Results from testing isolates from CA, FL, MD, MS, OH, and TX

Pathogen isolates (individuals) were obtained from single powdery mildew colonies (lesions) on field-grown leaves collected near the end of the growing season from research plantings in California (Yolo County), Florida (Manatee County), Maryland (Cecil County), Mississippi (Washington County), Ohio (Clark County), and Texas (Hidalgo County) in 2022 and/or 2023.

The leaf disk bioassay procedure was used.

Resistance to Endura (FRAC 7) was detected in FL (both years), CA (only 2023), MD (only 2022), and OH (2023). Resistance to Endura has been detected commonly in NY.

Resistance to Torino (FRAC U6) was detected in FL (only 2022), MD (only 2022), and OH (2023).

Resistance to Quintec (FRAC 13) was only detected in FL, and only in 2022.

All three isolates from FL that were resistant to Torino were also resistant to Quintec.  These isolates were not tested for resistance to Endura.  Isolates with resistance to these three fungicides plus QoI (FRAC 11) and MBC (FRAC 1) fungicides have been detected in NY.

Fungicides applied to the research plots did not have a clear and detectable impact on resistance occurrence.  However, it is important to note that sample size was small.  Torino and Quintec were applied to the research plots in 2023 but not in 2022 when an experimental FRAC 13 fungicide was tested. Resistance to Quintec (FRAC 13) was only detected in FL in 2023. Resistance to Endura was detected in five of the nine state-year sites although its active ingredient (boscalid) was not applied to the research plots either years.  Occurrence of fungicide resistance in the cucurbit powdery mildew pathogen detected in research plots reflects selection for resistance through fungicide usage in commercial crops in the area.

Isolates Collected in 2022

* 9 isolates total; not all tested

Isolates Collected in 2023