During the growing season there are so many disease problems to discuss that there is no time to explain the basics of how to diagnose a plant problem. Now that things have slowed down, we want to emphasize some of the information needed to obtain an accurate diagnosis.
It is very frustrating to receive a complaint from a client who says, "We were very disappointed with your diagnosis of our plant problem. We really expected a more exact answer, not just speculation." Read through this material and try to understand that a poor diagnosis is often directly related to the amount of information received about a plant problem. Let your clients know this, and your job will be much easier. We all enjoy those disease or insect problems that we can identify merely by looking at one leaf, but that is the rare exception.
Diagnosis is definitely an art--one that requires observing symptoms, facts, and clues to determine the cause or causes of a problem. Plant diagnosis is difficult because so many problems (such as pathogens, insects, and chemicals) cause similar types of symptoms. For example, a tree planted too deeply will exhibit decline symptoms very similar to those caused by clay soil and poor drainage. How do we determine this from a sample of stems and leaves? Jumping to conclusions when there is a lack of information is a sure way to provide a misdiagnosis--something we like to avoid. Listed here are a few steps for obtaining information that may help make the diagnostic procedure a bit easier. These suggestions may seem simplistic, but they are worthwhile! Most of the difficulties that we encounter in diagnosing a problem at the Plant Clinic stem from a lack of information.
- Correctly identify the plant. When only a few leaves comprise the sample, it is often difficult to identify the host. An ash tree leaflet looks much like the leaves of many perennial herbs. Ask the client for an identification. It is difficult to use reference materials without knowing the host. If we can identify to the exact cultivar, we can often determine more about specific disease resistance. Don't be afraid to ask. If the client cannot ID the plant, then get enough of a sample to have someone else help.
- List the symptoms. First, determine the symptoms as they relate to the entire area. Describe the pattern in the field, landscape, or planting area. Because the diagnostician often cannot go to the site, this information is essential. Find out whether all plants in the landscape are affected or only one species, whether the problem is worse on one side of the planting, etc.
What are the symptoms on one affected plant? Usually the response is "Refer to the enclosed sample." This won't do. Turf samples almost always turn yellow after two days in a mail truck. Fleshy leaves often turn brown and crisp compared to their condition before mailing. Have the client describe what is abnormal in very specific terms. An accurate diagnosis depends on specific symptoms described as they have occurred over time. For example: "The plant has grown poorly since it was planted two years ago May. It leafed out well and then developed yellowed foliage, starting at the bottom of the tree and moving upward. The leaves fall off with half-inch brown spots by July."
- Dig for facts. This category of information could go on forever. It is probably best to include weather conditions, soil characteristics, cultural practices, site-specific oddities, and anything else the client can suggest. Here are some examples, but keep in mind that we are looking for information from the time the problem occurred or earlier, not afterthe fact.
- Rainfall--amount and frequency; has this been different than past years?
- Wind and exposure patterns compared to other healthier plants
- Frost occurrences
- Soil pH in the root zone
- Type of soil or drainage
- Presence of compaction
- Is this a new home where topsoil has been removed?
- Was salt used nearby?
- Fertilizer type, timing, rates
- Watering practices: time of day, amount, frequency
- Pesticide use, especially herbicides, to the plant, to the lawn, or nearby
- Pruning practices
- Use of mulch
- Use of plastic or landscape cloth
- Age of plant, type of planting, site preparation
- What else is planted in this same area?
- In general, ask the client to list what has changed on the site since the problem occurred.
- Has there been any construction or changes in this area since the problem started?
- Have new traffic patterns been established on the site?
Use reference materials. Once you have gathered all of the above facts, use reference books. Often these texts will list problems by host. Look at photos, read descriptions, and compare with your sample.
Consult laboratory help when needed. If you have done all of the above, you have likely narrowed the possibilities to something infectious, to an insect problem, or to a suspected cultural problem. Often you just need to be able to rule out the pathogenic or insect possibilities but do not have the expertise or equipment to do so. That is where a lab such as the Plant Clinic is helpful. Often, though, by the time you get this far you have determined the cause of the problem or you have determined that control measures are the same for the possibilities identified, and lab help is not necessary.