Plant pathology disease triangle and disease cycle

1. Disease triangle
a triangle with the three essential factors (susceptible host, favorable environment for disease,
and pathogen) at the vertices.
OR
Definition:
The disease triangle illustrating the phenomenon of plant disease as the interior space of the
disease triangle is a conceptual model that shows the interactions between environment, the
host and infectious agent. This model can be used to predict epidemiological outcomes in
plant health and public health, both in local and global communities.
Factors affecting disease triangle:
There are three factors in triangle are interacting with b each other anyone from these factors
is missing the triangle is not complete. The occurrence of these parameters is necessary for
triangle.
Simply if there is no viable pathogen, or no susceptible host plant, or the environmental
conditions are not favorable.
• Host (crop and cultivar etc.)
• Pathogen (bacteria, fungi, virus and nematodes etc.)
• Environment ( temperature, humidity, pH and and watering etc.)
➢ Disease triangle
1. The host:
In our production system, regardless of planted crop (whether corn, cotton, grain sorghum,
rice, soybean, wheat) more often than not, but not in every field situation, a susceptible
cultivar is planted that does not contain resistance genes for a particular disease. Excluding
rusts (common rust of corn, southern rust of corn, soybean rust of soybean, peanut rust) in
this particular section since they are having to blow into the MS production system from
somewhere to our south and ALL corn and soybean varieties are susceptible to rusts. Wheat
rusts are similar in that they have to blow into MS; however, tolerant varieties
exist. However, for some foliar diseases, specifically in corn and soybean, there are plenty of
varieties available with some level of tolerance to a particular pathogen/disease (e.g., frogeye
leaf spot).

2. 2. The pathogen:
The pathogen is simply any particular organism (bacterial, fungal, or viral), but in this
particular situation I will continue to refer to the fungal organisms, that can cause a disease in
a particular crop plant. In some cases, the pathogen is present at a particular location
especially if a continuous crop has been grown. Most fungal organisms can overwinter in a
field between seasons on senesced plant material or even in the soil profile. The fact that
fungi can overwinter within a particular field is one major reason that rotation as well as
planting a resistant variety is such an important consideration. Again, in this particular
situation, rusts are the exception to this particular rule.
3. The environment:
The environment is probably the single most delimiting factor for the development of a
plant disease situation. Hence the side of the disease triangle for the environmental factor
being skewed. Without a conducive environment, a plant disease will not occur. In most
cases, a conducive environment involves high humidity, or free moisture• on the leaf surface
in the form of dew as well as moderate temperatures. Most fungi do not like the temperature
extremes. However, the environment must remain conducive for a specific amount of time
for disease to occur. Time is the key component lacking in the three-dimensional disease
triangle and the reason for the additional fourth component since a prolonged period of
exposure between the host and the pathogen in a conducive environment will be required
prior to disease incidence and symptom expression.
Mostly disease triangle is three dimensional some times we added time factor as a fourth
factor.
4. Time
Disease does not occur instantaneously, even in situations where a conducive environment
remains present for an extended period of time. Time is required for the pathogen to infect
the plant and produce characteristic lesions as well as produce sporulation (the asexual phase
of reproduction). A prolonged period of time can also elapse between infection of the plant
and symptom expression. Specific diseases where this likely tends to occur include soybean
rust.
In addition, in certain disease situations an extended
period of time may be required for infection, disease
development, and continued sporulation to occur.
As I said above, disease incidence is not instantaneous.
Several days may be required to go through several stages
of reproduction.

3. Disease cycle in plants
Definition:
The disease cycle is a chain of interconnected successive events of a pathogen’s infection in a
host plant. It usually coincides with the life cycle of the pathogen with a correlation to its host
and the environment.
• Each cycle includes two alternating phases; the parasitic phase and the survival or over-
summering or over-wintering phase.
The distinct events in a disease cycle are very much important as they provide us information
about how and when we should stop the spread of the disease easily. Besides, the prediction
about other disease cycles by other unknown or less known pathogens can be formulated using
the info of one of such cycle.
Steps in a disease cycle
• Inoculation
• Penetration
• Infection
• Invasion
• Colonization
• Dissemination
• Over-wintering and over-summering
Inoculation
Inoculation is the first contact of a pathogen with its host in a place where infection is
possible. The pathogen
Types of inoculum:
• Primary inoculum: An inoculum that survives by remaining dormant in the winter or summer
and causes the original infections in the spring or in the autumn respectively is called a
primary inoculum and the infections it causes are called primary infections.
• Secondary inoculum: An inoculum produced from primary infections is called a secondary
inoculum and it, in turn, causes secondary infection.
may be in any of its stages of its life cycle when being inoculated.
Most pathogens rely on rain, wind, insects or human to carry them to their host plants.
Penetration
Pathogens penetrate plant surfaces by direct penetration of cell walls, through natural openings,
or thorough wounds. It is to remember that, penetration doesn’t necessarily cause infection.
There are many cells that can be penetrated but yet not susceptible to infection. In this case,
the organism cannot proceed beyond penetration and die without producing disease.
Bacterial penetration

4. • Less frequently Enter mostly through wounds.
• through natural openings.
• Never directly through unbroken walls.
Viruses, viroids, mollicutes, fastidious bacteria, and protozoa
• Enter through wounds made by vectors or tools and other means.
Parasitic higher plants
• Enter their hosts by direct penetration.
Infection
When the inoculum starts procuring nutrients from the susceptible tissues of the host, the
infection starts. This is the first stage from where the disease starts to develop.
• As the pathogen d the time interval between inoculation and the appearance of disease
symptoms
is called the incubation period.
Invasion
Various pathogens invade hosts in different ways and to different extents. Some fungi, such as
those causing apple scab and black spot of rose, produce mycelium that grows only in the area
between the cuticle and the epidermis (subcuticular colonization). Others, such as those causing
powdery mildews, produce mycelium only on the surface of the plant but send haustoria into
the epidermal cells.
Most fungi spread into all the tissues of the plant organs (leaves, stems, and roots) they infect,
either by growing directly through the cells as an intracellular mycelium or by growing between
the cells as an intercellular mycelium.
Fungi that cause vascular wilts invade the xylem vessels of plants.
Colonization
Colonization starts when disease or the pathogen starts spreading in the host. Most of these
pathogens, whether inducing a small lesion, a large infected area, or a general necrosis of the
plant, continue to grow and branch out within the infected host indefinitely so that the same
pathogen individual spreads into more and more plant tissues until the spread of the infection
is stopped or the plant is dead. In some fungal infections, however while younger hayphae
continue to grow in new healthy tissues, the older parts of the hyphae die out. Thus the disease
spread exponentially as more fragments of the hypahe are in action.
Fungi causing vascular wilts, in turn, get benefit from the xylem vessel. They disperse their
spores and hyphal fragments throught the water conducting tissues and attack new susceptible
parts of the plant.
Dissemination
Some pathogens which have motile stages such as nematodes, oomycetes, zoosporic fungi, and
bacteria can easily disseminate very short distances on their own power. Fungal hyphae can

5. grow between tissues in contact and sometimes through the soil toward nearby roots for a few
to may centimeters. Both of these means of dissemination are quite limited. However, there are
some fungi which can expel their spores up to a few centimeter in the wind above their
sporophores.
Dissemination takes place by many ways like by air, water, insect and by humans.
Over-seasoning
Over-seasoning is an evolutionary developed technique followed by many pathogens to survive
the adverse period of their life cycle. It has been developed in the pathogens so that they can
complete their life cycle and can spread their infections in the next year. The pathogens may
survive this harsh period either as spores, sclerotia like hard over-seasoning structures or as a
whole. Pathogens may infect both perennial and annual plants. The mechanisms for over-
seasoning for pathogens actually differ in these two types of plants.
Annual plants die at the end of the year and then the pathogens of the annual plants become
hostless. In the hot humid climate, the annual plants die in the summer and over-summering of
pathogens occurs. On the other hand, in the areas with harsh winter, the plants die at cold and
over-wintering of pathogens takes place.
• The fungal pathogens usually survive this period by remaining as mycelia or spores in the
fallen debris or as sclerotia (a specialised hard mycelial structure).
• The resting spores and sclerotia may survive even in the soil without debris as they are hard
structures programmed to survive such conditions by accumulating foods inside them.
Perennial plants though live for many years may cause in the death of certain parts (such as
stems, leaves, buds, flowers, fruits etc.) at the end of the year. The death of whole plant or part
of it is usually occurred during the hot, dry climate of summer or cold condition of winter.
• Pathogens here may survive in the tissues of living plants in the over-seasoning period.
• They may also live in the debris formed by the death of certain parts of perennial plants.
• The deciduous trees shade their leaves and fruits in winter. Fungi affecting these portions
usually overwinter as mycelium or spore on fallen leaves or fruits or on the bud scales.