- Author: Steven A. Tjosvold
The first factor in the disease triangle to evaluate is the host plant. Know what a healthy plant looks like-- both its aboveground portions and roots-- so that abnormalities can be recognized. Identify the plant species or cultivars that the unknown disease is occurring on. Look for patterns where abnormalities are occurring. Are there certain species or many species? Are there patterns that could suggest abnormalities caused by abiotic factors such as excesses in temperature, light, fertilizer, or pesticide application?
The second factor is the pathogen. Identifying biotic diseases is sometimes facilitated if the pathogen is visible. Particularly with some fungi, distinctive mycelium and spores can be visible on diseased tissue. Sometimes the pathogen can be seen with the naked eye but sometimes a hand lens can aid in seeing them. Gray mold (Botrytis), powdery mildews, and rusts are good examples of diseases that have distinctive spores and mycelium that are usually quite visible. Often, however, diseased tissue must be cultured in the laboratory to get the pathogen to produce mycelium, spores, or spore-producing structures that could aid identification. A light microscope can help the plant pathologist see and identify the specific pathogen in the laboratory. With viruses or other similar microscopic pathogens an electron microscope is needed to see and identify the pathogen. Sometimes evidence of a secondary fungus, bacteria or insect is visible and not directly related to the primary cause of disease.
The third factor is the environment. The diagnostician can collect information on the environmental conditions associated with the disease. Does irrigation frequency, dew, greenhouse condensation support disease occurrence? Do temperatures support the pathogen? Have temperatures been excessively high or low for plant growth? Have there been excesses applied of fertilizer, soil amendments, or pesticides that have been associated with the occurrence of disease?
References that aid in the diagnostic process are often organized so that they use and describe these three factors. To use these references, you need to first identify the host plant. After host identification, you can often go to a section where the host plant's common diseases are listed. The disease symptoms are described along with a description of the pathogen's mycelium, spores, or spore producing structures. There usually is a description of the specific disease's life cycle and any environmental conditions that support infection and development of a disease. Together, the information helps to focus the diagnostic process and formulate a likely diagnosis of an unknown plant disease.
Next: Suggested references that aid in identifying diseases and pests of ornamental plants.
For a more comprehensive look at the diagnostic process see attachment below:
UCNFA News as PDF57650 diagnosis
- Author: Ben Faber
Hats Off.
The Orange County Master Gardeners have lived up to their name with their website information on citrus. It's a truly impressive information site for not only homeowners, but also growers:
http://mgorange.ucanr.edu/Edible_Plants/?uid=108&ds=530
The “Citrus Problem Diagnosis Chart” is especially work perusing:
- Author: Ben Faber
I get a call.
He: My trees not doing well.
Me: What's the problem?
He: It's yellow?
Me: Have you looked at the roots? Are there roots?
He: NO
Me: I'll be out next week, but in the meantime, look at the root system.
This is a pretty common exchange and when I got out, you find out that the emitter is clogged, the ground is soggy, there's weed whack damage, there's gopher damage, there's…………………. All kinds of things that pop up and until you see the context that the poor tree is in, it's hard to diagnose the problem. Too much fertilizer, black drip tubing that had water heated in the sun and burned the roots, the trunk buried in mulch, it's hard to imagine all the possibilities. But start with the roots and then go where that leads you.
So, here's the scenario.
You get a call/email.
He: What's wrong with my young tree? It snapped off in the wind. Here's a picture of what's left. Corroded, bulbous graft union. Incompatibility? Extensive decay that indicates a problem of long standing. The leaves are green though, so it means it was hanging in there until the wind blew. Was the trunk buried at planting leading to asphyxiation and crown rot? Is it some sort of wound that started it off? Oh, and about 1% of the planting is like that.
Me: OK, I better take a look at it. I'll get some samples and send them into Akif Eskalen at UC Riverside and bring our plant pathologist Jim Downer out to look at it with me.
So tune in to find out what we diagnose as the problem. In the meantime, if you have an unknown avocado or citrus problem that looks of root origin. You can contact Akif and send him a sample after following the “Sample Submission Form” at: http://eskalenlab.ucr.edu/