UNIVERSITY PARK, Pa. -- Few things are as frustrating for the home gardener as poor plant growth and nonproductive or dying plants. A major cause of plant problems in the garden is disease.
Many plant diseases can be minimized or eliminated if you know how to recognize them, says Dr. Alan MacNab, professor of plant pathology in Penn State's College of Agricultural Sciences.
"Unhealthy plants usually result from some form of external irritation," says MacNab. "Injury, or instantaneous irritation, can be caused by animal or insect chewing, lightning, freezing, burning, excess or inappropriate chemicals, and careless use of garden tools.
"Disease, on the other hand, is a product of continuous irritation. This can be caused by fungi, bacteria, viruses, nematodes, parasitic plants, temperature extremes, pesticides, improper pH, or incorrect amounts of water, light or nutrients."
MacNab says accurate diagnosis is the first step in overcoming a plant problem. Once the cause is known, steps can be taken to overcome the problem or prevent its spread.
When diagnosing a plant problem, there are several questions to consider:
- What are the symptoms? Usually, a few specific symptoms develop, which can indicate a specific problem.
- Are insects evident? Insect problems are easy to diagnose when insects are visible. But if insects are small or have left the plant, diagnosis must be based on symptoms or microscopic observations.
- What plants are affected? Organisms that cause problems prefer some plants over others. Most infest only related plants or one plant species. Adverse environmental conditions generally affect a wide range of plant species.
- What is the pattern of symptom development? Symptoms appearing on widely separated plants may indicate that an organism was introduced by seeds, transplants or flying insects. Symptoms appearing in only one part of the garden suggest problems specific to that area, such as soil-borne organisms, poor soil drainage or improper planting depth.
- Does the problem appear to be spreading? Many disease-causing organisms, such as mites, insects or microorganisms, can move from leaf to leaf or plant to plant.
- Did symptoms appear after you performed specific gardening practices? We often cause plant problems inadvertently. Excessive fertilization near plants can cause root burn and plant wilting. Deep cultivation or pulling large weeds near plants can damage roots and cause water stress, resulting in wilting and leaf curling. Herbicides can damage garden plants when a sprayer is contaminated from prior use or drift occurs when spraying a surrounding lawn.
- Did symptoms appear following specific weather conditions? Sunlight and wind can dry out "soft" transplants that are inadequately prepared for outdoor weather. Sunlight can burn tomatoes and peppers when their leaves fail to shade the fruit. Frequent rain promotes reproduction of some disease-causing bacteria and fungi. Wet soil can cause root rot and plant wilting.
If observation or laboratory analysis indicates that a microorganism is the culprit, MacNab says there are several steps you can take to control the problem or prevent its reoccurrence in the future:
- Choose a garden site with ample sunshine, air circulation and drainage. Many fungal and bacterial diseases are most serious in chronically wet conditions.
- Change the garden's location occasionally, and always practice crop rotation within your garden plot. Many diseases, especially soil-borne diseases, are most severe when the same or related crops are grown in the same area year after year.
- Fertilize and lime your garden as recommended by soil test results. Plants grown with balanced fertility and optimum pH are less susceptible to diseases than excessively weak or vigorous plants.
- Grow disease-resistant varieties when available and suitable for local use.
- Use disease-free seed and transplants. Obtain seed from a reputable company. Obtain transplants from a greenhouse operator who uses disease-free seed and soil.
- Control insects.
- Control weeds. Perennial weeds near the garden often are the initial source of viruses in the spring. Dense weeds in the garden promote fungal and bacterial diseases.
- Pull and destroy diseased plants soon after harvest. Compost or plow under crop refuse as soon as possible after harvest to promote decomposition and kill disease-causing organisms.
- Spray or dust appropriate crops with fungicides to control specific diseases. If you must use fungicides, be sure to read and follow label instructions.
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EDITORS: For more information, contact Dr. Alan MacNab at 814-863-7544 or Chuck Gill Chuck_Gill@agcs.cas.psu.edu 814-863-2713 814-865-1068 fax