Plant failures are common in the landscape. Some occur soon after planting, while others develop over time. What causes a landscape plant to fail is a question for many homeowners. Answers to this question will help not only to diagnose, but also to prevent similar problems in the future.
In Kentucky, the primary causes of plant failure in the landscape are environmental stresses and/or improper cultural practices. Most plant failures occur soon after planting. The transition from a container, where growth is controlled by conditions in a nursery, to the landscape can be very difficult. If an appropriate planting site and proper cultural conditions are not provided, plants will not establish adequately and are likely to die.
Frequently, insufficient irrigation during the establishment period is a major cause of plant failure. However, too much water due to severe weather, watering too frequently, or poor soil drainage may also cause plants to die or become diseased. Actual plant pathogens are generally a secondary cause of plant failure or decline in the landscape.
Plant pathogens may include fungi, viruses, bacteria, or nematodes with most problems caused by fungi. Disease development of this type also is subject to weather conditions and cultural practices. Commercial nurseries maintain healthy plants with regular fungicide programs. However, after transplanting into the landscape, unprotected plants may become diseased. Disease can develop soon after planting or years later, depending on the plant, the type of pathogen, and environmental stress.
The first steps in attempting to diagnose a plant problem are to determine (1) what the plant is, (2) what it is supposed to look like, and (3) what environmental conditions it requires. Keeping this information in mind should help you to learn what to look for in diagnosing plant problems. Landscape plant species are very diverse, and if the normal appearance of a plant is not known, diagnosis may not be possible. Signs and symptoms often confused with diseases include normal leaf variegation, corky ridges on stems, lack of flowers, and normal leaf drop. A less common cause of plant failure is insect damage. Sometimes it is difficult to distinguish disease symptoms from insect damage.
Once characteristics of the plant, environmental conditions, and cultural practices utilized are determined, observe signs and symptoms of plant pathogens. Plant pathologists usually group causes of plant failure into two categories: infectious (those caused by a pathogen), and noninfectious (caused by environmental stress or improper cultural practices). Identifying whether a problem is infectious or noninfectious is the first thing to determine when we diagnose a plant disease sample.
Environmental stresses and improper cultural practices that lead to plant failure are numerous. They include too much or too little water, soil or water that is too salty, pH that is too high or low, use of excess fertilizers or nutrient deficiencies, or chemical injuries (from herbicides, pesticides, runoff, or other pollutants). As a rule, noninfectious disease symptoms are distributed evenly over a large area or over several different plant species. They may also be associated with some specific location(s) or cultural practice, such as where herbicide applications were prepared, or with some environmental events, such as frost or a hailstorm. In contrast, infectious disease symptoms develop sporadically, are distributed unevenly, and usually are restricted to a particular plant species, or even cultivar. Therefore, field distribution of diseased plants and symptoms are an important initial observation for disease diagnosis.
If a disease is involved, the next question is, “What pathogen is causing the problem?” There are at least ten thousand fungi, and hundreds of bacteria, viruses and nematodes that could cause plant disease. To narrow this down requires some basic knowledge of morphology and biology of the individual groups of plant pathogens. At this stage, plant pathologists usually look for two things: disease symptoms and disease signs.
Disease symptoms are the changes in the plant from its normal appearance in response to the pathogen. Individual (groups of) pathogens may cause specific symptoms. For example, mosaic symptoms (irregular patches of discoloration) are usually associated with viral diseases. Spots and lesions are usually associated with fungal and bacterial diseases.
Detecting disease signs may also be helpful at this stage. Disease signs are vegetative and/or reproductive structures of plant pathogens left on the plants or plant parts. Some fungi and bacteria grow on the surface of leaves, stems, petals, etc., where they may be seen. The most obvious examples are rusts and powdery mildews, which can often be identified with the naked eye from the massive amounts of spores or white fungal threads on the plant surface.
Sending a sample to a diagnostic lab through the Extension Service is recommended when you are uncertain about what is responsible for the plant problem.
For more information, contact the Christian County Cooperative Extension Service, 270-886-6328.
