Biotic plant problems are caused by living organisms, such as fungi, bacteria, viruses, nematodes, insects, mites, and animals. Abiotic disorders are caused by nonliving factors, such as drought stress, sunscald, freeze injury, wind injury, chemical drift, nutrient deficiency, or improper cultural practices, such as overwatering or planting too deep. Unfortunately, the damage caused by these various living and nonliving agents can appear very similar. Even with close observation, accurate diagnosis can be difficult. For example, browning of leaves on an oak tree caused by drought stress may appear similar to leaf browning caused by oak wilt, a serious vascular disease, or the browning cause by anthracnose, a fairly minor leaf disease.

When the cause of a plant health problem is not readily diagnosed, it's important to take a systematic approach and carefully consider site conditions, weather conditions, care of the plant, and the known biotic disease agents of that plant. The first important step is to determine the identity of the plant and its requirements for healthy growth.

There are a few clues to look for that will help you distinguish between abiotic and biotic disease problems.

• Abiotic damage often occurs on many plant species. Drought stress or chemical drift will likely cause damage on several types of plants in a yard or garden. In contrast, biotic disease problems are more limited to a certain species. The fungi that cause tomato leaf blight do not cause damage on sweet corn, for example.
• Abiotic damage does not spread from plant to plant over time. Biotic diseases can spread throughout one plant and also may spread to neighboring plants of the same species. Wind-blown rain is a common way for disease agents to spread from plant to plant.
• Biotic diseases sometimes show physical evidence (signs) of the pathogen, such as fungal growth, bacterial ooze, or nematode cysts, or the presence of mites or insects. Abiotic diseases do not show the presence of disease signs.

An important take-home message is to remember that there may be several factors, abiotic and biotic, contributing to a plant health problem. For example, older trees that are stressed by drought conditions are often troubled by fungal canker diseases. Another example is the presence of decay fungus at the base of the tree. The primary problem may have been mower damage, which subsequently allowed entry of the fungus. The identification of the primary problem and other contributing factors is a necessary step in managing the problem or avoiding it in the future.

For guidance on diagnosing tree problems, refer to the Iowa State University bulletin SUL 3 "Diagnosing Tree Problems." This publication is available for $1.00 from your local county extension office or from the Iowa State University Publications Distribution Center, 515-294- 5247.

This article originally appeared in the 9/12/2003 issue.