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PLANT PATHOLOGY |
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KEY CONCEPTS
I. Definition of Plant Pathology
II. Plant Pathogens
A. Fungi1. Powdery mildews
III. Diagnosis of Plant Diseases
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PLANT PATHOGENS
[illus. of various plant pathogens entering or in plant tissue]
To help you develop a "disease vocabulary," here are some of the common pathogen groups. Pathogens are grouped taxonomically into related groups. It is helpful to know the type of pathogen in order to prescribe a control. Specific diseases are discussed in the chapters dealing with the plant. Plant disease names are often very colorful and descriptive, as in fireblight, black knot and clubroot. Many disease names indicate something about the appearance of the symptoms.
[illustration of infected plant showing spotting, p. 116? redrawn from Rx for Your Vegetable Garden]
Fungi (plural of fungus) are the largest group of plant pathogens. Fungi often develop into colonies, like bread mold. Some fungi develop into large structures, such as mushrooms. Fungi spread by spores carried by wind, water or animals. They enter host plants directly or through natural openings or wounds. They cause damage by producing substances that change or destroy plant tissues.
Common fungal diseases include powdery mildew, rust, leaf spot, blight, root and crown rots, damping-off, smut, anthracnose, and vascular wilts.
These fungal diseases are very HOST-SPECIFIC. It is common to see powdery mildew on lilacs, roses, zinnias, melons, beans, cucumbers, turfgrass and many other plants. The symptom is a white or gray, powdery growth on leaves and stems. Powdery mildew will not usually kill a plant; however, it may weaken plants, and it is unsightly. Providing adequate sunlight and air circulation to lower relative humidity and using fungicide sprays for high-profile plants help control powdery mildew.
Rusts are also host-specific. They produce masses of easily noticed, orange or dark red spore masses on leaf tissue. Rusts are commonly seen in our region on turfgrass, hollyhocks, snapdragons, hawthorns and the small fruits. Rusts usually develop during cool weather. They are spread by wind and splashing water. Rusts require water to reproduce and infect host plants. Controls include watering early in the day and applying protective fungicidal sprays.
Rose growers are very familiar with black spot, a common leaf-spotting disease of roses. Scab on crabapple and pyracantha are leaf-spotting diseases that can cause defoliation. Botrytis flower blight occurs on annuals and some perennials during wet seasons. Anthracnose of shade trees is a blight frequently seen in Ohio on sycamores, maples, ashes and oaks. Fungi-causing tree diseases are spread by wind and splashing water. Grow resistant cultivars, maintain plant vigor, enhance leaf drying by proper pruning and water early in the day to help avoid leaf spotting and blights. Fungicides provide protection from infection when applied before infection by fungal spores.
These diseases, which include water mold diseases, attack a wide variety of plants. They cause damage to roots and crowns of plants. Damage to some of the root system results in poor growth, yellowing or stunting of the plant. Root rot fungal pathogens are found in almost all soils. However, they do not survive as well in well-drained soils. These organisms can live in soil for years in a dormant state. Proper cultural practices, such as correct planting depth and improving drainage, are important controls.
Cankers may be compared to sores on a stem or twig. The tissue may be raised or sunken, discolored or split open. Twig cankers on junipers and leaf and stem blight of pachysandra are common examples. Most of the canker diseases are not controllable with fungicides. Maintaining plant vigor, protecting from injury, removing diseased parts and growing plants suitable for the site help prevent canker problems.
Dutch elm disease is the classic wilt disease. The fungus invades the vascular system of the plant and clogs it, stopping the flow of water and nutrients. Verticillium wilt is common on trees, shrubs, vegetables and fruits. Discoloration of the vascular tissue sometimes can be seen when you cut diagonally through a branch. There are no effective chemical controls for wilts. Growing resistant species, maintaining plant vigor and sanitation provide the only controls.
Smut fungi produce large amounts of black sooty spores which show up as unsightly dark brown or black blotches on leaves, stems and fruit. Corn smut is the common example in Ohio. Resistant cultivars and protective sprays provide control.
Bacteria are microscopic, single-celled organisms that multiply rapidly. They produce chemicals that destroy plant tissue or cause abnormal growth. Bacteria enter through wounds, stomata or other natural openings. Bacteria can cause leaf spotting as on English ivy. Fireblight on crabapples, pears, mountain ash, hawthorn, cotoneaster and pyracantha is a bacterial disease. Bacteria also cause diseases, such as crown gall on stone fruits and euonymus and wilts in geraniums and cucurbits (cucumbers, squash and melons).
Disease-free stock and resistant cultivars plus sanitation help prevent bacterial diseases. When cutting out bacterial infections, sterilize pruners with 70% alcohol between cuts. Streptomycin and fixed copper sprays may help slow the spread and development of bacterial diseases. However, chemical controls for bacterial diseases are less reliable than for many fungal diseases.
Mycoplasmas are disease agents that were not accurately identified until the 1960's. Previously many mycoplasma-caused diseases were thought to be caused by viruses. A group of diseases caused by mycoplasmas is called YELLOWS diseases. Yellows disease is usually spread from one host plant to another by an insect vector. Leafhoppers are common vectors for mycoplasmas.
Viruses are organisms so small that they can only be seen under an electron microscope, magnified 2,000 to 3,000 times. Viruses multiply only within living cells of host plants. Viruses are spread by insects, nematodes and humans. Symptoms include vein banding, mosaic, flecking or spotting on foliage and abnormal growth, similar to herbicide damage. Tobacco mosaic virus on tomatoes and leaf mosaic on dahlias are common viral diseases. Control is limited to removing and destroying infected plants.
Nematodes are microscopic, worm-like organisms. They most commonly feed on plant roots, but some nematodes invade leaf tissue. Nematodes suck out liquid nutrients and inject damaging materials into plants. They injure plant cells or change normal plant growth processes. Symptoms of nematodes include swelling of stems or roots, irregular branching, deformed leaves, lack of blossoming and galls on roots. Nematodes can facilitate the entry of viruses and fungi into plants.
Control efforts are difficult and provide limited success. Chemical controls must be done by professional pesticide applicators. A specialized soil test for nematodes indicates the type of nematode. This test is available through The Ohio State University Plant and Pest Diagnostic Clinic.
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