Why the Results Vary when Grapevine Samples are Submitted for Disease Testing

By: Judit Monis, Ph.D. - Vineyard and Plant Health Consultant

You probably know that there are many options when it comes to laboratory testing services. It can be confusing to the grower, vineyard manager, and/or nursery staff to decide which laboratory to choose

I am always asked by clients: “why the results of samples submitted from the same vineyard block yield different results at different laboratories?” There are many different reasons and I will try to clarify some aspects associated with laboratory issues (method/techniquess used) and sample collection that will affect the final disease diagnostic results. Finally, I will introduce the concept of standardization of diagnostic methods used for the detection of grapevine pathogens. After reading this article, my hope is that you hire a knowledgeable plant pathologist who can determine your best options based on your needs and walks you through the process.

Description of Most

Common Laboratory Techniques:

Microbiological Culture: Fungal and bacterial pathogens can be cultured and isolated in specialized media. However, microorganisms could compete among each other. Microbe(s) that grow faster will outcompete microbes that grow slower, making the diagnosis of certain bacterial or fungal pathogens difficult. The diagnosis could be biased or the laboratory may not be able to report the disease causal agent unless sophisticated molecular methods are used in combination with culturing methods. However, in some cases, the identification of the fungal taxonomic family (i.e., species of the Diatripaceae or Botryosphaeriaceae family isolated from a canker) or bacterial genus (Agrobacterium species isolated from a typical gall) may be sufficient to decipher the cause of the problem. Phytoplasmas (a special type of bacteria that lack cell walls) and viruses cannot be cultured and their identification must be carried out using molecular and serological methods.

ELISA, PCR, RT-PCR, qPCR: ELISA is the abbreviation for “enzyme-linked immuno-sorbent assay, and consists the binding of a protein (coat protein, in the case of a virus) on a plastic test plate that was coated with specific antibodies. A positive reaction is seen when there is a change of color in the wells of the test plate (colorimetric enzymatic reaction). ELISA detection is limited to the amount of virus present in the sample, therefore not prone to lab contamination. During the Coronavirus pandemic you probably heard in the general media talk about antibody tests. ELISA, although different from the rapid home COVID 19 tests based on immunochromatography, is an antibody test. PCR, is the abbreviation for polymerase chain reaction (this is a molecular based test). The technique allows the multiplication nucleic acid from the concentration of pathogen present in the vine. The process is specific, and utilizes copies of small portions of the pathogen’s genome (called primers) to start the copying process. The amplification is repeated many times, with each copy making more copies, so after the completion of an appropriate number of PCR cycles, more than a billion copies of the nucleic acid is produced. For RNA viruses the detection is done using RT-PCR (RT stands for reverse transcription, a molecular