Plant Phytopathology Notes for B. Sc. -I paper-3

Plant Phytopathology

The study of the diseases of plants, its causes, processes of development, consequences and the structural and functional changes caused by the disease. In Greek, ‘Phyto’ means plant, ‘Pathos' means suffering and ‘Logos’ means knowledge. Ø Disease A disease may the defined as sustained physiological or structural unbalance in the plant as a whole or a part of it, caused by certain external agencies. Any abnormal condition of the body that causes discomfort, dysfunction or distress to the affected plant is called disease.

Agrios (1997) defined disease in plants as a series of invisible and visible responses of plant cells and tissues to a pathogenic micro-organism or environmental factor that result in adverse changes in the form, function or integrity of the plant and may lead to partial impairment or death of the plant or its parts. Agencies that cause disease may be unfavourable environmental conditions (non-pathogenic) or it may be a pathogen like bacteria, fungi, virus, etc.

Disease triangle and disease pyramid

The interactions of three components of disease i.e. the host, the pathogen, and the environment is called the disease triangle.

A plant becomes diseased in most cases when it is attacked by a pathogen, or less commonly, when it is affected by an abiotic agent. So, 3 components must be present simultaneously to cause a disease. These are:

 • Susceptible host

• Virulent pathogen in the active stage

• Suitable environment

The interaction of these 3 components have been often visualised as a triangle, referred to as ‘disease triangle’. Each side of the triangle represents one of the 3 components. The length of each side is proportional to the sum total of the characteristics of each component that favours disease. The area of the triangle represents the potential amount of the disease. If the plant is resistant, wrong age or widely spaced, the host side is small or zero and amount of disease is less or zero. If the plant is susceptible, at right age or closely planted, the host side is long and amount of disease will be more. Similarly, more virulent, abundant and active the pathogen, the longer the pathogen side would be and the amount of disease would be greater. And, the more favourable the environmental factors that help infection, the longer the pathogen side would be and the amount of disease would be greater

 

 Koch postulates

Robert Koch (1882) laid down the conditions to identify the real causal agent of a disease. These are called Koch postulates which are as follows-

 

Koch’s postulates: Robert Koch enunciated certain rules or criteria that should be satisfied before the identity of the disease-producing organism in a particular disease could be established. These rules are known as Koch’s postulates. Koch’s postulates

§ A specific organism must always be associated with a disease.

 § The causal organism must be isolated from the diseased plant (first isolate) and grown in pure culture and accurately described.

§ The organism must be identified.

§ The inoculations with inocula from pure culture of the causal organism must reproduce the disease in the same species or variety of plant from which the causal organism was isolated.

§ The causal organism must be re-isolated (second isolate) from the plant in which disease has been produced by inoculating with inocula from pure culture. The physiological and morphological characters of the first and second isolates should be compared. Drawbacks

§ In case of some pathogens like some phytoplasmas, fastidious vascular bacteria, protozoa, fungi and viruses, culture or purification of the pathogen is not yet possible. The pathogen often cannot be re-introduced in the host to reproduce the disease (Expowdery mildew). Thus with these pathogens, Koch’s postulates cannot be followed.

§ In case of root diseases, inoculation and isolation of the pathogen are both difficult.

 § It do not include the following concepts: o Asymptomatic carrier state. o Many viruses do not cause illness in all infected individuals (Ex-Polio virus cause paralysis in 1% of those infected.

 o Infection with the same virus may lead to markedly different diseases.

 o Multiple causation

o One syndrome may have different caused under different settings.

 o Diseases may be caused due to nutrient deficiencies without involvement of pathogens. Modifications Here are Koch’s postulates for the 21st century as suggested by Fredricks and Relman:

 § A nucleic acid sequence belonging to a pathogen should be present preferentially in the diseased organs or gross anatomic sites and not in those organs that lack pathology.

 § Fewer, or no, copy numbers of pathogen-associated nucleic acid sequences should occur in hosts or tissues without disease.

§ With resolution of disease, the copy number of pathogen-associated nucleic acid sequences should decrease or become undetectable. With clinical relapse, the opposite should occur.

 § The nature of the microorganism inferred from the available sequence should be consistent with the known biological characteristics of that group of organisms.

§ Tissue-sequence correlates should be sought at the cellular level: efforts should be made to demonstrate specific in situ hybridization of microbial sequence to areas of tissue pathology and to visible microorganisms or to areas where microorganisms are presumed to be located.

 § These sequence-based forms of evidence for microbial causation should be reproducible

 

Classification of Plant Disease

Based on the nature of the causal agent, the plant disease is classified into

·         Non-parasitic or abiotic diseases– These are induced by unfavorable environmental conditions such as mineral deficiencies or excess, low or high temperatures, improper water, oxygen, and light relations. Examples are the black heart of potatoes, heat canker of flax, scald of apples, etc.

·         Parasitic diseases– These are caused by the attack of some living agents called the causal agents which may be a plant, an animal, or a virus. In this case, the diseased plant is called a host and the causal organism is called a pathogen. Most plant diseases are caused by fungi, bacteria, algae, angiosperms, nematodes, and viruses.

According to occurrence, they are divided into three categories

·         Endemic diseases– A disease that is regularly present in a certain region or a country in a moderate to severe form is called endemic disease. Example-Wart disease in potato.

·         Pandemic diseases diseases occur over very large regions, sometimes globally, affecting multiple crops and plants. Example- Chestnut blight.

·         Epiphytotic disease (epidemic) – It is the kind of infectious disease that spreads widely but occurs periodically. The causal agent may be regularly present in the environment but its rapid development may only occur periodically and thus this type of disease is responsive to environmental variations. Example- rust, blight, mildews.

·         Sporadic disease– These are the plant diseases that occur at irregular intervals or occasionally. Example leaf blight, wilt etc.

Based on perpetuation and primary infections, plant diseases are classified as

·         Soil-borne diseases– There are several pathogens whose infectious agents are present in soil and primary infection takes place through soil. They infect through roots. Examples of pathogens are- Phytophthora, Fusarium, etc. Examples of diseases are- Root rot, wilt, and seedling blight.

·         Seed-borne diseases– In this type of disease, the pathogens survive as dormant mycelium in the seeds or other propagating materials of host plants. Example of pathogen- Ustilago

·         Examples of disease- damping off, smut, etc.

·         Airborne diseases– In this type, the pathogens infect the host plant through air and bring primary as well as secondary infection. The spores of such pathogens are disseminated by wind. Examples of pathogens- are Puccinia, Rhizopus, etc. Examples of diseases are blight, rust, and powdery mildew, etc.

·         Vector-borne disease– In this type, the diseases are transmitted by vectors such as insects that carry pathogens from one plant to another.

Based on the extent of infection it is of two types

·         Localized– In this type, only a particular part of the plant is affected. Example root rot, leaf spot etc.

·         Systemic– In this type, the pathogen spreads throughout the plant body. Example Downey mildew.

Based on pathogen generations

·         Monocyclic disease/ simple interest disease– In this type of disease the pathogens have only one generation in one cropping season. Example- Loose smut of wheat.

·         Polycyclic disease/ compound interest disease– In this type of disease the pathogen has more than one generation in one cropping season. Example- Late blight of potato.

·         Polyetic disease– These are polycyclic diseases but complete their disease cycle in more than one year or above. Example- Cedar apple rust.

Based on symptoms, they are of mainly three types

1. Necrosis- Death or killing of the host tissues due to the pathogen is called necrosis. The types of necrotic symptoms are-

·         Spots- Spots are localized brown or black-colored areas surrounded by purple color margins. Spores may occur as fungal leaf spots or bacterial leaf spots.

·         Blights- These are rapid discoloration of infected parts like leaves, stems, or twigs in response to the pathogen infection and followed by death of these parts.

·         Blasts- These are the sudden death of the unopened buds or inflorescences.

·         Scorch- Scorches are blight-like but occur in irregular patterns usually along the leaf margins.

·         Cankers- These are localized necrotic areas with open wounds often sunken in the woody tissues like stems surrounded by living tissues. Canker is a dead lesion.

·         Scabs- A scab is an epidermal infection which are mostly rough, and crust-like.

·         Rots- Rots are infections of various soft tissues of the host and may involve various parts of the plant. Rots represent the dead and decomposed tissues due to disintegration by the enzymes secreted by the pathogen.

·         Anthracnose- This destroys the collenchyma and cambium layers and is difficult to distinguish from the cankers. In this case, the lesions are sunken in the center with raised margins.  

·         Wilts- Wilts are secondary symptoms in which the leaves and shoots lose their turgidity and droop.

·         Damping off- It is the rapid death and collapse of the seedlings at the soil line.

·         Mildews- These are chlorotic or necrotic areas covered with pathogens due to their vigorous growth. There are two types of mildews- powdery and downy mildews. Powdery mildews appear as white or brown dust on the leaves and stems of the host. Downy mildews are deeply affected by narcotic and chlorotic lesions on the leaves and stems.

·         Streak and stripe- These are elongated necrotic areas along the stem and leaf veins.

·         Ring spots- These are symptoms of viral infections found on leaves and stems which may be present in single or in groups.

·         Rusts- These are small, local pustules of various colors on stems or leaves giving a rusty appearance.

·         Smuts- These are shooty or charcoal-like powdery masses appearing on different parts.

·         Scald- Scald is the blenching of epidermal and adjacent tissues of fruits and leaves.

·         Dieback- Dieback results from severe necrosis of fruits and branches beginning from the tip region that advances downwards. Dieback is seen in old plants.

2. Hyperplasia and hypertrophy- Hyperplasia is the abnormal increase in the cell number due to rapid division. The rapid growth in cell divisions is called hyperplasia.

Different hypertrophic and hyperplasic symptoms seen in plants are-

·         Galls/tumors- These are irregular growth or elongations of parts due to fungal or bacterial infections.

·         Leaf curls- These are thickened, swollen, distorted, and curled down leaves due to hypertrophy on the upper surface.

·         Warts- These are small protuberances developed on galls/tumors giving a discolored or warty appearance.

·         Witches broom- It is a special type of overdevelopment where a large number of special branches arise from a single localized area.

·         Enations- These are small outgrowths occurring on veins and midribs on the lower side of leaves.

3. Hypotrophy and hypoplasia- Hypotrophy is the reduction in growth and hypoplasia is a reduction in a number of cell divisions. Different hypoplasic symptoms are as follows-

·         Chlorosis and yellowing- It is the distortion of green color due to lack or reduction of chlorophyll synthesis, the leaves and the entire plant look pale green and yellow.

·         Mosaic- It is the irregular color development where a mixing of green with other colors is found giving a mosaic appearance.

·         Dwarfing and stunting- This symptom is manifested by proportionate reductions in the growth of all parts as a result the plant becomes dwarf-stunted or underdeveloped.

·         Leaf curling- This is the upward curling of leaf lamina due to reduced growth of veins in comparison to lamina growth.

·         Pallor- This is due to the destruction of chlorophyll in the leaves. It occurs due to the presence of parasites in this pale area or around it.

·         Little leaf- This represents the reduced growth of the lamina due to virus infection.

·         Rosette- The length of the internode is shortened due to reduced growth.

Based on duration and severity

·         Acute diseases– These diseases develop rapidly and cause severe damage over a short period. Example- Fire blight.

·         Chronic diseases– These diseases progress slowly, causing long-term damage and a decline in plant health. Example- Oak wilts.

Based on the part affected

·         Foliar disease– These diseases primarily affect the leaves. Example- leaf spots.

·         Stem diseases– These diseases primarily affect the stems. Example- stem rust.

·         Root diseases– These diseases impact the root system. Example nematode infestations.

·         Fruit and seed diseases– These diseases affect the reproductive part of the plant leading to issues like poor fruit development and seed viability. Example Seed smut.

 

Some important Phytopathology terms and definitions

Disease– A malfunctioning of host cells and tissues that results from continuous irritation by a pathogenic agent or environmental factor and leads to the development of symptoms. (G.N. Agrios, 1997).

Disorder– Non-infectious plant diseases due to abiotic causes are called disorders.

Pathogen– An entity; usually a micro-organism that can incite disease. A pathogen is any agent that causes disease. 

Parasite– It is a living organism that derives the nutrition for its growth from another living organism. A parasite may not always cause a disease.

Pathogenicity– It is the ability of a pathogen to cause a disease.

Pathogenesis– It is a chain of events that leads to the development of disease in the host or a sequence of progress in disease development from the initial contact between the pathogen and its host to the completion of the syndrome.

Sign- It is the pathogen or its parts or products seen on a host plant.

Symptoms– It is the external or internal reactions shown by the plant as a result of a disease.

Syndrome– A set of varying symptoms characterizing a disease are collectively called a syndrome.

Biotroph– An organism that can live and multiply on another living organism by obtaining its food from living tissues on which it completes its life cycle.

Hemibiotroph– The parasite that attacks living tissue as a biotroph but continues its life cycle even after the death of the host tissue is called hemibiotroph or facultative saprophytes.

Alternate host– It is a host that helps in the completion of the life cycle of the pathogen and its survival and belongs to a different group.

Perthotrophs (Necrotrophs) – A parasite is a necrotroph when it kills the host tissues in advance of penetration and lives saprophytically.

Inoculum- It is part of the pathogen in different forms; such as spores, dormant mycelium, etc. which on contact with a susceptible host plant causes infection and is called inoculum.

Incubation period- The period between penetration of a host by a pathogen and the first appearance of symptoms on the host is called the incubation period.

Penetration- The invasion of the host tissue by the pathogen is called penetration.

Hypersensitivity– Excessive sensitivity of a plant to pathogens is called hypersensitivity.

Infection- It is the establishment of the pathogen within the tissue of the host plant after penetration is called infection.

Disease cycle– A series of events between infection and the complete development of a given disease is called a disease cycle.

Epidemiology– It is the study of the rate of multiplication of pathogens and the spread of a disease caused by it in a plant population.