DISEASE FOCUS: Mixed infections cause problems with accurate disease diagnosis

by Deborah Mathews

Often I am told by a grower that they sent a sick plant sample to a lab and it came back as pathogen X, Y or Z.  They treated for that cause, but problems are still present; can I take a look? What I usually find is that there are mixed infections of several pathogens, even different pathogen types, and when coupled with some horticultural issues, the cure may not be a simple one.  It is easy for a typical diagnostician to see the “individual trees” but not the “forest” in this case. If a pathogen is dominant, or grows out faster in a culturing assay, it can mask the presence of one or more other pathogens. As a result, further assays are not conducted, which can miss another important players in the disease cycle. 

Another problem is that many clients send samples that are almost dead to the lab. By that time, the actual pathogen that was causing the problem has died and secondary opportunistic organisms are the ones detected, not the actual pathogen that started the problem. This is very common with diagnoses of Fusarium since many species of this fungus are saprophytic and feed on already dead tissues, but are not pathogenic themselves. Commercial testing labs usually only identify organisms to the genus level, not species, so it is difficult to assess the causal agent in these cases. 

I commonly find that field-grown plants that are wilting have more than one pathogenic fungus or water mold (Phytophthora or Pythium) in the root system.  Recently I tested some rosemary plants that were showing wilting symptoms in warm temperatures and some dieback. Results from culturing assays showed that the root balls contained Pythium, Rhizoctonia and Fusarium; an antibody lab test revealed a fourth pathogen, Phytophthora. This or similar pathogen combinations complicate fungicide applications since many fungi require treatment with a product that has a distinct mode of action, which may not be effective against a broad range of pathogens.  Plants may still be able to thrive if only one pathogen is present, but if there are multiple infections, there may be a compounding effect (synergism). The presence of a plant virus and/or environmental factors such as drought or extreme heat cause further stress on the plant.  This makes the end result much worse than the effect of any individual pathogen alone.

Fig. 1. Palm frond with canker caused by Phytophthora palmivora. Ideal isolation area of advancing disease margin is shown inside red box. D. Mathews

Some good ways to provide optimal samples for diagnosis are:

• Send the entire plant, roots and shoots wrapped separately, early in symptomatic development, not already half dead.
• For larger specimens where a subsample needs to be sent, try and collect young feeder roots. If lesions are present, remove a section with a healthy portion of the plant and the margin with the canker or dead portion (fig. 1).  The pathogen will most easily be recovered from the advancing margin, not the already dead part.
• Send two to three examples of the problem if possible to help ensure successful isolation.
• To avoid cross contamination, wear disposable gloves while collecting samples or wash hands well between plants.

So take care in diagnosing your plant disease problems. Finding the solution is not always easy.

Deborah Mathews is UC Cooperative Extension Specialist/Plant Pathologist for Ornamental Crops, Department of Plant Pathology and Microbiology, UC Riverside.