CAUSES OF COMMON PLANT DISEASES
By Bob Chapman
All garden plants can be attacked at one time or another by disease. Currently over 80,000 different diseases are known. Causes of plant diseases may be conveniently divided into two groups: those caused by a parasite (bacteria, viruses, fungi and parasitic flowering plants) and those caused by unfavorable growing conditions. Fortunately for the gardener, only a few cause the majority of problems.
Factors leading to infections of diseases in plants
In general, plants respond to optimum growing conditions by resisting disease. Plants that are stressed can favor disease. Stresses include:
- Water stresses: droughts or flooding
- Temperature extremes: Chilling or high temperatures of young plants can cause stress.
- Compacted soils. Compaction leads to oxygen stress.
- Nutrient imbalances, excesses or deficiencies cause stress. A lack of some minor nutrients weakens a plant, causing a lack of vigor or a yellowing of the leaves.
- Heavy insect populations. Lack of controls allows insects to harm plants.
- Applying pesticides inappropriately.
- Smog causes stress in plants.
- Improper pruning. Cuts too large, failing to thin fruits properly.
- Transplant shock or stress.
Three main causes of disease in plants
There are three types of diseases that can infect our plants: bacteria, viruses and fungi.
Bacteria
Several hundred types of bacteria cause plant disease. The most common types of bacterial disease are soft rots, leaf spots or blotches, blights, stem rots or cankers, wilts and galls. Many of the bacteria that cause rots in ripening fruits or vegetables are secondary invaders, gaining entrance through the wounds caused by other diseases or pests.
Bacteria, the simplest of plants, overwinter or oversummer on or inside plants, seeds, plant refuse, garden tools or in soil. Some may even live for several months or longer in the bodies of living insects. Fortunately, most disease-causing bacteria are quickly killed by high temperatures, dry conditions and strong sunlight. Many bacteria in the soil, capable of causing plant disease, are inhibited by antibiotic substances secreted by other soil-inhabiting organisms, chiefly bacteria and fungi.
Fireblight is a common bacterial disease that infects apples, crabapples, pears, ornamental pears, Pyracantha, flowering quince, Loquat, Cotoneaster and raspberries. Flowers and leaves wilt, turn brown and hang on the twig, not dropping. The branch may bend over at the top in a distinctive shepherd's crook shape. To control fireblight, spray with Lilly-Miller's Microcop and Sta-stuk just before blossoms open and every 5-7 days during blossoming. The bacteria enter the tissues through the newly opened buds. Once inside there is no control. You must spray at the time the buds open to control the disease. Remove infected twigs and branches by cutting into live wood 6 inches below the infected part. Destroy, do not compost, the diseased trimmings.
Clean gardening practices and using disease-free plants are the chief methods of control of most bacterial diseases.
Viruses
Viruses are complex protein molecules, which infect, multiply, mutate and otherwise act like living organisms when in living plant or animal tissue. The most common types of virus-caused diseases are mosaics, yellows, curly top, spotted wilt, ringspots, stunting and the blocking of water-carrying tissues. Certain variegated plants such as camellia blossoms and leaves, Abutilon and Rembrandt tulips are inherently virus-infected. Some plant viruses are quite infectious, being spread easily from diseased to healthy plants by mere contact. Others are transmitted only by the feeding and plant- to-plant movement of insects, primarily aphids, leafhoppers and thrips. Prevent these insects from invading the garden.
Viral infections are not treatable by the home gardener.
Fungi
Fungi are simple plants that lack chlorophyll. They obtain their food from living plants and animals or from decaying, non-living, organic material. Most fungi are inconspicuous, but certain mildews, molds and mushrooms are known to almost all gardeners.
Fungi are usually spread by spores and these and are found in many different shapes, sizes and colors. Spores are easily carried by wind, water, insects, man, animals, plant parts (seeds and bulbs) and equipment. Dormant, or resting, spores allow the fungus to withstand unfavorable growing conditions such as extreme heat, cold, drying and flooding. Spores of some fungi may lie dormant for a number of years. Most of the fungi need moisture on the leaf surface to germinate, grow and penetrate the tissues. When they inhabit the soil they are very difficult to kill.
Fungi cause the majority of infectious or parasitic plant diseases. These include mildews, rusts, smuts, scabs, leaf spots, cankers, blights, root rots, stem rots, fruit rots, wilts and leaf galls.
There are many fungicides on the market to control these widespread common diseases.
Maintain good growing practices as a first line of defense against plant diseases
A good growing environment keeps plants strong, vigorously growing and productive. Maintaining a proper environment for a plant helps it overcome attacks by diseases.
- Properly preparing the soil, following correct planting procedures and the addition of organic matter to the soil is a major step in the prevention program.
- A gardener must maintain optimum soil moisture and keep enough nutrients in the soil for plants to use. Vigorous plants are able to resist many infections.
- Plants must have the correct amount of light. Deprived of adequate light the plant may become weak, spindly and not a vigorous grower.
- To grow at all a plant must have the correct soil temperature.
- Prevent mechanical injury from gardening equipment. Abrasions allow the entry of harmful pathogens. Lawn mowers and string trimmers are the chief culprits.
- Accidentally spraying with improper pesticides. Herbicidal contact with green tissues may not show immediate harm, but with time, hormonal disturbances in the plant can affect the growth and the production of a crop.
- Keep the garden free of debris that might harbor disease organisms.
Points to remember
- There are three main pathogens that cause plant diseases: bacteria, fungi and viruses. The majority of diseases affecting gardeners are the fungal diseases.
- Often the first sign of infection is seen on the leaves. These can be spots, blotches, distortions, wrinkles, streaks or other abnormalities. Usually the disfiguring is caused by a fungus.
- Plants have defense mechanisms to counteract disease attacks. If the plant is under stress for any reason, its defense mechanisms can be overcome. Keep the garden in optimum growing conditions to keep the plants growing vigorously.
- There are natural predators to aid in the control of disease. These help the gardener avoid problems.
- Most fungal diseases need moisture on the leaves to germinate, grow and penetrate the tissues.
- Many fungicides act as protectorants, coating the leaves to prevent infections in addition to controlling fungal diseases.
- Some diseases are difficult to diagnose, requiring a hand lens or a lab analysis. Fortunately, most common ones are easily recognized and do not need special diagnostic tools.
- Keep the garden clean of debris and weeds.
- Spray only as needed, using the right pesticide for the identified disease.