Plant Pathogens

Plant Pathology/ Phytopathology (Phyton = plant, Pathos = suffering, ailment, Logos = study / discourse / knowledge) i.e the study of nature, development and management of plant diseases.

Definition: Plant pathology is a branch of Agricultural science which deals with cause, etiology, resulting losses, and management of plant diseases.

What are Plant Pathogens?

Plant Pathogens is an entity usually a micro-organism that can incite disease in susceptible plants. It is also referred to as an incitant, causal agent, or causal organism.

Plant Disease (disease i.e not ease): It is the malfunctioning of host cells and tissue that results from continuous irritation by a pathogenic agent (plant pathogens) and leads to the development of symptoms.

A plant disease is any physiological or structural abnormality that is caused by a living organism. Organisms that cause disease are referred to as ‘pathogens’ (i.e plant pathogens), and affected plants are referred to as ‘hosts.’ Many organisms rely on other species for sources of nutrients or as a means of survival but are not always harmful to the host. For example, saprophytic organisms obtain nutrients from dead organic material and are a vital part of many ecosystems. Plant pathogens, on the other hand, utilize hosts for nutrients and/or reproduction at the hosts’ expense. Disease-causing organisms include fungi, oomycetes (fungus-like organisms called water molds), bacteria, viruses, nematodes, phytoplasmas, and parasitic seed plants.

Once a plant pathogens infect a host, symptoms often develop. Symptoms are the outward changes in the physical appearance of plants. Symptoms take time to develop, and thus, disease development may be delayed for several days, weeks, months, or even years after initial infection occurs. Examples of symptoms include wilt, leaf spots, cankers, rots, and decline.

Physical evidence of plant pathogens (called ‘signs’) may also be observed on diseased tissue. Examples of signs include fungal fruiting bodies, bacterial ooze, nematode cysts, and fungal mycelia. Both symptoms and signs are utilized in making disease diagnoses.

Conditions for disease development

Disease development is dependent upon three conditions: a susceptible host plant, a favorable environment, and a viable pathogen. All three of these factors must be present for the disease to occur.

Host plant genetic makeup determines its susceptibility to disease. This susceptibility depends upon various physical and biochemical factors within the plant. A plant’s stature, growth habit, cuticle thickness (a protective outer layer on plant tissues), and shape of stomata (small openings that allow water, oxygen, and carbon dioxide in and out of plant tissues) are a few physical factors that influence disease development. Plants may also produce biochemical compounds that limit or prevent colonization or infection. The growth stage and ability to deter plant pathogens can also impact plant susceptibility to disease. For example, young leaves are often more susceptible to infection than mature leaves.

The environment plays an important role in disease development. Plant pathogens generally require specific environmental conditions for infection and spread. Most plant pathogens require high humidity and moderate temperatures. Other plant pathogens, such as bacteria and water molds, require surface water for spread. In some disease cycles, environmental conditions influence the development of symptoms. For example, extreme temperatures or drought can cause plant stress; this loss of vigor can increase host susceptibility to both infection and disease development. Other environmental factors affecting disease can include those resulting from planting and maintenance practices. For example, high-density plantings can have higher relative humidity, while overhead watering increases leaf surface moisture needed by plant pathogens to infect plant leaves.

Plant Pathogens must be present and viable to infect plants and cause disease. Removal of infected plant parts and other remnants of pathogens makes them unavailable for infection. Fungicides also reduce amounts of inoculum (infective propagules) available for infection. Many plant pathogens, however, have developed specialized structures that ensure survival during adverse conditions. For example, several water molds and fungi are capable of surviving in soil for many years until conditions are favorable for infection. Plant Pathogens may also survive winter temperatures and other harsh conditions in infected plant tissue. If a susceptible host and favorable environment are not available, some pathogens can assume a dormant state for many years.

Importance of Plant Pathogens:

  1. Plant diseases caused by microorganisms are of paramount importance to humans because they damage plants and plant products on which humans depend on food, clothing, furniture, and housing.
  2. Millions of people all over the world still depend on their plant products for their survival.
  3. Plant pathogens reduce the quality and quantity of plant produce eg. Wheat bunt caused by Tilletia species.
  4. Results in increased prices of products to consumers.
  5. Results in severe pathological effects on humans and animals that eat plant products.
  6. Destroy the beauty of the environment by damaging plants around the home, park streets, and forests.
  7. The pesticides used to control disease, pollute the water and environment.
  8. Reduce crop yields.
  9. Cause financial loss i.e the money spent on plant protection chemicals.
  10. Changes agricultural pattern.
  11. Influences the industries i.e lack of raw material.
  12. Some plant diseases even change the food habits of the human population.

Examples of serious diseases that lead to famines:

Irish famine (1845) – late blight of potato by Phytophthora infestans destroyed millions of hectares of potato fields thus people switched over to other food crops.

Bengal famineBipolaris oryzae (1942), West Bengal, India

Coffee rustHemileia vastatrix (1868), Srilanka

Wheat rustPuccinia graminis f.sp.trici (1940) U.S.A

Southern corn leaf blightHelminthosporium maydis, U.S.A

Important Plant Pathogenic organisms

  1. Fungi: Fungi are eukaryotic, spore-bearing, achlorophyllous organisms that generally reproduce sexually and asexually and whose filamentous, branched somatic structures are typically surrounded by cell walls consisting chitin or cellulose or both with many organic molecules.
  2. Bacteria: Bacteria are extremely minute, rigid, essentially unicellular organisms free of true chlorophyll and generally devoid of any photosynthetic pigment, most commonly multiplying asexually by simple transverse fission, the resulting cell, being of equal or nearly equal in size.
  3. Fastidious vascular bacteria (RLO’s): Fastidious vascular bacteria are similar to bacteria in most respects but are obligate parasites or can not be grown on routine bacteriological media.
  4. Mollicutes (phytoplasma and spiroplasma): Phytoplasmaare pleomorphic, wall-less prokaryotic micro-organisms, that can infect plants and can not yet be grown in culture. Spiroplasma is helical, wall-less prokaryotic micro-organisms that are present in the phloem of diseased plants, often helical in culture, and are thought to be a kind of mycoplasma and can be cultured on artificial medium.
  5. Virus: A sub-microscopic, obligate parasite consisting of nucleic acid and protein that multiplies only intra-cellularly and is potentially pathogenic.
  6. Viroids: Small, low molecular weight ribonucleic acids (RNA) that can infect plant cells, replicate themselves, and cause disease in plants.
  7. Algae: Algae are eukaryotic, photosynthetic, uni, or multicellular organisms, containing chlorophyll and a few algae mainly green algae that cause plant diseases.
  8. Flagellated protozoans: Protozoa are microscopic, non-photosynthetic, eukaryotic, flagellate motile, single-celled animals.

GENERAL CHARACTERISTICS OF FUNGI

FUNGI: Fungi are eukaryotic, spore-bearing, achlorophyllous, heterotrophic organisms that generally reproduce sexually and asexually and whose filamentous, branched somatic structures are typically surrounded by cell walls containing chitin or cellulose or both with many organic molecules and exhibiting absorptive nutrition. Somatic structures: Thallus/ Soma Commonly called a vegetative body or fungal body. A thallus( pl. thalli) is a simple, entire body of the fungus devoid of chlorophyll with no differentiation into stem, roots, and leaves lacking vascular system.

Hypha (hypha=web) ( pl. hyphae): Hypha is a thin, transparent, tubular filament filled with protoplasm. It is the unit of a filamentous thallus and grows by apical elongation.

Mycelium( pl. mycelia): A network of hyphae ( aggregation of hyphae) constituting the filamentous thallus of a fungus. It may be colorless i.e., hyaline or colored due to the presence of pigments in the cell wall. The mycelium may be ectophytic or endophytic.

plant pathogens
Image of fungal mycelia

Types of fungal thalli:

1.Plasmodium (plasma = molded body): It is a naked, multinucleate mass of protoplasm moving and feeding in an amoeboid fashion. Eg. Plasmodiophora brassicae.

2.Unicellular thallus: consisting of a single cell. Eg.Chytrids, Synchytrium.

3.Multicellular or filamentous thallus: The majority of fungi i.e., a true fungi are filamentous, consisting of several branched, thread-like filaments called hyphae.Eg.Many fungi, Alternaria. Fungi based on reproductive structures:

Holocarpic (holos=whole+karpos=fruit): If the thallus is entirely converted into one or more reproductive structures, such thallus is called holocarpic thallus. Eg.Synchytrium

Eucarpic(Eu=good+karpos=fruit): If the thallus is differentiated into a vegetative part that absorbs nutrients and a reproductive part that forms reproductive structures, such thallus is called eucarpic thallus. Eg.Pythium

Ectophytic fungus: If the fungal thallus is present on the surface of the host plant, it is called ectophytic.Eg. Oidium. Endophytic fungus: If the fungus penetrates the host cell/present inside the host, it is called endophytic.Eg. Puccinia.

Endophytic fungus may be intercellular (hypha grows in between the cells), or intracellular ( hypha penetrates host cell).Eg.Ustilago, or vascular (xylem vessels) Eg. Fusarium oxysporum.

Intercellular hyphae produce special organs called haustoria which penetrate the host cell and absorb food. These are absent in intracellular hyphae. Endophytic intracellular mycelium absorb food directly from protoplasm without any specialized structures.

In ectophytic mycelium, haustoria are produced in epidermal cells.

Septation in Fungi :(septum=hedge/partition) ( pl.septa) Some fungal hyphae are provided with partitions or cross walls which divide the fungus into several compartments /cells. These cross walls are called septa.

Aseptate hypha/coenocytic hypha: ( Koinos=common,kytos=hollow vessel) A hypha without septa is called aseptate /non-septate/ coenocytic hypha wherein the nuclei are embedded in cytoplasm.Eg. lower fungi like Oomycetes and Zygomycetes.

Septate hypha: A hypha with septa or cross-walls is called septate hypha.

Eg.common in higher fungi like Ascomycotina, Basidiomycotina and Deuteromycotina

General types of septa:
1. Based on formation:

a)Primary septa: These are formed in direct association with nuclear division (mitotic or meiotic) and are laid down between daughter nuclei separating the nuclei /cells. Eg. Higher fungi like Ascomycotina and Basidiomycotina.

b)Adventitious septa: These are formed independent of nuclear division and these are produced to delimit the reproductive structures. Eg. lower fungi like Oomycetes and Zygomycetes in which septa are produced below gametangia (sex organs) which separate them from the rest of the cells.

2. Based on construction:

a)Simple septa: It is most common which is a plate-like, with or without perforation.

b)Complex septa: A septum with a central pore surrounded by a barrel-shaped swelling of the septal wall and covered on both sides by a perforated membrane termed the septal pore cap or parenthood. Eg. Dolipore septum in Basidiomycotina.

3. Based on perforation:
a)Complete septa: A Septum is a solid plate without any pore or perforations. Eg. Adventitious septa in lower fungi.

b)Incomplete septa: A septum with a central pore. Fungal tissues: Plectenchyma : ( plekein=to weave+enchyma=infusion)

Fungal tissues are called plectenchyma i.e., mycelium becomes organized into loosely or compactly woven tissue. This tissue composes various types of vegetative and reproductive structures.

Types of plectenchyma:
1.Prosenchyma: It is a loosely woven tissue. The component hyphae retain their individuality which can be easily distinguishable as hyphae and lie parallel to one another.Eg. Trauma in Agaricus.

2. Pseudoparenchyma: It is compactly woven tissue. It consists of closely packed cells which are isodiametric or oval resembling parenchymatous cells of plants and hence the name. The component hyphae lose their individuality and are not distinguishable as hyphae. Eg. Sclerotial bodies of Sclerotium and rhizomorph of Armillariella.

You can also read about what are diseases occur in rice or paddy crop.