Landscape - More on Diagnosing Landscape Problems

This is a continuation of the series on diagnosing landscape plant problems.

Can you diagnose plant failure by yourself? The answer is 'Yes' for most situations. The do-it-yourself guidelines below provide a list of what to look for, basic tools, and step-by-step procedures. These procedures will guide you in examining a sample, and in narrowing down the possible causes of the problem as closely as necessary to make decisions for disease management.

What to look for. Plant pathologists usually group causes of plant failure into two categories: infectious (that is caused by a pathogen(s)), 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 can 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 general 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 hail storm.

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 is an important initial observation for disease diagnosis. It is not possible for professional diagnosticians to examine spatial distribution of every disease sample received. Thus, you are in a better position to make these observations than diagnosticians are.

If a disease IS involved, the next question is, "What pathogen(s) 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 are usually associated with viral diseases. Spots and lesions are usually associated with fungal and bacterial diseases. Viruses rarely cause root rots or cankers (stem lesions). Thus, learning symptoms that tend to be associated with the different groups of pathogens is an important step toward correct diagnosis of plant disease problems.

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.

Sometimes signs of fungal pathogens may be observed on diseased plant parts after placing them in a humid chamber for a day or two. A plastic bag with a moist paper towel works well for this. Some bacteria may be released from infected plant tissue where they may ooze visibly from wounds under wet conditions. Pathologists sometimes use this trait to test for bacterial disease in the lab by cutting a small piece of infected tissue and placing it in a beaker with tap-water. Bacteria in the plant tissue may ooze into the water, making it cloudy.

A similar procedure may be used to examine certain nematode diseases. Nematodes are large enough to be seen with a hand lens after they are released into water. Thus, disease symptoms and signs along with some preliminary observations can help narrow down the causal agent of many disease samples to specific pathogen groups.

Substantial additional examinations are needed to determine exactly what species causes a disease problem. Justification for this extra effort depends on disease management options. Further identification is justified only for fungal diseases in situations in which a fungicide(s) must be applied to keep the disease under control. There are several fungicides labeled for control of fungal diseases, but some are only effective in controlling a specific group of fungi.
There are two types of fungi: true fungi, and oomycetes (water molds), that cause plant diseases. These different groups of fungal pathogens have different physiologies, so fungicides that can effectively control diseases caused by true fungi may have no impact on those caused by oomycetes. Major oomycete pathogens include Phytophthora and Pythium species, which are primarily responsible for root rot of numerous plants. Also included are the species that cause downy mildews of many crops.

Consulting a professional diagnostician or sending a sample to a diagnostic lab is recommended when you are uncertain about which group of fungal pathogen is responsible for the plant problem. Detailed examinations may be useful for helping with cultural recommendations, or for fungicide recommendations if the services of a licensed pesticide applicator are utilized. However, there are few options for control of bacterial, nematode and viral diseases in the landscape. Thus, diagnosing which group of pathogen is causing the problem is most likely all you need.

Extracted from "A Guide to Diagnosing Diseases of Landscape Plants" by Chuan Hong, Extension Specialist; Tom Banko, Associate Professor; and Marcia Stefani, Research Specialist; Virginia Tech. For the full fact sheet with pictures go to http://www.ext.vt.edu/pubs/plantdiseasefs/450-800/450-800.html