Thesis (M.S. in plant pathology)--Washington State University, May 2009. / Title from PDF title page (viewed on May 26, 2009). "Department of Plant Pathology." Includes bibliographical references (p. 35-38).

Thesis (M.S.)--Missouri State University, 2009. / "May 2009." Includes bibliographical references (leaves 44-50). Also available online.

This study was undertaken to determine the sensitivity of grape downy mildew (DM, Plasmopara viticola) and powdery mildew (PM, Erysiphe necator) to commonly used single-site fungicides in Virginia and nearby states. DM and PM isolates were collected from 2005 to 2007. In grape leaf disc bioassays, 92% of the DM isolates were QoI (azoxystrobin)-resistant, but none were resistant to mefenoxam. Eighty-two percent of the PM isolates were QoI-resistant, but none were resistant to boscalid and quinoxyfen. The frequency of the G143A point mutation, which confers high levels of QoI resistance, was quantified in DM and PM isolates by real-time PCR. Most of the QoI-resistant DM and PM isolates contained >95% of the 143A allele. QoI-sensitive DM isolates contained less than 1% of 143A. One out of 145 and 14 out of 154 QoI-resistant DM and PM isolates (able to grow on azoxystrobin concentration ï ³ 1 µg/ml), respectively, contained less than 1% 143A. Most PM isolates exhibited reduced sensitivity to five DMI fungicides when compared to a sensitive subgroup (n=9) and compared to published reports for unexposed populations; the resistance factor (median EC50 of the entire isolate collection divided by median EC50 of sensitive subgroup) was highest for tebuconazole (360) and myclobutanil (350), followed by triflumizole (79), triadimefon (61), and fenarimol (53). Sensitivities to all five DMI fungicides, but also azoxystrobin, were moderately to strongly correlated (pairwise r-values ranging from 0.60 to 0.88). / Master of Science in Life Sciences

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