Fundamentals of Plant Pathology

CAUSES OF PLANT DISEASES

Causes of Plant Diseases

• Plant diseases are caused by a variety of pathogens.
• The word pathogen can be broadly defined as any agent or factor that incites ‘pathos’ or disease in an organism. Thus in strict sense, the pathogens do not necessarily belong to living or animate groups.

Abiotic (Inanimate) factors

• They include mainly the deficiency or excess of nutrients, light, moisture, aeration, abnormality in soil condition, atmospheric impunities etc. Examples are: Black tip of mango (due to SO2 toxicity), khaira disease of rice (due to Zn deficiency), whiptail of cauliflower (Mo deficiency), hollow and black heart of potato (due to excessive accumulations of CO2 in storage), bitter pit of apple (due to Ca deficiency).

Mesobiotic causes

• These are the disease incitants which are neither living nor non-living. They are considered to be on the threshold of life. They are:
• Viruses: They are infections agents made up of one type of nucleic acid (RNA or DNA) enclosed in a protein coat. Examples of viral diseases of plants are: potato leaf roll, leaf curl of tomato and chillies, and mosaic disease of many plants.
• Viroids: They are naked, infectious strands of nucleic acid. They cause diseases like potato spindle tuber, citrus exocortis, chrysanthemum stunt, cadang cadang of coconut palm, star crack of apple, etc.

Biotic (Animate) causes
This category includes the pathogens which are animate or living or cellular organisms. They are:

• Prokaryotes like bacteria which are unicellular prokaryotic microorganisms lacking true nucleus. Examples of diseases caused by true bacteria are: brown rot or wilt of potato, soft rot of potato and vegetables, , citrus canker, etc.

i) Phytoplasma are wall-less prokaryotes and cause diseases like peach X.
ii) Fastidious bacterium, Xylella fastidiosa causes almond leaf scorch, Pierce’s disease of grapevine.

• Eukaryotes are the organisms with true nucleus.

i) Fungi: Potato wart, powdery mildew, rust, smuts, red rot of sugarcane (nearly 80% of plant diseases are caused by fungi).
ii) Straminopiles (Oomycetes): Downy mildews, late blight of potato, white rust of crucifers, damping off etc.
iii) Protozoa: Hart rot of coconut palm and phloem necrosis of coffee.
iv) Algae: Red rust of mango or papaya or litchi
v) Metazoan animals (Nematodes): Root knot of vegetables, ear cockle of wheat, citrus decline etc.
vi) Parasitic flowering plants (Phanerogamic plant parasites): Dodder, Striga, Orobranche, Loranthus, Phoradendron, etc.