February 2022: Optimizing Your Diagnosis at the PDDC

Diseased Leaves Branches and Fruits IconHere at the Plant Disease Diagnostics Clinic (PDDC), we strive to provide the best possible diagnoses of plant disease problems.  PDDC staff use a range of techniques to do this including visual and microscopic examination of plant tissue, incubation of tissue in moist chambers to coax fungal pathogens to sporulate, culturing techniques to grow pathogens from tissue, serological tests to detect proteins specific to certain pathogens, and DNA/RNA (i.e., genetic material) detection techniques.  The PDDC’s ability to provide a high quality diagnoses using these techniques however, is dependent on receiving a high quality sample to work with.

Here are some pointers on how to provide an optimal sample so that PDDC staff can provide an accurate, timely diagnosis.

Submit a sample when you first see a problem

Diagnosing plant diseases as early as possible allows for greater flexibility in management of diseases once they are identified.  So, contact PDDC staff as soon as you see a problem.  That said, disease-causing organisms may not produce structures that we need to see to make a diagnosis (e.g., fruiting bodies, spores) early in disease development.  We may need to work with you and have you repeatedly sample and submit materials over time to accurately diagnosis your plant disease problem.

Consider submitting photos prior to a physical sample

Sometimes, we can provide a reasonably accurate diagnosis based on photos (tar spot anyone?).  If not, photos can provide useful information on the sort of physical sample that you can send to the PDDC for a more detailed analysis.  You can use the online form at https://pddc.wisc.edu/digital-diagnosis/ to submit photos.  If for some reason that form doesn’t work for you, feel free to send photos to the clinic email address (pddc@wisc.edu).  If we are able to provide a diagnosis from your photos, there will be a $20 digital diagnosis fee.  If we make a preliminary diagnosis from the photos but suggest a follow-up physical sample submission for additional testing, the $20 digital fee will be credited towards any lab fees for your follow-up sample.

When submitting a physical sample, make sure you provide the appropriate plant part

If you have any questions about what to submit, contact the PDDC for advice.  Submitting the wrong plant part can lead to an inaccurate or delayed diagnosis.

For herbaceous plants (e.g., herbaceous ornamentals, vegetables), sending in an entire plant may be the best course of action.  In some situations, symptoms that you see on the leaves of plants are an indication of a problem in the root system.  With trees and shrubs, submitting an entire plant is not practical.  For these plants, submitting subsets of leaves/needles, branches and/or roots will likely be more appropriate.

Whatever you end up sending, send A LOT.  Often, we need to perform multiple tests to diagnose a problem.  We don’t want to run short on tissue.  For leaves, send half a dozen to a dozen (or more) showing a range of symptoms.  For branches, send three or more symptomatic branches (with attached leaves or needles where appropriate).  For roots, send a large handful of the small, fibrous roots.

In particular, appropriate branch selection can be critical for accurately diagnosing vascular wilt diseases such as Dutch elm disease, oak wilt and Verticillium wilt.  For these diseases, select branches that have recently wilted/died back.  DO NOT submit branches that easily snap off.  These branches have been dead too long and cannot be accurately tested for vascular wilt pathogens.  If you suspect Verticillium wilt, select symptomatic branches from as low on the tree as possible as the pathogen that causes this disease infects through the roots.  Choose branches that are roughly one inch in diameter.  Larger diameter branches (particularly from oak trees) tend to have thick bark that is difficult to remove without contaminating underlying tissue where vascular wilt pathogens reside.  This contamination will interfere with growing pathogens from branch tissue and can also interfere with DNA-based tests for pathogen detection.  Sometimes, clients like to send in trunk sections for testing for vascular wilts.  These sorts of samples can work, but only if the trunk slices are no more that about one inch thick.  We need to be able to easily pop off the bark from these slices with minimal contamination of the wood underneath.

Submit your sample as quickly as possible to the PDDC

If possible, collect samples just before you mail them or drop them off in person.  If there is going to be a delay in submission, keep samples as cool as possible.  High temperatures can kill certain pathogens and can degrade herbaceous plant tissues, leading to what we not-so-affectionately refer to as “slime in a bag”.  Degraded samples make diagnostic testing more difficult, if not impossible.  Mail samples by overnight mail when possible.  If mailing via regular mail, please mail early in the week (Monday or Tuesday) so that samples do not sit around in a mail facility over a weekend.  Click here for details on how to package samples to make sure they arrive at the PDDC in good shape.

Help us, help you

Providing us with a high quality sample can go a long way in us providing you with a high quality diagnosis.  Let’s work together to make this happen.  If you have questions about submitting a sample (or about plant diseases in general), feel free to contact the PDDC by phone at (608) 262-2863 or email at pddc@wisc.edu.  Also check out the PDDC website (https://pddc.wisc.edu) for additional details on sample submission.  Feel free to follow the PDDC on Twitter and Facebook (@UWPDDC) or email me to subscribe to the PDDC listserv, UWPDDCLearn, to receive updates on clinic services and educational materials.

Good luck and happy plant disease sleuthing!