were detected in a Virginia vineyard. In 2014, the first year of this study, 65% of powdery mildew isolates from parts of this vineyard that received no further quinoxyfen treatments had the QLR type of resistance, and this declined only slowly to 46% by the third year. Field trials were conducted in 2014, 2015, and 2016 to determine the efficacy of quinoxyfen in the presence of QLR. Powdery mildew control by quinoxyfen on both grape clusters and leaves was similar to, or only slightly less, than that provided by the standard anti-powdery mildew fungicides myclobutanil and boscalid in all three years. In order to gain a better understanding of the mode(s) of action and resistance mechanism(s) of quinoxyfen, gene expression of QLR and sensitive isolates, both in the presence and absence of quinoxyfen, was analyzed by nucleic acid sequencing. This study confirms previous research suggesting that quinoxyfen interferes with the important biological process signal transduction, and revealed additional gene targets of quinoxyfen. The phosphites are a group of fungicides commonly used to control grape downy mildew. Control failures after phosphite application have occasionally been suspected, and downy mildew isolates from vineyards with and without suspected control failures were tested in laboratory bioassays to determine if any level of resistance could be demonstrated. There was a limited range of sensitivity, and none of the isolates showed a notable loss of sensitivity. A field trial was conducted to determine the efficacy of one phosphite fungicide, Prophyt, applied at 14-day intervals under conditions favorable for disease development. Prophyt provided poor downy mildew control, suggesting that it has to be applied more frequently. Next-generation sequencing technologies were utilized to identify genetic markers for clade identification and generated a draft genome assembly of grape downy mildew, which improves the understanding of grape downy mildew genome. Grape downy mildew isolates collected from a vineyard in Virginia where mandipropamid provided poor control of downy mildew were bioassayed. The isolates tolerated mandipropamid rates well above the field rate, showing that they were indeed resistant. The mutation that confers mandipropamid resistance on other continents was found in the PvCesA3 gene of two resistant isolates.