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                                          Note Taking Guide
                                           Topic 2043

                                           Plant Pathology
                                           By Katie Wagar

•   •      Introduction
•   •      Damage caused by plant pests
•   •      Plant Disease ID
•   •      Plant Pathology
•   •      Nematodes

Plant Pathology Introduction
••    Plant diseases are important to humans, because they cause damage to plants and plant
••       The yield and quality of plants are reduced by a wide array of plant diseases
        – – A plant disease is the complex of symptoms caused by a pathogen on a plant.
••    A plant pathologist is a person who studies plant diseases and works to diagnose and control
     – – Some plant diseases are easily controlled by one or another method.
••    For other plant diseases, however the cost of control is as high or higher than the expected
    value of the crop.


••    Plant diseases can be caused by fungi, bacteria, viruses, nematodes, insects and several other
    organisms. These are called pathogens. Various organisms can transport (vector) pathogens
    from infected to healthy plants.
     – – As agriculturalists, we are concerned with these organisms because of the damage they
       do to crops.

Damage caused by plant pests (plant pathogens)
   – – Dwarfing of growth
   – – Yellowing of foliage
   – – Leaf spotting
   – – Blasting of grain heads
   – – Stem cankers
   – – Fruit rot
   – – Seed decay
   – – Damping off (destruction of seedlings near the soil line)
     –   –   Wilt
     –   –   Defoliation
     –   –   Root rot
     –   –   Galls

••    The organisms that cause these symptoms include fungi, bacteria, nematodes and viruses.

••    Fungi are microscopic plants that lack chlorophyll and conductive tissues.
     – – Fungi produce diseases like stem rust, corn smut, powdery mildews, brown rot and
       damping off.
     – – Fungi reproduce mainly by means of spores.
     – – Fungi are particularly damaging to plant propagation operations.
••    Bacteria are microscopic, single-celled organisms.
     – – They cause diseases such as galls, leaf spots, soft rots, scabs and systemic disorders.
     – – Bacteria are a significant cause of plant disease, because they can multiply very rapidly
       when proper environmental conditions are present.
     – – Bacteria are as damaging to plant propagation operations as are fungi.
••    Viruses are pathogenic particles that infect most higher plants and animals.
     – – In plants they cause such symptoms as stunting, leaves with yellow mosaic patterns,
       flower break and vein clearing (veins are chlorotic--i.e., lack green color; without
     – – Virus can multiply only in living cells.
     – – Nematodes are very small round worms belonging to the animal phylum Nemata.

Plant Disease Identification
    – – Beginning students may not be able to accurately identify the exact pathogen causing
      the plant symptoms.
    – – Much experience is needed to become an expert plant pathologist.
    – – However, the ability to collect samples, observe symptoms and record observations will
      be an excellent experience for a beginning student.


Plant Pathology
•   •      Affect plants
•   •      Are Parasitic
•   •      Types of pathogens
•   •      Controls for Bacterial infections
•   •      Plant Breeding
•   •      Examples of Bacterial plant diseases
•   •      Fungi
•   •      Examples of Fungi
•   •      Viruses
•   •      Other methods of control
•   •      Chemcial substances

Affect Plants
••       There are many ways in which plant disease pathogens can affect plants
        – – They can suppress the chlorophyll content.
        – – They can reduce the leaf area.
        – – They can curb the movement of solutes and water through the stems.
        – – They sometimes reduce the water-absorbing capacity of the roots.
        – – They suppress the translocation of photosynthates away from the leaves.
        – – They sometimes promote wasteful use of the products of photosynthesis as in the
          formation of galls.

Are Parasitic
••       Most pathogens are parasitic - they invade the host and obtain food from it.
        – – Many are submicroscopic, making identification difficult.
        – – By definition, plant pathogens are capable of spreading from one host to another.
        – – The most important plant pathogens are bacteria, fungi, viruses, and nematodes.
        – – Most of the plant pathogens have a saprophytic (living in dead or decaying organic
          matter) existence apart from their host plants during most of the year.

Types of Pathogen
••       Bacteria are small, single celled, microscopic organisms.
        – – Of the 1600 known bacterial species about 200 have been found to cause plant disease.
        – – Most plant infecting bacteria are rod-shaped and most have thread-like structures
          (Flagella) that propel them through liquids.
        – – Anything that moves and comes in contact with bacteria may spread them to other
           • • This includes farm equipment, rain, plant material, seeds, birds, insects, nematodes,
              and people.
           • • To start an infection in a plant they must enter a natural opening or a wound..
        – – Bacteria are significant as pathogens because of their ability to multiply rapidly.
           • • They divide by binary fission. (One bacterium divides in half and becomes two
          • • Under proper environmental conditions this division takes place every twenty
          • • At this rate 1 bacterium can give rise to 17 million cells within 12 hours if food,
            moisture, and temperature are favorable.
     –   – Bacterial diseases can be seen as galls, leaf spots, soft rots, scabs and systemic disorders

Controls for Bacterial Infections
••   Controls for bacterial infections include the use of antibiotics, Bordeaux mixtures, and fixed

Plant Breeding
••    Plant breeding has produced many plant varieties that are resistant to bacterial infection

Examples of Bacterial Plant Diseases
••   Examples of bacterial plant diseases are crown gall, fireblight, walnut blight, deep phloem
   canker of walnuts, soft rots of vegetables, and bacterial wilt of cucumbers.

••    Fungi are small, usually microscopic, plants that lack chlorophyll and conductive tissues.
     – – Unlike green plants, they do not photosynthesize their own food, so they depend on
       living or dead plant or animal tissue.
     – – Only about 8,000 fungi species are known to cause plant diseases of the 100,000
       species on earth.
     – – All plants can be attacked by some type of fungi.
     – – Each of the parasitic fungi can attack one or many kinds of plants.
     – – Some fungi grow and multiply by living on their host plant during their entire life.
     – – Other fungi can multiply on dead organic matter as well as living on plants.
     – – Fungi reproduce mainly by means of spores.
         • • These spores are special reproductive bodies made up of one or a few cells.
         • • The spores perform the same job as seeds in higher plants.
         • • Some fungi produce up to 5 types of spores to complete a single life cycle.
         • • Dissemination of fungi can be by wind, rain, insects, irrigation or flooding,
           contaminated seed, infected plants, animals, tillage equipment and pruning shears and
         • • A few have motile spores and others can grow to neighboring plants by their hyphae
           (thread-like strands).
         • • The vegetative body (Mycelium) of fungi is made up of very small filaments or
           threads called hyphae.
             – – These branch and grow in all directions through their food supply.
             – – They absorb food from the cells of their host plants.
         • • Fungi are controlled by several methods.
             – – Chemical sprays (fungicides) have been used with success for many years.
             – – Soil pasteurization and the use of fumigants works well in some instances.
             – – The development of resistant species and cultivars continues to show success
               and more promise.
            – – Crop rotation, good soil drainage, proper handling of the crop, and low
              temperature storage are all helpful in controlling various fungi.

Examples of Fungi Diseases
••   Examples of fungus diseases are stem rust of wheat, corn smut, powdery mildews, rusts,
   brown rot, damping off, and Dutch Elm disease.

    – – Viruses are pathogenic particles that infect most higher plants and animals.
        • • Virus particles are extremely small (20 to 250 millimicrons).
        • • Viruses are not cells, nor do they consist of cells.
        • • They can be seen only with an electron microscope.
        • • They cannot grow or multiply except when they are within a host cell or insect vector
        • • These particles move from one plant to another by vectors (carriers).
            – – Also virus is spread to new plants when any asexual method is used for
               propagation (once virus is in the mother plant, all plants started from it will likely
               have virus).
            – – Most commonly aphids, leafhoppers, thrips, or mites are the vectors.
        • • Viruses can often be identified by their symptoms:
            – – Mosaic patterns on leaves.
            – – Yellows (leaves lacking chlorophyll).
            – – Stunting.
            – – Ringspots.
            – – Flower break.
            – – Vein clearing (veins are chlorotic).
        • • These symptoms may be accompanied by other symptoms elsewhere on the same
        • • The best way to control a virus is to keep it out of an area through quarantine,
          inspection, and certification programs.

Other Methods
   – – Other methods of control include:
           – – Use virus free tubers, budwood, scionwood and other propagating materials.
           – – Breeding resistant varieties.
           – – Eradication of diseased plants.
           – – Controlling insect vectors is rarely effective.
   – – Removal of weeds which serve as virus hosts.
Chemical Substances
••    As yet, no chemical substances (viricides) are available for controlling virus diseases.
     – – Examples of virus diseases are Tobacco mosaic, Curly Top of Sugar Beets, Barley
       Yellow Dwarf, Necrotic Ring Spot of Stone Fruit, Tristeza Disease of Citrus, and
       Blackline of Walnuts.
     – – Nematodes are plant parasites belonging to the animal kingdom that are studied in plant
        • • Nematodes are small round worms (1/64" to 1/8").
        • • They are, in general, eel-shaped, round in cross section, with smooth unsegmented
        • • They have no legs or other appendages.
        • • Plant parasitic nematodes characteristically have a stylet (spear) as a feeding
        • • Their method of feeding is to puncture the cell with their stylet, secrete fluids (saliva)
          into the cell, then withdraw the cell contents.
   –   – They are spread by any way that soil is moved (e.g., farm equipment, water, animals,
       wind, nursery plants). Major damage to plants occurs from nematodes feeding on roots.
   –   – The above ground symptoms shown by a plant infected with nematodes is not very
        • • Reduced growth, nutrient deficient leaves (yellow), excess wilting are symptoms.
        • • Reduced yields and poor quality products are the results.
   –   – Below ground symptoms are more distinctive; these include the following:
        • • Galls produced on roots and tubers.
        • • Generally a lack of feeder roots.
        • • Root lesions.
        • • Injured root tips.
        • • Excessive root branching (sprangling).
   –   – Control measures are very seldom complete, usually only reducing populations.
   –   – Controls that are successfully used include:
        • • The use of fumigants.
        • • Use of resistant varieties.
        • • Crop rotation to non host plants.
        • • Summer fallow.
   –   – At present all chemicals must be used pre-plant; none that are available for use today
       can be used after the crop is planted (post-plant).
        • • Most commonly found nematodes include Root-Knot, Root Lesion, Cyst, and
          Burrowing Nematode:



   COMMON NAME                    SCIENTIFIC NAME                        PRINCIPAL HOST
Cyst Nematodes                  Heteroda                         Small grains, corn, carrot,
                                                                 crucifers, soybean, pea, hop, wheat,
                                                                 Irish potato, tomato, eggplant,
Root Knot Nematodes             Meloidogyne                      1700 hosts including: forage, small
                                                                 grains, grasses, fruit, vegetables,
                                                                 field crops, nursery, weeds
Root Lession Nematodes          Pratylenchus                     Potato, corn, peanut, cotton,
                                                                 pineapple, avocado, tobacco, olive,
                                                                 apple, wheat, oats, strawberry,
                                                                 tomato, lily, walnut, grape, fig,
                                                                 citrus, rose
Burrowing Nematode             Radopholus                      Citrus, ornamentals, avocados

   – – Nutrition affects the rate of growth and the state of readiness of plants to defend
     themselves against attack by pathogens.
       • • High nitrogen fertilization increases the susceptibility of some plants to bacterial and
         fungal diseases.
       • • Proper nutrition of phosphorous, potassium, calcium, and micronutrients have been
         shown to aid a plants resistance to certain diseases.
       • • In general, plants receiving a balanced nutrition, are more capable of protecting
         themselves than plants with either excessive or deficient amounts of nutrients


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