Diagnosis of Plant Problems
Ethel M. Dutky
Cooperative Extension Service
University of Maryland
Gail E. Ruhl
Cooperative Extension Service
When plants fail to grow properly the gardener is faced with a challenge to determine the cause of the problem so that effective corrective action may be taken. In the United States, each state has a land grant university which has as its primary mission to support U.S. agriculture through research and educational programs aimed at the "primary producers of food and fiber". Although home gardeners are not the original intended audience, most land grant universities have programs to assist home gardeners in plant production and pest and disease control. There are perhaps both a plant diagnostic laboratory and a soil testing laboratory which act as part of the team serving agriculture in your state. This article is intended to assist you in locating and effectively using existing resources in your state or region.
Many problems can be prevented most easily before the plants are put into the ground. A careful site analysis can detect problems with pH, drainage, and the presence of plant parasitic nematodes. The best time to treat for nematodes, correct poor drainage, incorporate organic soil amendments to improve internal soil drainage, and correct a pH that is unsuitable for rhododendrons and azaleas is before the plants are installed. The severity of pests and diseases is greatly influenced by site factors such as drainage, exposure to drying wind, sunlight, pH and fertility. Part of the recommendation to correct growth, disease and pest problems is always to provide optimum growing conditions.
Tests that should be part of a site analysis include:
1. Basic soil test for pH, fertility, and soil texture.
2. Soil assay for plant parasitic nematodes.
3. On site test for drainage.
4. In some cases a soil test for soluble salts may be needed, if salt contamination is suspected.
Strategies to Diagnose Problems
There are six categories of problems or major causal agents you might encounter when dealing with plant disorders: insects (and mites), animals (rodents and birds), infectious diseases, chemicals (including nutritional imbalances), environmental factors and management practices. If you can narrow your problem to one or two of these categories, you may get an idea of where to look for specific identification and control remedies.
The cause of most problems can be determined by following a logical series of diagnostic steps. First, know what is normal. This may sound silly; however, before one can diagnose a problem one should know what a healthy plant looks like. One might mistakenly identify normal variegated leaves for a virus problem or a dwarf variety as having improper growth. Thus, it is important to know the normal growth habits of the plant in question. Second, stand back and observe the overall pattern of injury - on the entire planting, on individual plants and on individual plant parts, such as leaves. The pattern of injury noted can aid in the diagnosis of the problem. A uniform pattern of injury, such as the browning of leaf margins on a majority of the leaves on a plant is most often the result of an abiotic or non-infectious causal agent, such as environmental (drought) or site (root restriction) stress. A random pattern of injury on individual plants, plant parts or the entire planting can usually be traced to a biotic causal agent such as an infectious disease (fungal leaf spots), insect (leaf notches from black vine weevil feeding), mite (bronzing) or rodent (scattered dead plants due to chewing activity) damage. Third, check to see what plant part is injured. For instance, if browning is noted on the leaves, is the leaf the primary site of the actual damage or is that just where the symptom is appearing due to an injury on some other part of the plant, such as the stem or roots? Fourth, collect information about the symptoms seen. Note the color of the plant parts affected, the stage of growth affected (young or old tissues), and any other notable abnormalities. Finally, ask questions. There are many different kinds of questions one can ask to help narrow down the scope of possible causal agents (see Figure 1).
Figure 1: Questions To Ask I. Cultural Practices 1. History of the Problem? 2. Previous Plantings? 3. Pesticides/Fertilizers? 4. Varietal Differences? 5. Watering Practices? 6. Sanitation? II. Weather Conditions 1. Last Summer/Fall/Winter? 2. Present Time? 3. At Time Pesticides Were Applied? 4. Last Week? III. Area/Site Problems 1. Drainage? 2. Soil Type? 3. pH/Salts/Nutrients? 4. Exposure? 5. Topography? IV. Neighboring Plants 1. Similar Problems? 2. Same Types of Plants?
One can formulate a tentative diagnosis by comparing reports of pests, diseases and other problems known to occur on the plant affected. In most cases you will find a description that closely fits the symptoms you are seeing. You may need to collect additional information and make closer examinations to confirm your tentative diagnosis. It is usually in the latter stages of problem-solving that experts and laboratories are consulted. Share your tentative diagnosis with the "experts". Once you have confirmed your tentative diagnosis, you can consider control and corrective options.
Your role in collecting information and providing a good sample is essential to diagnosis. Most plant diagnostic laboratories have data forms; be sure to fill out the form completely. Include additional information such as diagrams of distribution (patterns) on the site, drawings of symptoms, etc. Your tentative diagnosis will also be a guide as to what plant material should be submitted to the clinic. For example, if you suspect a root disease, then the entire plant, or at minimum a large root sample, must be included, along with branch and leaf samples. Specific instructions as to sample submission are available from the diagnostic clinics in your state. Hand delivery or shipping by next day delivery are important to preserve sample quality. Plant samples decompose rapidly in the mail, especially in hot weather.
As you become more experienced, you will develop a sense for the cause of most problems. In general, pests and diseases appear slowly, and are not uniformly distributed on the plant or on the site; some plants will have more symptoms than others. Most plant diseases are very host-specific; only one species of plant, or often only certain cultivars will be affected. Many pests are also very host specific, but others, such as spider mites, feed an a wide variety of plants. In contrast, most problems caused by non-living agents will appear suddenly and affect a wide variety of plants on the site. Examples of non-living agents include cold, drought, heat, herbicide and air pollution.
Tools for Diagnosis
Your knowledge and experience are your most valuable tools for diagnosing problems. Reference books, especially those with color pictures, are useful assistants. A magnifying glass and sharp knife are essential for collecting the evidence to confirm or deny your tentative diagnosis. Sturdy plastic bags will be useful in packing and shipping plant and soil samples. Keep accurate notes of your observations, and mark samples clearly.
We have not included specific fees in this article because fees vary widely and may change each year. Although some university labs do not charge, the trend is to charge a fee for both soil testing and sample diagnosis. Generally the fees are lower for residents of the state, and higher for out-of-state samples. All private labs charge. Consult with your local labs for current fee structures.
Other Diagnostic Tests
There are now some "high tech" diagnostic kits which identify the presence of certain plant pathogenic fungi, bacteria and viruses. These kits utilize sero-logical (immunological) reactions similar to the technology used in rapid "home pregnancy tests". The sample is prepared by grinding, placed in specific reagents, and then treated with a series of other reagents, giving a final yes or no reaction. These serological tests are routinely used to index propagation stock for certain bacteria, and viruses. Serological tests for several plant pathogenic fungi are presently widely used in the high maintenance turf market; they are new to ornamental plant production systems, but have great potential for Integrated Pest Management programs in commercial production systems. Their cost may make them less attractive for home gardeners. Presently, a reliable test for the fungus Phytophthora, available from Agri-Diagnostics, Cinnaminson, N. J., would be potentially useful to rhododendron and azalea growers.
Quality plants are the result of quality care. With close attention and a little training, most plant disorders can be corrected before they permanently affect the plant. When diagnosing a problem it is important to remember not to jump to conclusions. Always ask questions - and listen to answers! The secret is to collect enough information to identify the problem. Sometimes symptoms can be confusing. Many disorders have similar symptoms or two disorders may work together to give symptoms that are not typical of either. In addition, the damage may be the result of a previous problem which does not show up until months later. These factors emphasize the need for examining a plant carefully and considering all possibilities. Diagnosing plant problems is not always easy, but the time spent will bring worthwhile dividends!