See also: Disease Symptom Chart
An analysis of diseased plant samples received from the local landscape industry and homeowners reveals environmental stresses and/or improper cultural practices are the primary causes of plant failure in the landscape.
The majority of plant deaths occur soon after planting in the home garden. Transitioning from the controlled nursery conditions of container grown plants into the unprotected home landscape can be difficult for a new plant.
Appropriate planting sites and proper cultural conditions must be provided or the plant will not develop, and likely to die. Planting appropriate plants for the climate and location are critical. Providing the proper soil is also vital.
In areas where heavy clay is the norm, digging a hole and plopping in a container grown plant that is growing in a earthy loam is a doomed recipe. As anyone with clay soil knows, water does not percolate through it. Digging the hole for the plant is like putting a bucket in the ground and all the soil water runs into it and drowns the new plant. The planting hole needs to be at least twice as wide as the container plant, the removed clay should be mixed with equal parts rotted compost, and peat to provide an adequate area for the water to reach the new plant, but not stand.
Watering is frequently the culprit. Insufficient irrigation during establishment is a major cause of plant failure as is too much water due to severe weather, watering too frequently, or poor soil drainage. Salinity of the soil and the quality of well water used for irrigation are other common concerns depending on your geographic area.
Actual plant diseases are generally a secondary cause of plant failure or decline in the landscape.
Plant pathogens 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 develop after a year or more, depending on the plant, the type of pathogen, and environmental stress.
A less common cause of plant failure is insect damage. Sometimes it is difficult to distinguish disease symptoms from insect damage. It is also difficult to make a disease diagnosis on an unknown or new plant species. Landscape plant species are very diverse, and if the normal appearance of a plant is not known, diagnosis may not be possible.
Other signs and symptoms that are often confused with diseases include normal leaf variegation, corky ridges on stems, lack of flowers, and normal leaf senescence and drop, particularly associated with the springtime leaf senescence of broadleaf evergreens or fall needle senescence of conifers.
The first steps in diagnosing plant problems are to determine:
What plant is it?
What is the plant supposed to look like?
What environmental conditions does the plant require?
To learn what to look for in diagnosing plant problems you need to keep the above information in mind. Plant characteristics, environment, and cultural practices should be determined first, followed by observation of signs and symptoms of plant pathogens.
There are at least ten thousand fungi, and hundreds of bacteria, viruses and nematodes that can cause plant disease. Narrowing this down requires some basic knowledge of morphology and biology of individual groups of plant pathogens.
At this stage, plant pathologists usually look for two things:
Symptoms are changes in the plant's appearance from how it should normally appear. Individual pathogens may cause specific symptoms.
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).
Symptoms tend to be associated with the different groups of pathogens so identification of the symptoms is an important step toward correct diagnosis of plant disease problems.
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 plastic bag with a moist paper towel. 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 cause of many disease samples to specific pathogen groups.
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 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.
Identify the plant(s) involved.
Inquire into site history of plant disease or other problems.
Look for patterns of symptoms on plant parts or whole plants.
Assess spatial distribution of disease symptoms in the landscape.
Examine disease symptoms and disease signs using a hand lens when necessary.
Compare disease symptoms and signs to the images in reference books and those posted on the Internet.
Perform simple examinations for potential bacterial and nematode diseases with the aid of basic tools listed above.
Consult with a local Extension agent, then a professional diagnostician, if necessary,
or send a sample to a public or private diagnostic lab, depending on the nature of the diseases
to be examined.