Plant Pathology Basics

Course Information

• Course title: BIOL 3025: Molecular Plant Pathology
• Lecture: 1 - Introduction
• Instructor: Dr. ANGELA T. ALLEYNE

Learning Outcomes

• Define "plant disease"
• Describe different types of pathogens
• Explain the disease triangle
• Explain Koch's Postulates
• Differentiate between disease signs and symptoms
• Describe common plant disease symptoms

Plant Pathology

• The Irish Potato Famine (1845-1851) marked a turning point in plant pathology, highlighting the importance of understanding plant diseases.
• Widespread "stinking" rotten potato mass in Europe played a role, as well as the fact that the microbes responsible for the rot were found to be responsible for the destruction of potatoes.
• This led to the acceptance of the germ theory of disease.

Microbial Phytopathogens

• Microbial phytopathogens include bacteria, fungi, viruses, nematodes, and phytoplasmas.
• These pathogens enter the plant through various ways, such as natural openings, injuries, and direct penetration.
• Specific examples of microbial phytopathogens:
• Xanthomonas
• Streptomyces
• Ralstonia
• Xylella
• Agrobacterium
• Pseudomonas
• Clavibacter
• Phytoplasmas

Prokaryotic Cells

• Prokaryotic cells, like bacteria, are characterized by their lack of a nucleus and membrane-bound organelles.
• Key components: cell wall (peptidoglycan), capsule, fimbriae (pili), cytoplasm, ribosomes, flagellum, and DNA.

Fungal Phytopathogens

• Fungal phytopathogens are a diverse group impacting plants.
• Specific examples: Colletotrichum, Cercospora, Ustilago, Puccinia, Phytophthora, Erysiphe, Claviceps, Fusarium

Fungi Structure

• Fungi are characterized by their hyphae, mycelia, conidiophores, and conidia (spores).
• These structures facilitate the spread and infection of the fungus, playing a critical role in successful pathogenicity.

Viruses

• Viruses are a variety of plant pathogens.
• Examples: Tobacco mosaic virus (TMV), Tomato spotted wilt virus (TSWV), Tomato yellow leaf curl virus (TYLCV), Cucumber mosaic virus (CMV), Potato virus Y (PVY), Cauliflower mosaic virus (CaMV), African cassava mosaic virus (ACMV), Plum pox virus (PPV), Brome mosaic virus (BMV), Potato virus X (PVX).
• Structure: a nucleic acid (RNA or DNA) enclosed within a protein coat; some have a segmented genome, and some are spherical others are rod-shaped.

Common Plant Pathology Terms

• Biotroph: pathogen that survives by living on living tissues of the host plant
• Necrotroph: pathogen that kills cells of the host plant and obtain nutrients
• Hemibiotroph: pathogen that combines both biotrophic and necrotrophic growth mechanisms
• Host: organism on or in which another organism lives
• Pathogen: organism that causes disease
• Host-defense: mechanisms by which a host plant resists or prevents the growth of pathogens.
• Pathogenicity: ability of a pathogen to cause disease.
• Virulence: relative capacity of a pathogen to cause disease, ranges from minimal damage to death of the plant.

Pathogenesis

• Disease is characterized by an abnormal change in the physiological or developmental processes of a host plant, often due to persistent biotic or abiotic factors.
• The disease triangle includes factors like the pathogen, environment, and host plant interaction.

Definition of Disease

• Disease arises from a dynamic relation between an organism (pathogen) that interferes with the normal processes of a host (plant).
• This interaction is detrimental, resulting in observable damage or symptoms.
• Plant stresses or injuries are not considered diseases; they are not dynamic.

Pathogen

• Pathogens can be either animate or inanimate, including microbes, viruses, and viroids.
• The definition of pathogen has evolved over time, reflecting advancements in our understanding of disease-causing agents.

Koch's Postulates & Exceptions

• These are steps used to prove that a certain microbe causes a specific disease.
• Exceptions exist for microbes tough to culture in the lab. Molecular methods can often replace earlier steps.

Signs and Symptoms

• Symptoms: visible responses of the plant to the disease (e.g., discoloration, wilting, abnormal growth).
• Signs: microscopic or macroscopic structures produced by the pathogen (e.g., fungal spores, bacterial ooze).

Common signs of plant disease

• Exudates or ooze; Wilt; Stem rust
• Examples: Bacteria oozing from leaves (sign of infection), or mold growth on a plant (sign of infection)
• Symptoms: change in appearance, necrosis, or death of parts of a plant, underdeveloped tissues, or organs, overdeveloped tissues or organs
• Examples: Mosaic (discoloration in leaf), blight (rapid wilting and death of plant parts), rot (decay and softening of plant parts), cankers (lesions on stems or branches).

Examples of Plant Diseases & Causative Agents

• Various plant diseases and the organisms responsible were mentioned in the presented material.
• Target spot lesion on tobacco caused by Rhizoctonia solani
• Bacterial leaf spot on lettuce caused by Xanthomonas campestris pv. vitians.
• Mosaic on squash leaves
• Peanut leaf spots caused by Tomato spotted wilt virus.
• Powdery mildew on apple blossoms caused by Podosphaera leucotricha
• Stem rust on barley caused by Puccinia graminis
• Herbicide injury on bean leaves;
• Canker on an apple tree caused by Nectria galligena
• Brown rot on peaches caused by Monilinia fructicola
• Verticillium wilt of cucumber caused by Verticillium dahliae.
• Crown gall on Euonymous caused by Agrobacterium tumefaciens.
• Brown rot of lettuce caused by Botrytis cinerea

Other Key Concepts

• Time is a critical element in epidemiology, as disease initiation, development, and spread are all influenced by time.
• Abiotic & Biotic factors influence plant disease, as do the interactions within the disease triangle.
• Pathobiome: a new concept emphasizing the roles of native microbial community members that may contribute to disease. This includes aspects of the plant microbiome.
• Disease Triangle: A model to visualize how factors like pathogen, host susceptibility, and environmental conditions contribute to disease.

Etiology & Epidemiology

• The study of disease cause and the factors influencing spread are crucial in plant pathology (etiology and epidemiology).