Damage caused by plant pests
Plant Disease ID
Plant diseases are important to humans, because they
cause damage to plants and plant products.
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 them.
◦ Some plant diseases are easily controlled by one or another
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.
◦ 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)
◦ Root rot
Plant pathogens can cause various symptoms to appear on affected
The organisms that cause these symptoms include fungi,
bacteria, nematodes and viruses.
Fungi are microscopic plants that lack chlorophyll and
◦ 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
Bacteria are microscopic, single celled organisms.‑
◦ They cause diseases such as galls, leaf spots, soft rots, scabs and
◦ Bacteria are a significant cause of plant disease, because they can
multiply very rapidly when proper environmental conditions are
◦ Bacteria are as damaging to plant propagation operations as are
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 chlorophyll).‑‑
◦ Virus can multiply only in living cells.
◦ Nematodes are very small round worms belonging to the animal
◦ 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
◦ However, the ability to collect samples, observe symptoms and
record observations will be an excellent experience for a
Types of pathogens
Controls for Bacterial
Examples of Bacterial
Examples of Fungi
Other methods of
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
◦ They sometimes reduce the water absorbing capacity of the‑
◦ They suppress the translocation of photosynthates away from
◦ They sometimes promote wasteful use of the products of
photosynthesis as in the formation of galls.
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,
◦ 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.
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‑
◦ Anything that moves and comes in contact with bacteria may
spread them to other areas.
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
◦ Bacteria are significant as pathogens because of their ability to
They divide by binary fission. (One bacterium divides in half and
becomes two bacteria.)
Under proper environmental conditions this division takes place
every twenty minutes.
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 include the use of
antibiotics, Bordeaux mixtures, and fixed coppers.
Plant breeding has produced many plant varieties that
are resistant to bacterial infection
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
◦ 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
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
Dissemination of fungi can be by wind, rain, insects, irrigation or
flooding, contaminated seed, infected plants, animals, tillage
equipment and pruning shears and knives.
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
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 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
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 cell.
These particles move from one plant to another by vectors
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).
Vein clearing (veins are chlorotic).
These symptoms may be accompanied by other symptoms elsewhere
on the same plant.
The best way to control a virus is to keep it out of an area through
quarantine, inspection, and certification programs.
◦ Other methods of control include:
Use virus free tubers, budwood, scionwood and other propagating
Breeding resistant varieties.
Eradication of diseased plants.
Controlling insect vectors is rarely effective.
◦ Removal of weeds which serve as virus hosts.
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 pathology.
Nematodes are small round worms (1/64" to 1/8").
They are, in general, eel shaped, round in cross section, with smooth‑
They have no legs or other appendages.
Plant parasitic nematodes characteristically have a stylet (spear) as a
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 specific.
Reduced growth, nutrient deficient leaves (yellow), excess wilting are
Reduced yields and poor quality products are the results.
◦ Below ground symptoms are more distinctive; these include the
Galls produced on roots and tubers.
Generally a lack of feeder roots.
Injured root tips.
Excessive root branching (sprangling).
◦ Control measures are very seldom complete, usually only
◦ Controls that are successfully used include:
The use of fumigants.
Use of resistant varieties.
Crop rotation to non host plants.
◦ At present all chemicals must be used pre plant; none that are‑
available for use today can be used after the crop is planted
Most commonly found nematodes include Root Knot, Root Lesion,‑
Cyst, and Burrowing Nematode:
Cyst Nematodes Heterodera Small grains, corn, carrot,
crucifers, soybean, pea, hop,
wheat, irish potato, tomato,
Pratylenchus 1700 hosts including: forage, small
grains, grasses, fruit, vegetables,
field crops, nursery, weeds
Pratylenchus Potato, Corn, peanut, cotton,
pineapple, avocado, tobacco, olive,
apple, wheat, oats, strawberry,
tomato, lily, walnut, grape, fig,
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
In general, plants receiving a balanced nutrition, are more capable
of protecting themselves than plants with either excessive or
deficient amounts of nutrients