Research funded by American Vineyard Foundation (AVF) found that in Washington, Oregon and California, over 90% powdery mildew samples tested were found to be resistant to QoI (FRAC Class 11) and about 60% were resistant to both QOI and DMI (FRAC Class 3) (2017 data). The QoI resistance is attributed to a single point glycine to alanine mutation at position 143 of cytochrome b gene (G143A) of Erysiphe necator.  A change from tyrosine to phenylalanine in the cytochrome 450 gene (Y136F) is associated with tolerance higher DMI fungicide.

What is Qualitative Resistance?  It is like a light switch on or off.  For resistance to quinone outside inhibitor (QoI) fungicides (FRAC Group 11), the identification of the G143A mutation in Cytochrome b indicates that QoI resistance is present.  This is a classic case of qualitative resistance where resistance comes from the modification of a single major gene (Cytochrome b), Erysiphe necator subpopulations are either sensitive or fully resistant to the fungicide.  The FRAME Research team has only found the G143A in the CYTb gene from over 400 samples sequenced.  

What is Quantitative Resistance?  It is like getting a suntan.  A slow progression in tolerating higher and higher dose or the gradual erosion of disease control over time.  For resistance to DeMethylation Inhibitors (DMI) fungicide (FRAC group 3), the FRAME Research team has found Y136F mutation to be associated with most resistant isolates. In addition, there is a correlation of copy number of Y136F to DMI tolerance level.  CYP51 mutation copy number is important.  Isolates with high copy number of CYP51 with Y136F mutation is 99% likely to be highly tolerant.  If these mutations are present, in the short time, the disease may be managed by increased dosage.  However, eventually, even the highest manufacturer's suggested rates and/or shortest application intervals may fail to adequately control the disease.    

Here at Agri-Analysis, we use a combination of qPCR and DNA sequencing to accurately determine the G143A and Y136F mutations and estimate the relative abundance of mutated vs wild type E. necator from field samples. Please contact us at info@agri-analysis.com for more details and questions.