• Pathogenesis (Mechanism of Disease Development)

Pathogenesis refers to the sequence of events through which a pathogen interacts with a host, invades plant tissues, and causes disease. It includes:

• Stages of Pathogenesis

1. Inoculation – Initial contact between the pathogen and the plant surface (via air, water, insects, or soil).
2. Penetration – Pathogen enters the plant through natural openings (stomata, lenticels), wounds, or direct penetration.
3. Infection – The pathogen establishes itself inside the plant and starts producing toxins, enzymes, or manipulating plant hormones.
4. Incubation Period – The time between pathogen entry and symptom appearance.
5. Invasion & Colonization – The pathogen spreads through plant tissues, affecting physiological functions.
6. Reproduction – Formation of spores, bacterial cells, nematodes, or viral particles.
7. Dissemination – The spread of pathogens to new plants via wind, water, vectors, or soil.
8. Survival (Overseasoning) – Pathogens persist in plant debris, soil, alternate hosts, or insect vectors during unfavorable conditions.

• Causes of Plant Diseases Plant diseases are caused by biotic (living) and abiotic (non-living) factors.

1. Biotic Causes (Infectious Diseases)

These diseases are caused by living pathogens that multiply and spread.

• Fungi – Cause diseases through spores, enzymes, and toxins (g., Powdery Mildew, Rusts).Bacteria – Enter through wounds or stomata and produce toxins (e.g.,
• Blight of Rice).
• Viruses – Require a living host and are transmitted by vectors (g., Tobacco Mosaic Virus).
• Phytoplasma & Spiroplasma – Transmitted by insects, affecting plant phloem (g., Little Leaf of Brinjal).
• Nematodes – Soil-borne microscopic worms that damage roots (g., Root-Knot Nematodes).
• Parasitic Plants – Obtain nutrients from host plants (g., Cuscuta – Dodder).

2. Abiotic Causes (Non-Infectious Disorders)

These are caused by environmental stress and cannot spread from one plant to another.

• Nutrient Deficiencies/Toxicities – Lack or excess of essential nutrients. (e.g., Iron deficiency causes chlorosis in plants).
• Temperature Stress – Extreme heat or cold damages plant tissues. (e.g., Frost injury in wheat).
• Water Stress – Drought reduces plant growth, while waterlogging leads to root rot.
• Chemical Injury – Excess pesticides or pollutants harm plants. (e.g., Herbicide drift causing leaf burn).
• Air Pollution – SO₂, O₃, and NO₂ damage plant cells. (e.g., Acid rain causing leaf spots).

III. Factors Affecting Disease Development

Plant disease development depends on three key factors known as the Disease Triangle:

1. Host Factors (Plant Susceptibility)

• Genetic Resistance/Susceptibility – Some plants are resistant, while others are highly susceptible.
• Growth Stage – Young seedlings are more vulnerable to diseases than mature plants.
• Nutritional Status – Deficient or excess nutrients influence disease severity. (e.g., Nitrogen excess promotes fungal growth).

2. Pathogen Factors (Virulence & Survival)

• Pathogen Type – Fungi, bacteria, viruses, nematodes, etc., have different infection mechanisms.
• Reproductive Rate – Pathogens that reproduce quickly cause rapid outbreaks.
• Toxin/Enzyme Production – Some pathogens produce toxins that kill host cells. (e.g., Alternaria produces host-specific toxins).

3. Environmental Factors (Favorable Conditions for Disease)

• Temperature – Moderate to high temperatures favor bacterial and fungal diseases.
• Humidity & Rainfall – High moisture promotes fungal diseases like rusts and mildews.
• Wind – Helps in the spread of fungal spores and bacterial cells.
• Soil Conditions – Poor drainage leads to root diseases, while high organic matter may favor pathogen survival.

• Disease Forecasting & Management

Since disease development depends on host, pathogen, and environment, forecasting systems use weather data to predict outbreaks and help in early control.

Preventive Measures:

• Use resistant varieties.
• Follow crop rotation and sanitation.
• Proper irrigation and fertilization to maintain plant health.
• Apply fungicides, bactericides, and biocontrol agents when necessary.