Author: Chuan Hong, Extension Specialist; Tom Banko, Associate Professor; and Marcia Stefani, Research Specialist; Virginia Tech
Publication Number: 450-800, Posted January 2003
In Virginia, analysis of plant disease samples received from the local landscape industry and homeowners reveals that environmental stresses and/or improper cultural practices are the primary causes of plant failure in the landscape. The majority of plant failures occur soon after planting. The transition from container-growth, under controlled conditions in a nursery, to the landscape can be very difficult. If an appropriate planting site and proper cultural conditions are not provided, the plant will not establish adequately, and is 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. Salinity of the soil and the quality of well water used for irrigation are other common concerns in the eastern part of the mid-Atlantic region.
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 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 attempting to diagnose a plant problem are to determine (i) what the plant is, (ii) what it is supposed to look like, and (iii) what environmental conditions it requires.
Keeping this information in mind should help you to learn what to look for in diagnosing plant problems. Characteristics of the plant, environmental conditions, and cultural practices utilized should be the first things to determine, followed by observation of signs and symptoms of plant pathogens.
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 [Figure 1A]. 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 [Figure 2]. In contrast, infectious disease symptoms develop sporadically, are distributed unevenly [Figure 1B], 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 [Figure 3] 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) [Figure 4]. 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 [Figure 5] or white fungal threads [Figure 6] 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 [Figure 7]. 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.
Basic diagnostic tools [Figure 8]:
Step-by-step procedures:
There are two types of private services. Some companies provide both diagnostic kits and direct diagnostic services; others provide only direct diagnostic services. Table 1 is a partial list of these companies with their contact addresses. These companies provide a wide range of test kits (service code A in Table 1) or direct diagnostic services (service code B) for viral diseases.
Bacterial and fungal diseases are gradually being added to their service list. These private services are important alternative resources to help meet your plant disease diagnostic needs.
Additional resources available include a wealth of VCE publications (www.ext.vt.edu), disease symptom images, and current VCE chemical control recommendations posted on the Internet. These data can be freely accessed at any time and without time limits. The Internet can be an unparalleled information resource for plant disease diagnoses. It can also be an endless source of frustration, as well as an enormous waste of time if searching in the wrong locations for information for disease diagnosis. To help landscape and nursery personnel access and use the wealth of information available to meet the industry's diverse needs, researchers at Virginia Tech's Hampton Roads Agricultural Research and Extension Center developed a Digital Nursery Workshop in 1999. This workshop is available through the Center's website at http://arecs.vaes.vt.edu/arec.cfm?webname=hampton §ion=projects&subsection=2266. The workshop is listed under "programs." This workshop is a good place to get started for people who like to surf the Internet. Selected sites were chosen for their relevance to nursery and landscape species. The workshop highlights the best of university and commercial information sites for disease, insect, and weed control resources. Information contained at these sites is considered reliable, accurate, and up-to-date.
Table 1. Partial list of companies providing plant disease diagnostic kits and/or direct diagnostic services
| Company | Location | Telephone | Fax | Service | |
|---|---|---|---|---|---|
| Agdia | Indiana | 800-622-4342 | 574-264-2153 | info@agdia.com | A, B |
| Hydros | Massachusetts | 508-540-2229 | 508-540-2224 | hydros@capecod.net | A, B |
| Neogen | Kentucky | 800-477-8201 | 859-255-5532 | inform@neogen.com | A, B |
| MMI | Georgia | 706-548-4577 | 706-548-4891 | B |
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