Soil and Plant Testing Laboratory
573-882-0623

Don't always blame the dirt: Diagnosing plant health problems

Do your garden plants come up only to quickly die? Is grass difficult to establish in certain parts of your yard? A favorite tree or bush becomes sick and dies. The MU Soil and Plant Testing Laboratory often receives soil samples with complaints such as those above with the implication that something is wrong with the soil to cause their garden plants, grass or trees to die. This is probably logical since the soil's processes operate at an unseen level, and we tend to blame what we don't see or understand.

Taking a soil sample then seems to be a good idea toward determining the source of the problem, but the evidence sometimes suggests a problem other than the soil. Nutrient deficient plants may look quite sick, but nutrient deficiencies are seldom the primary cause of plant death. Soils can develop conditions that can be toxic to plant growth (very low or very high soil pH, or high salt levels) and can be identified by a soil test. Often nutrient deficiencies and unfavorable soils conditions predispose plants to insect and disease or weaken the plant so that it cannot survive adverse environmental conditions. It is sometimes this combination that causes plants to die. In other situations, the cause for the decline or death is unrelated to soil factors and could be directly caused by an insect, fungus, environmental factors or mechanical injury. So before grabbing a spade or probe to collect a soil sample from a trouble spot, consider a few other possibilities.

There are many reasons why plants decline or die. The problems may be related to a biotic (living) agent (insect, fungus, viral, bacteria or nematodes) or an abiotic (non-living agent) (temperature and moisture extremes, improper soil pH or chemical injury). In some situations, the specific symptoms are diagnostic and in the hands of a trained professional—area extension agronomy or horticulture specialist or the MU Plant Diagnostic Clinic—the problem can be fairly consistently identified. There are times, however, when the diagnostic process can be complicated because a combination of factors led to the decline. This may be difficult to determine unless there is a great deal of background information submitted with the sample. Diagnoses may be accompanied by suggestions to manage, alleviate or diminish the problem. Sometimes the diagnosis is regretfully the recognition of a terminal condition. While far from being comprehensive, the following steps may be helpful when you are trying to determine why a plant is not growing well or why it has died.

Accurately identify the plant
Each plant has its own cultural requirements. Azaleas require a low soil pH, yews do not tolerate poor soil drainage conditions and dogwoods do not grow well in full sunlight. Placing a plant in a site that is not suited for optimal growth may put the plant under stress and make it less able to deal with fluctuating moisture conditions, insect and disease pressures or other stresses. Because pathogens (fungi, virus, bacteria, nematodes, insects) are typically attracted to specific plants species, knowing the identity of the plant will narrow the list of potential problems to a manageable number.
Know what is normal for a healthy plant of the same age and variety
Too often a normal coloration or growth on a plant is mistaken for a fungal or insect problem. Observing other plants of the same species and age and species helps one identify normal growth. A common example of a normal process that is mistaken for disease is the yellowing of the older needles of conifers; specifically white pine.
Know the common problems
There are many references that list the common problems of plants and are commonly available at local nurseries and garden centers. These resources are a must have for any gardener. Also visit the Extension Plant Diagnostic Web site for a listing of the common plant problems in Missouri.
Distinguish between biotic (living) and abiotic (non-living) factors .
If the cause of the problem is due to a biotic agent, the patterns of damage will differ from those made by an abiotic agent. Typically, the damage from a biotic agent such as a fungus or insect will be limited to one plant species or closely related species. The damage will be scattered on the plant. Most biotic agents will leave signs of their existence. Insects will leave webbing, frass, chewing marks or cast skins. Fungi produce fruiting structures, resting structures and mycelium. Close examination of damaged areas is usually needed to observe these signs. When conditions are conducive to the needs of these organisms, their populations grow and inflict greater damage that spreads and worsens with time.
In contrast, the damage made by an abiotic agent, such as a cold snap or herbicide exposure, will affect many species and create more uniform damage on plant parts. For example a white pine injured by spray from deicing salts applied to a road will show the most damage on the side of the tree facing the road. The damage from abiotic agents causes damage at one point in time and does not continue to progress. Therefore a frost event on daylilies may cause the exposed leaves to die, but the leaves that emerge after the event would be symptom free.
Examine damaged plant parts for symptoms and signs
Examine the overall plant for symptoms (the plant's response to the stress). You may observe wilting, yellowed leaves, spots on the needles, rotted stems etc. Symptoms by themselves are not necessarily diagnostic, since many problems can result in similar symptoms. However, the presence of symptoms when combined with knowledge of the common problems that occur on a plant can be useful in arriving at a diagnosis.

The death of one's tree, bush, lawn or garden plants can be sorrowful. The loss can be aesthetic or economic, or it can be disquieting, given the care or management attended to the plant. The University of Missouri's Plant Diagnostic Services, which includes the Soil and Plant Testing Laboratory, the Plant Diagnostic Clinic and the Plant Nematology Laboratory can be useful to determine the cause of the plant(s) death. Hopefully this discussion will help direct the efforts of individuals seeking to answer why their plant(s) died, and not waste time, money or effort pursuing the wrong cause by blaming what so often seems to first come to mind. When the proper samples are submitted, it is not uncommon to forward/exchange samples between labs when potential diagnoses are negative and there exists the possibility that the other lab could be helpful. Contact your local extension specialist for specific information regarding submission of samples, or submit samples directly to one of the labs.

Updated 8/19/05