history, loss of crop yields from disease have had
severe effects on the human race.
- The Irish potato
famine in the 1800's killed 1.5 million people,
one fourth of Ireland's total population. The
crop was lost to late blight of potato, a fungal
- In the mid 19th
century downy mildew grape disease was
accidentally introduced in Europe. Downy mildew
of grape almost destroyed the vineyards in
France, Germany and Italy.
- Ergot of rye
grain causes hallucinations and sometimes deadly
illness. Massive European epidemics caused by
ergot of rye occurred frequently during medieval
- In the early
20th century the American Chestnut tree was
wiped out by an Asian blight disease. The
chestnut was a major source of lumber at the
are at least 50,000 diseases of crop plants. New
diseases are discovered every year. About 15% of the
total U.S. crop production is lost annually to
infectious diseases despite improved cultivars and
disease control techniques. Damage from disease has
not been eliminated. Disease-causing organisms
(pathogens) multiply and mutate rapidly. They
develop genetic resistance to chemical controls and
have the ability to infect new hybrids. Good
gardening practices and an understanding of plant
pathology are the first line of defense against
In this chapter you
will learn that plant health is affected by disease
and environmental factors.
DEFINITION OF PLANT PATHOLOGY
study of plant diseases is known as plant pathology.
Infectious diseases are caused by living organisms
called pathogens. Noninfectious diseases caused by
environmental stress and damage by weather and other
environmental factors also will be covered.
Indirectly, environmental factors that cause a plant
to be stressed may result in the plant's gradual
decline. Decline results in the plant being more
susceptible to disease organisms. Because of this,
diagnosing plant diseases can be tricky. The real
cause of a problem may be the stress factors, with
the disease simply being a secondary factor.
critical factors or conditions must exist for
disease to occur: a SUSCEPTIBLE HOST PLANT, a
PATHOGEN, and the right mix of ENVIRONMENTAL
CONDITIONS. The relationship of these factors is
called the disease triangle.
only a part of the triangle exists, disease will not
occur. Understanding the disease triangle helps us
understand why most plants are not affected by the
many thousands of diseases that exist.
Pathogens are microorganisms that cause disease.
Because they are living, they are called BIOTIC
(bye-AH-tick) agents or causes. Pathogens can be
FUNGI (FUN-geye), BACTERIA, VIRUSES, MYCOPLASMAS
(MY-crow-plas-mahs) or NEMATODES (KNEE-ma-toads).
Each has a different life cycle, which includes an
pathogens are host-specific to a particular plant
species, genus or family. For instance, blackspot of
rose will not attack marigolds or lettuce. Some
diseases, such as the powdery mildews, produce
similar SYMPTOMS on different plants. However, the
fungi involved are usually host-specific. The rose
powdery mildew fungus will not infect zinnias or
turfgrass or vice-versa.
susceptible host has a genetic makeup that permits
the development of a particular disease. The genetic
defense against a disease is called disease
resistance. This resistance can be physical
characteristics of the plant (fuzzy or waxy leaf
surfaces), chemical characteristics (enzymes that
kill pathogens and lack of enzymes) and growth
patterns (ability to block off diseased tissue or
Plants also may be disease-tolerant. Even though
infected with a disease, they can grow and produce a
good crop or maintain an acceptable appearance. The
plant outgrows the disease and symptoms are not
apparent or at a damaging level.
important to remember that plants labeled as
disease-resistant are resistant only to a particular
disease. They are not resistant to all diseases.
Resistance does not mean immunity. Under extreme
circumstances, resistant plants may be infected by
the disease to which they have resistance.
disease to occur, the host plant must be at a stage
of development that allows it to be susceptible to
infection. For example, damping-off only affects
seedlings. Botrytis is primarily a disease of buds,
although it also can occur on flowers and leaves.
Also, it is important that the pathogen be in a
proper stage of its development to infect host
Certain environmental conditions must exist for
disease pathogens to cause infection. The specific
conditions vary for different pathogens. High
moisture and specific temperature ranges, for
example, are necessary for many fungal diseases.
These conditions must continue for a critical period
of time while the pathogen is in contact with the
host for infection to occur.
Moisture, temperature, wind, sunlight, nutrition and
soil quality affect plant growth. If one of these
factors is out of balance for the culture of a
specific plant, that plant may have a greater
tendency to become diseased. For example, lilacs
growing in shade are more likely to be infected with
powdery mildew than those growing in full sunlight.
Environmental conditions also affect the growth and
spread of disease pathogens. Very dry or wet weather
will have an accompanying set of diseases that
thrive under these conditions.
Moisture in the plant environment can include
humidity, dew, rainfall or water from irrigation.
Moisture is critical to the spread of most plant
diseases. Familiar diseases, such as black spot,
fireblight and apple scab require moisture to spread
to and infect new host plants.
Constantly wet foliage from overhead watering is a
condition that promotes disease development.
Seedlings grown indoors in soggy, unsterilized
potting medium and pots are more prone to
damping-off, a fungal disease.
disease pathogen has a specific temperature range
for growth and activity. There are warm-weather and
cool-weather diseases. Many powdery mildew diseases
are late summer, warmer temperature diseases.
Temperature affects how rapidly pathogens multiply.
temperature can also be critical for disease
infection. Cool, wet soils promote fungal root
diseases. Temperature extremes can cause stress in
host plants, increasing susceptibility.
WIND AND SUN
combination of wind and sun affects how quickly
plant surfaces dry. Faster drying generally reduces
the opportunity for infection. Wind can spread
pathogens from one area to another, even many miles.
Wind and rain together can be a deadly combination.
Windblown rain can spread spores from infected plant
tissue, blowing these pathogens to new host plants.
Sunlight is very important to plant health. Plants
that do not receive the right amount of sunlight to
meet their cultural requirements become stressed.
This may make them more susceptible to infection.
SOIL AND FERTILITY
type can affect plant growth and also development of
some pathogens. Light sandy soil low in organic
matter favors growth of many types of nematodes.
Damping-off disease increases in heavy, cold,
water-logged soils. Soil pH affects pathogen
development in some diseases. Clubroot of cabbage
occurs in soils with a low pH, for example. High
soil pH is a factor in the development of scab on
Fertility affects a plant's growth rate and ability
to defend against disease. Excessive nitrogen
fertilization can increase susceptibility to
pathogen attack. It causes formation of SUCCULENT
tissue and delays maturity. This can contribute to
certain patch diseases in lawns. Nitrogen deficiency
results in limited growth and plant stress which may
cause greater disease susceptibility.