CHAPTER 4

DISEASES IN PLANTS AND ANIMALS

Learning Outcomes
It is expected that students will be able to
 know the types of plant diseases and the factors for causing diseases
 study the signs and symptoms of plant diseases and types of pathogens
 increase knowledge about how to control diseases in plants
 understand the different types of non-infectious and infectious diseases in animals
 learn the cardiovascular disease as an example of non-infectious diseases
 know the non-modificable risk factors and modificable risk factors for cardiovascular
disease
 learn the knowledge about prevention and treatment of cardiovascular disease
 study the malaria disease as an example of infectious diseases
 gain the knowledge about transmission of malaria through observing life stages of
Plasmodium
 understand how to treat and prevent malaria
 learn the history of worldwide control of malaria

4.1 PLANT DISEASES

A plant disease is usually defined as abnormal growth and/or dysfunction of a plant.
Diseases are the result of some disturbance in the normal life process of the plant. Diseases may
be the result of living and/or non-living causes. Biotic diseases are caused by living organisms (e.g.,
fungi, bacteria, and viruses). Abiotic diseases are caused by non-living environmental conditions
(e.g., soil compaction, wind, frost, soil salt damage, and girdling roots).
Plant disease is any abnormal condition that alters the appearance or function of a plant. It
is a physiological process that affects some or all plant functions. Disease may also reduce yield
and quality of harvested product. Most plant diseases, around 85 percent are caused by fungal or
fungal-like organisms. However, other serious diseases of food and feed crops are caused by viral
and bacterial organisms. Certain nematodes also cause plant diseases. Nematodes are very tiny
roundworms. They are plant parasites and help distribute bacteria and fungi through the soil as
well as along roots. Some plant diseases are classified as "abiotic," or diseases that are non-
infectious and include damage from air pollution, nutritional deficiencies or toxicities, and grow
under less than optimal conditions.
Infectious plant diseases are caused by a pathogenic organism such as a fungus, bacterium,
mycoplasma, virus, viroid, nematode, or parasitic flowering plant. An infectious agent is capable of
reproducing within or on its host and spreading from one susceptible host to another. Non-
infectious plant diseases are caused by unfavourable growing conditions, including extremes of
temperature, disadvantageous relationships between moisture and oxygen, toxic substances in
the soil or atmosphere, and an excess or deficiency of an essential mineral. Because non-
infectious causal agents are not organisms capable of reproducing within a host, they are not
transmissible. The study of plant diseases is called plant pathology. Pathology is derived from the
two Greek words pathos (suffering, disease) and logos (discourse, study). Plant pathology thus
means a study of plant diseases (Figure 4.1).

4.1.1 Disease Causing Factors

 Plant disease is caused by a pathogen (biotic or infectious disease) or an environmental
factor (abiotic or non-infectious disease).

Biotic or infectious diseases

Biotic diseases can spread throughout one plant and also may spread to neighbouring plants
of the same species. These include biotic problems caused by living organisms such as pathogens,
nematodes, insects and other arthropods. Biotic diseases sometimes show physical evidence
(signs) of the pathogen, such as fungal growth, bacterial ooze, or nematode cysts, or the presence
of mites or insects. Many plant problems, especially biotic problems, if not recognized and
controlled early in their development, they can result in significant economic damage for the
producer. Therefore, timely and accurate diagnoses are required so that appropriate pest and
disease management options and other corrective measures can be implemented.

Abiotic or non-infectious diseases

Abiotic diseases are caused by nonliving factors, such as drought stress, sunscald, freeze
injury, wind injury, chemical injury, nutrient deficiency, or improper cultural practices, such as
overwatering or planting conditions. These diseases are caused by conditions external to the
plant, not due to living agents. They cannot spread from plant to plant but are very common and
should be considered when assessing the health of any plant. Examples of abiotic disease include
nutritional deficiencies, soil compaction, salt injury, ice, and sun scorch.
Abiotic damage often occurs on many plant species. Drought stress or chemical injury will
likely cause damage on several types of plants. In contrast, biotic disease problems are more
limited to a certain species. For example, the fungi that cause tomato leaf blight do not cause
damage on sweet corn. Abiotic damage does not spread from plant to plant over time. Abiotic
diseases do not show the presence of disease signs.
Plant diseases are their occurrence and severity result from the impact of three factors. They
are the host plant, the pathogen and the environmental conditions. This is represented with the
disease triangle as shown in Figure 4.2.

4.1.2 Signs and Symptoms of Diseases

Signs of plant disease are physical evidence of the pathogen. For example, fungal fruiting
bodies, bacterial ooze, or nematode cysts. Signs can also help with plant disease identification.
Signs are the actual organisms causing the disease. Signs include conks, fruiting bodies, mildew,
mushrooms, mycelium, rhizomorphs, slime flux or ooze, spore masses, insects and/or their frass.
Symptoms are visible effects of disease on plants. Any detectable changes in colour, shape,
and/or functions of the plant in response to a pathogen or disease-causing agent is a symptom.
Symptoms of disease are the plant's reaction to the causal agent.
Plant symptoms include blight, canker, chlorosis, decline, dieback, distortion, gall or gall-like,
gummosis, leaf distortion, leaf scorch, leaf spot, mosaic, necrosis, stunting, wilt, witches" broom,
insect feeding injury.
Viruses cause mottling, spots, mosaic-like patterns, crinkling, and other malformations on
leaves and fruits, and may stunt plants. Because viruses are systemic, infected plants must be 1
rogued or discarded (Figure 4.3). For example, common virus diseases in Colorado include curly
top virus of tomatoes, cucumber mosaic virus in tomatoes and vine crops, tomato spotted wilt
virus, and a variety of greenhouse plant viruses.

4.1.3 Types of Pathogens

Plant pathogens are very similar to those that cause disease in humans and animals.
Infectious plant diseases are caused by living organisms that attack and obtain their nutrition from
the plaat they infect. The parasitic organism that causes a disease is a pathogen. Pathogens can
spread from plant to plant and may infect all types of plant tissues including leaves, shoots, stems,
crown, roots, tubers, fruits, seeds and vascular tissues.

Groups of plant pathogens - fungi

Fungi and fungal-like organisms (FLOS) cause more plant diseases than any other group of
plant pest with over 8,000 species shown to cause disease. FLOS are organisms like Pythium and
Phytophthora and cause downy mildew. All plants are attacked by one species or another of
phytopathogenic fungi. Individual species of fungi can parasitize one or many different kinds of
plants (Figure 4.4). Most pathogenic fungi belong to the following genera: Alternaria, Aspergillus,
Botrytis, Fusarium, Puccinia, Rhizopus, and Sclerotinia (Table 4.1).

Groups of plant pathogens - bacteria

Not all bacteria are bad for plants and soil. In fact, most are beneficial. However, there are
approximately 200 types of bacteria that cause diseases in plants. They are most active in warm
and humid environments. Bacteria that cause plant diseases are spread in many ways. They can
be splashed about by rain or carried by the wind, birds or insects (Figure 4.5). Most plant
pathogenic bacteria belong to the following genera: Agrobacterium, Erwinia, Pseudomonas,
Streptomyces and Xanthomonas (Table 4.2).
Groups of plant pathogens - viruses
Viruses are intracellular (inside cells) pathogenic particles that infect other living organisms.
They cause human and animal diseases such as influenza, polio, rabies, smallpox, and warts.
Unlike bacteria and fungi, viruses are not spread by water or wind. Instead, they must physically
enter the plant. One of the most common vectors of viruses are insects. Examples of some viruses
are tobacco mosaic, tomato mosaic, barley yellow dwarf, potato leaf roll, tomato spotted wilt, and
tobacco ringspot (Figure 4.6).
Mosaics are characterized by the formation of light green, yellow, or white spots
intermingled with the normal green aerial plant structures (Figure 4.7). Ringspots are
characterized by the appearance of chlorotic or necrotic rings on the leaves (Figure 4.8). These
primary symptoms may be accompanied by a variety of other symptoms in specific viral plant
diseases.

Groups of plant pathogens - nematodes

Nematodes are simple, multicellular animals and are bilaterally symmetrical, soft-bodied (no
skeleton), non-segmented roundworms. Most nematode species that attack plants are
microscopic. A number of genera and species of nematodes are highly damaging to a great range
of hosts, including foliage plants, agronomic and vegetable crops, fruit and nut trees. Plant
parasitic nematodes have a stylet, a piercing mouthpart. The presence of a stylet is the key
diagnostic sign differentiating plant parasitic nematodes from all other types of nematodes. Based
on the plant part, the nematode invaded can be classified into wheat nematode: Anguina tritici,
spring dwarf nematode: Aphelenchoides fragariae, Stubby- root nematodes: Trichodorus sp., and
root-knot nematodes, Meloidogyne spp. (Figure 4.9).

4.1.4 Control of Plant Diseases

Plant diseases caused by infectious pathogens have seriously affected human society and
nature through their damages to food production, economic development. It is need to be
controlled to maintain the quality and abundance of food, feed, and fibre produced by growers
around the world. To prevent, mitigate or control plant diseases, different approaches can be
used. There are three plant diseases control approaches: biological, physical and chemical.

Biological control
Biological control is control of plant diseases using living microorganisms. Four main
mechanisms involved in the biocontrol are (i) the biological agent (antagonist) may parasite the
other organism, (ii) antagonist may secrete metabolites (antibiotics) harmful to the pathogens
(antibiosis) (iii) antagonist may compete with the pathogens for nutrients or space (competition)
and (iv) may cause death of the parasite by producing enzymes (lysis).

Physical control
Physical methods of disease prevention and control are based on the physiological tolerance
of disease agents to adverse conditions such as high or low temperature, absence of moisture,
presence of deleterious irradiation and the removal of pathogen sources or presence of physical
barriers to prevent contact between the disease agent and the host.

Chemical control
A variety of chemicals are available that have been designed to control plant diseases by
inhibiting the growth or by killing the disease-causing pathogens. Chemicals used to control
bacteria (bactericides), fungi (fungicides), and nematodes (nematicides) may be applied to seeds,
foliage, flowers, fruit, or soil.

4.2 ANIMAL DISEASES

A disease is an illness or disorder of the body or mind that leads to poor health; each disease
is associated with a set of signs and symptoms. All animals, including human, suffer from one kind
of disease or another during their lifespan. Animal diseases can be divided into non-infectious
diseases and infectious diseases.

4.2.1 Non-infectious Diseases

Non-infectious diseases are those that are not caused by pathogens, so that they cannot
spread from one person to another. In fact, they are caused by factors such as aging, genetics,
malnutrition, lifestyle and environment. Examples of non-infectious diseases include
cardiovascular disease, cancer, chronic respiratory disease, diabetes, and Alzheimer's.

Cardiovascular disease (CVD)

Most cardiovascular disease starts with atheroma formation. If the endothelium, the inner
lining of the wall of the artery, is damaged by high blood pressure, an inflammatory response.
where by white blood cells (mostly macrophages) move into the area. These white blood cells and
lipids from the blood clump together under the endothelium to form fatty streaks. Overtime,
more white blood cells, lipids and connective tissue build up and harden to form a fibrous plaque
called atheroma. This plaque partially blocks the lumen of the artery and restricts blood flow,
which causes blood pressure to increase. The hardening of arteries, caused by atheroma, is called
atherosclerosis (Figure 4.10).
In angina, plaques build up slowly in the coronary arteries, reducing blood flow to the parts.
of the heart muscle beyond the plaques. Often symptoms are first noticed during exercise, when
the cardiac muscle is working harder and needs more oxygen. The narrowed coronary arteries
cannot supply enough oxygenated blood and the heart muscle resorts to anaerobic respiration.
This causes a gripping pain in the chest that can extend into the arms, particularly the left one,
and the jaw, and often also causes breathlessness. The symptoms of angina subside once exercise
stops, but the experience is painful and frightening (Figure 4.11).
Oxygen is supplied to the heart muscle by the coronary arteries. If the coronary artery
becomes completely blocked by a blood clot, an area of the heart muscle will be totally cut off
from its blood supply and will not receive any oxygen. This causes myocardial infarction, more
commonly known as a heart attack, due to damage and death of the heart muscle. Symptoms
include pain in the chest as in angina and upper body, shortness of breath and sweating. If large
areas of the heart are affected, complete heart failure can occur. Death may be instantaneous
without any previous symptoms or may happen after several days of feeling tired and suffering
symptoms mistaken for indigestion (Figure 4.12 A and B).
A stroke is caused by an interruption to the normal blood supply to an area of the brain. This
may be due to bleeding from damaged capillaries or a blockage cutting off the blood supply to the
brain. A blockage is usually caused by a blood clot, an atheroma or a combination of the two.
Sometimes, the blood clot forms somewhere else in the body and is carried in the bloodstream
until it gets stuck, blocking an artery in the brain. The damage happens very quickly. A blockage in
one of the main arteries leading to the brain causes a very serious stroke that may lead to death.
If it happens in one of the smaller arterioles the effects may be less disastrous.
The symptoms of strokes vary, depending on how much of the brain is affected. Very often,
the blood is cut off from one part or one side of the brain only. Symptoms include dizziness,
confusion, slurred speech, blurred vision or partial loss of vision (usually one eye) and numbness.
In more severe strokes, there can be paralysis, usually on one side of the body (Figure 4.13).
Deep vein thrombosis (DVT) is the formation of a blood clot in a vein deep inside the body
and it usually happens in leg veins. It can be caused by prolong inactivity such as during long-haul
flights, and the risk increased with age (Figure 4.14).

Risk factor for cardiovascular disease

The risk factors may be non-modifiable or modifiable.
Non-modifiable risk factors for cardiovascular disease
The three main risk factors for CVDs which cannot be changed are:
 Genes: there is a genetic tendency (trend) in some families, and also in some ethnic
groups, to develop CVDs. These trends can include (i) arteries which are easily damaged,
(ii) a tendency to develop hypertension which can cause arterial damage and make CVDs
more likely, and (iii) problems with the cholesterol balance of the body.
 Age: as a person gets older, the blood vessels begin to lose their elasticity and to narrow
slightly. This can make more likely to suffer from CVDs, particularly heart disease.
 Gender: men are more likely to suffer from heart disease (and other CVDs) than women.

Modifiable risks factors for cardiovascular disease

The development of atherosclerosis is linked to many types of CVD.
Lifestyle can affect the risk of developing atherosclerosis in the future. Epidemiological studies
have shown links with smoking, diet and weight, lack of activity and high blood pressure.
Smoking - Studies have shown that smokers are far more likely to develop atherosclerosis than
non-smokers with a similar lifestyle. Studies found that the substances in tobacco smoke: (i)can
damage the artery linings, which makes the build-up of plaques more likely, (ii) can cause the
arteries to narrow, raise the blood pressure and increase the risk of atherosclerosis, and (iii)
smoking also changes the balance of lipoproteins in the blood.
Inactivity - Sedentary lifestyle can lead to heart diseases. It also causes obesity, high blood
pressure and diabetes. Regular exercise helps to prevent these adverse health effects. It also
lowers blood cholesterol levels, balance lipoproteins and reduce stress. And thus, lower the risk of
developing atherosclerosis and CVDs.
High blood pressure - A healthy blood pressure is around 120 mmHg during systole and around 80
mmHg during diastole. If the blood pressure is regularly above 140/90 mmHg, it is a sign of high
blood pressure or hypertension and can also be a sign of atherosclerosis. The blood pressure goes
up when the walls of the arteries become less flexible due to the build-up of the plaques. This
means that raised blood pressure can be the result of atherosclerosis. Measuring blood pressure
is used as an indicator of both the heart and blood vessels.
Diet and obesity - An increasing number of studies suggest that being overweight does not
directly affect the risk of developing CVDs, but it is a very important indicator of risk. Most
scientists think that the best predictors of future CVDs are: (i) where fat is stored on the body, (ii)
how much exercise a person does and (iii) the levels of different fats in the blood.
Two other factors which are often a direct result of being overweight do increase the risk of
atherosclerosis and CVDs. These are:
 high blood pressure which increases the risk of damage to blood vessel linings, and so of
plaque formation
 type 2 diabetes- this can result in damage to the lining of the blood vessels which increases
the risk of plaque formation.
Many studies are starting to find that an increased risk of developing a disease is often due
to a combination of factors.
Prevention of cardiovascular disease
 Eating a balanced diet with a variety of fats and plenty of fruit and vegetables helps prevent
atherosclerosis. Not smoking and maintaining a healthy weight- a body mass index (BMI) of 18.5-
25 kgm is the ideal range. It is needed to control high blood pressure and type 2 diabetes. Taking
regular exercise strengthens the heart muscle and helps to control blood pressure and cholesterol
level. It also reduces constant stress. Thus, all these factors contribute towards the prevention of
cardiovascular disease.
Treatment
Antihypertensives drugs that such as beta blockers, sympathetic nerve inhibitors, ACE
inhibitors are some of the commonly prescribed drugs to reduce blood pressure. They reduce the
risks of CVDs and also reduce the risk of damage to the kidneys and eyes from high blood
pressure. But there are risks. If the treatment is not monitored carefully, the blood pressure may
become too low. That can lead to falls and injuries which, particularly in elderly patients, can be
serious and even life-threatening.
To lower the level of cholesterol in the blood, a group of drugs known as statin is used. They
also improve the balance of LDLs (low-density lipoprotein) to HDLS (high-density lipoprotein) and
reduce inflammation in the lining of the arteries. Both functions reduce the risk of atherosclerosis
developing. Plant stanols and sterols compounds are now widely sold in spreads and yoghurts,
which also reduce the levels of LDLs in the blood.
Anticoagulants (e.g., warfarin and heparin) are a class of medicines used to reduce blood
clotting. Platelet inhibitory drugs such as aspirin are also a type of anticoagulant. These drugs
prevent platelets clumping together. As they reduce the formation of blood clots, the chance of a
blood vessel becoming blockage can be reduced.
4.2.2 Infectious Diseases
Infectious diseases are diseases that are caused by organisms known as pathogens. They are
sometimes called communicable diseases as they are passed from infected to uninfected people.
Some also affect animals and are passed from animals to humans. The way in which a pathogen
passes from one host to another is called the transmission cycle.
The prevalence of a disease is the number of people who have that disease at any one time.
Diseases that are always in populations are described as endemic. An epidemic occurs when there
is a sudden increase in the number of people with a disease. A pandemic occurs when there is an
increase in the number of cases throughout a continent or across the world. The death rate from
different diseases is referred to as mortality. Some diseases of worldwide importance are shown
in
Malaria
Transmission of malaria
The features of this disease are summarized in Table 4.4. Most cases of malaria are caused
by one of four species of the protist Plasmodium, whose life cycle is shown in Figure 4.15. Genetic
analysis of infections shows that some species of Plasmodium that cause malaria in monkeys also
affect humans.
Female Anopheles mosquitoes feed on human blood to obtain the protein they need to
develop their eggs. If the person they bite is infected with Plasmodium, they will take up some of
the pathogen's gametes with the blood meal. Male and female gametes fuse in the mosquito's gut
and develop to form infective stages, which move to the mosquito's salivary glands. When the
mosquito feeds again, it injects an anticoagulant from its salivary glands that prevents the blood
meal from clotting, so that it flows out of the host into the mosquito. The infective stages pass
from the mosquito's salivary glands into the human's blood together with the anticoagulant in the
saliva. The parasites enter the red blood cells, where they multiply.
The female Anopheles mosquito is therefore a vector of malaria and it transmits the disease
when it passes the infective stages into an uninfected person. Malaria may also be transmitted
during blood transfusion and when unsterile needles are re-used. Plasmodium can also pass
across the placenta from mother to fetus.
Plasmodium multiplies in both hosts, the human and the mosquito; at each stage there is a
huge increase in the number of parasites, and this improves the chances of infecting another
mosquito or human host. If people are continually re-infected by different strains of malaria they
become immune. However, this only happens if they survive the first five years of life, when
mortality from malaria is very high. The immunity only lasts as long as people are in contact with
the disease. This explains why epidemics in places where malaria is not endemic can be very
serious, and why malaria is more dangerous in those areas where it only occurs during and after
the rainy season. This often coincides with the time of maximum agricultural activity, so the
disease has a disastrous effect on the economy: people cannot cultivate the land when they are
sick (Figure 4.15, 4.16 A and B).
Treating malaria
Quinine and chloroquine-are anti-malarial drugs used to treat infected people. They are also
used as prophylactic (preventative) drugs, stopping an infection occurring if a person is bitten by
an infected mosquito. Prophylactic drugs are taken before, during and after visiting an area where
malaria is endemic. Chloroquine inhibits protein synthesis and prevents the parasite spreading
within the body.
Proguanil is another prophylactic, has the added advantage of inhibiting the sexual
reproduction of Plasmodium inside the biting mosquito. Where anti-malarial drugs have been
used widely, there are strains of drug-resistant Plasmodium-the drug is no longer effective against
the pathogen.
Mefloquine is a new drug used in South America, Africa and New Guinea. However,
mefloquine is expensive and sometimes causes unpleasant side-effects such as restlessness,
dizziness, vomiting and disturbed sleep. Resistance to mefloquine has developed in some areas,
notably the border regions of Thailand. The antibiotic doxycycline is also used as a prophylactic
drug. Artesunate, a drug derived from the plant compound artemisinin, is used in combination
with mefloquine to treat infections of P. falciparum.
Preventing malaria
There are three main ways to control malaria:
 reduce the number of mosquitoes,
 avoid being bitten by mosquitoes and
 use drugs to prevent the parasite infecting people.
It is possible to kill the insect vector and break the transmission cycle. Mosquitoes lay their
eggs in water. Larvae hatch and develop in water but breathe air by coming to the surface. Oil can
be spread over the surfaces of water to make it impossible for mosquito larvae and pupae to
breathe. Marshes can be drained and vegetation cleared.
Two biological control measures that can be used are:
 stocking ponds, irrigation and drainage ditches and other permanent bodies of water with
fish which feed on mosquito larvae e.g., mosquito fish Gambusia affinis
  spraying a preparation containing the bacterium Bacillus thuringiensis, which kills
mosquito larvae but is not toxic to other forms of life
However, mosquitoes will lay their eggs in any small puddle or pool, which makes it
impossible to completely eradicate breeding sites, especially in the rainy season.
The best protection against malaria is to avoid being bitten. People are advised to sleep
beneath mosquito nets and use insect repellents. Soaking mosquito nets in insecticide every six
months has been shown to reduce mortality from malaria. People should not expose their skin
when mosquitoes are active at dusk.
Worldwide control of malaria
In the 1950s, the World Health Organization (WHO) coordinated a worldwide eradication
programme. Although malaria was cleared from some countries, the programme was not
generally successful. There were two main reasons for this:
 Plasmodium became resistant to the drugs used to control it
 mosquitoes became resistant to dichloro-diphenyl-trichloroethane (DDT) and the other
insecticides that were used at the time, such as dieldrin
The reasons for the worldwide concern over the spread of malaria are:
 an increase in drug-resistant forms of Plasmodium
 an increase in the proportion of cases caused by P. falciparum, the form that causes
severe, often fatal malaria
 difficulties in developing vaccines against malaria
 an increase in the number of epidemics, because of climatic and environmental changes
that favour the spread of mosquitoes
 the migration of people from areas where malaria is endemic, for economic and political
reasons
Malaria is still one of the world's biggest threats to health: 40% of the world's population
lives in areas where there is a risk of malaria. Between 2000 and 2011, control measures have
achieved a decrease in mortality rates of about 25% across the world, and 33% in the WHO's
African region.
Control methods now concentrate on working within the health systems to improve
diagnosis, improve the supply of effective drugs and promote appropriate methods to prevent
transmission. Several recent advances give hope that malaria may one day be controlled. The
introduction of simple dip stick tests for diagnosing malaria means that diagnosis can be done
quickly without the need for laboratories. The whole genome of Plasmodium has been sequenced,
and this may lead to the development of effective vaccines. Several vaccines are being trialed, but
it is not likely that a successful vaccine will be available for some time. Drugs are used in
combination to reduce the chances of drug resistance arising.
Three factors may lead to improvements in the control of malaria:
 use of modern techniques in gene sequencing and drug design
 development of vaccines targeted against different stages of the parasite's life cycle and
 a renewed international will to remove the burden of disease from the poorest parts of
the world, allied to generous donations from wealthy individuals and foundations.