Calculation of phytoplasma titer
Phytoplasma concentration (number of copies of phytoplasma 16S rDNA gene per µl) was calculated automatically from the quantification cycle (Cq) values by the use of a cloned 16S rDNA gene standard ranging from 101 to 109copies in qPCR with the internal manufacturer’s software (referred to as estimated qPCR concentration). Samples with Cq values higher than 30 were considered as tested negative (see Table S1; value of control DNA from healthy trees maintained under insect-proof conditions). Calculation of phytoplasma cells per gram wet weight of extracted leaf tissue and phloem scrabbings was performed by multiplying the assessed qPCR concentration by the applied volume of extraction buffer (Y) and the volume of HPLC water used for DNA resuspension, by dividing by the number of 16S rDNA operons (2) and the wet weight of extracted leaf tissue (X):
\begin{equation} \text{Phytoplasma\ titer}\ \left[\frac{\text{cells}}{g}\right]=qPCR\ concentration\ \left[\frac{\text{copies}}{\text{µl}}\right]*(Y*50*\frac{1}{2}*\frac{1}{X})\ \left[\frac{\text{µl}}{g}\right]\nonumber \\ \end{equation}