History of Plant Pathology

Ancient history

• Since organized agriculture developed 4000 years ago, plant diseases and pests were recognized, with symptoms such as blight, wilt, and root rot being documented.
• The Rig Veda (1500-500 BC) mentions the germ theory of disease, acknowledging microorganisms as causal agents of diseases—an idea that Europe only accepted around 120 years ago.
• Vraksha Ayurveda, an 11th-century book by Surapal, classifies plant diseases into two groups:
  1. Internal diseases (likely physiological disorders)
  2. External diseases (probably infectious diseases)
• Ancient texts like the Bible, Shakespeare’s works, and Christian literature frequently reference plant diseases such as rust, smut, mildew, and blight.
• Theophrastus (286 BC), a Greek philosopher, recorded his observations about plant diseases in Enquiry into Plants. However, his findings were based on imagination rather than experimentation.
• For 2000 years after Theophrastus, plant diseases were attributed to divine powers, religious beliefs, superstitions, and celestial influences.
• The Romans celebrated the Robigalia festival (around 700 BC) to appease Robigo, the Rust God, in an attempt to prevent wheat rust.
• Farmers in France (1660) and the United States (early 18th century) enacted laws mandating the destruction of barberry bushes, recognizing their association with wheat rust.

Early scientific developments

• 1674: Antony van Leeuwenhoek observed microorganisms under a microscope for the first time.
• 1729: Pier Antonio Micheli, the Father of Mycology, observed fungal spores and demonstrated that they could grow into new fungi. However, his findings were largely ignored due to the prevailing belief in Spontaneous Generation.
• 1743: John Needham, an English clergyman, defended Spontaneous Generation by boiling meat broth in corked flasks.
• 1775: Lazarro Spallanzani challenged Needham’s theory, proving that microbial decay could be prevented by heating and sealing flasks, though his findings were refuted due to faulty techniques.
• 1753: Carl Linnaeus, a Swedish scientist, introduced binomial nomenclature in Species Plantarum.
• 1793: C.H. Persoon published Synopsis Methodica Fungorum, the basis for the nomenclature of Uredinales, Ustilaginales, and Gasteromycetes.
• 1821-1832: E.M. Fries, the Linnaeus of Mycology, published Systema Mycologicum, a key work in fungal classification.

Contributions to plant disease understanding

• 1775: Tillet, a French botanist, studied bunt or stinking smut of wheat. He observed that wheat seeds with black powder (fungal spores) produced more diseased plants and suggested salt and lime treatments to reduce disease incidence.
• 1767: Felice Fontana distinguished the red and black stages of wheat rust and made microscopic drawings.
• 1807: Benedict Prevost discovered the life cycle of bunt fungus and proved that its spores caused the disease. He also demonstrated that copper sulfate treatment controlled smut.
• 1845: Late blight of potato devastated Ireland and parts of Europe, drawing attention to plant diseases. Early literature on this event is found in Gardener’s Chronicle.
• 1846: M.J. Berkeley, a British mycologist, supported the parasitic theory of plant diseases.
• 1857: Speers and Schneider experimentally proved that Botrytis infestans (later renamed Phytophthora infestans) was the causal agent of potato blight.
• 1861, 1863: Anton de Bary, the Father of Modern Plant Pathology, provided definitive proof that Phytophthora infestans caused late blight of potato.

Modern experimental plant pathology

• Heinrich Anton de Bary (1831-1888) laid the foundation of modern experimental plant pathology with the following contributions:
  • Confirmed Prevost’s findings in 1853.
  • Experimentally proved that Phytophthora infestans was the cause of late blight (1861).
  • Studied rusts, smuts, downy mildews, and rots.
  • Reported the heteroecious nature of rust fungi (1885).
  • Identified the role of enzymes and toxins in Sclerotinia sclerotiorum (1886).
  • First to report that lichens are a symbiotic relationship between a fungus and an alga.
  • Published Morphologie und Physiologie der Pilze Flechten und Myxomyceten (1866, 1884), a major fungal classification work.
  • Trained over 60 prominent scientists who advanced plant pathology worldwide.

Important scientists and their contributions

• J.G. Kuhn (1858): Published Diseases of Cultivated Crops, Their Causes and Their Control, recognizing plant diseases as caused by environmental and parasitic agents (insects, fungi, and parasitic plants).
• Louis Pasteur (1860s): Disproved Spontaneous Generation, proving that microorganisms cause disease and establishing the Germ Theory of Disease.
• Robert Petri (1887): Developed artificial nutrient media for microbial culturing.
• Brefeld (1875-1880s): Advanced fungal culturing techniques and systematics.
• M.S. Woronin (Russia): Studied club root of crucifers (Plasmodiophora brassicae).
• H.M. Ward (England): Studied coffee rust and proposed the bridging host theory.
• A. Fisher (Switzerland): Worked on bacterial plant pathology and was involved in the Fisher-Smith controversy regarding the role of bacteria in plant diseases.

Early Developments and the Role of Fungi in Plant Diseases

The study of plant diseases dates back centuries, with observations of diseased plants recorded as early as the 17th century. However, significant advancements in understanding plant pathology came with the development of microscopy and germ theory. The role of fungi in plant diseases was one of the earliest discoveries, leading to the identification of various pathogenic fungi responsible for devastating agricultural losses.

Notable contributions include:

• 1807: Tillet identified the role of spores in smut disease.
• 1853: Anton de Bary demonstrated that the late blight of potato was caused by Phytophthora infestans.
• 1865: Pasteur’s work on fermentation supported the idea that microorganisms could cause disease.

These findings laid the foundation for modern plant pathology, leading to the classification of plant pathogens into different groups such as fungi, bacteria, viruses, viroids, phytoplasma, and fastidious vascular bacteria.

Role of Other Plant Pathogens

Bacteria as Plant Pathogens

• 1882: T.J. Burrill of the USA first reported that a plant disease (fire blight of apple and pear) was caused by a bacterium (now known as Erwinia amylovora).
• 1883: Wakker showed that the yellows disease of hyacinth was also caused by a bacterium.
• 1895: E.E. Smith of the USA made significant contributions to the discovery of plant diseases caused by bacteria and is regarded as the Father of Phytobacteriology.
• 1893-1894: Smith studied crown gall disease and considered it similar to cancerous tumors in humans and animals.
• 1977: Chilton and his team demonstrated that Agrobacterium tumefaciens transforms normal plant cells into tumor cells by inserting a part of its plasmid into the plant chromosome.

Viruses

• 18th Century: Leaf roll disease of potatoes and broken tulip patterns were observed.
• 1886: Adolf Mayer introduced the term ‘mosaic’ and showed that the disease was infectious.
• 1892: Dimitri Ivanowski proved that the causal agent of tobacco mosaic disease could pass through bacteria-proof filters.
• 1898: Beijerinck, the Father of Plant Virology, described the causative agent of tobacco mosaic disease as a “contagious living fluid” and named it a virus.
• 1935: Stanley crystallized the tobacco mosaic virus and won a Nobel Prize.
• 1936: Bawden and Pirie discovered that the virus consists of both proteins and RNA.
• 1939: Kausche et al. viewed tobacco mosaic virus under an electron microscope.
• 1956: Gierer and Schramm demonstrated that viral nucleic acid is responsible for infection and replication.

Viroids

• 1971: Diener and Raymer identified potato spindle tuber disease as being caused by a small, circular RNA molecule, which they called a viroid.
• 1982: The term virusoid was introduced for circular RNA molecules associated with some plant viruses.

Phytoplasma and Rickettsia-like Organisms

• 1967: Doi et al. and Ishiie et al. independently observed Mycoplasma-like organisms (MLOs) in the phloem of plants with yellows and witches’ broom symptoms.
• 1973: Certain mycoplasmas like Spiroplasma citri (causing citrus stubborn disease) were successfully cultured.
• Diseases caused by phytoplasma include aster yellows, mulberry dwarf, potato witches’ broom, and sandal spike.

Vascular Bacteria

• Observed in diseases such as: Pierce’s disease of grapevine, phony peach disease, and citrus greening.
• Xylem-limited bacteria: Xylella fastidiosa (causing citrus variegated chlorosis, almond leaf scorch).
• Phloem-limited bacteria: Candidatus Liberibacter (causing club leaf of clover, citrus greening, bunchy top of papaya).

Flagellate Protozoa

• 1909: Lafont observed flagellate protozoa in latex-bearing plants (Euphorbiaceae), though they were not pathogenic.
• 1931: Stahel identified flagellates in the phloem of coffee plants, causing wilting.
• 1976: Flagellates were found in the phloem of coconut palms affected by hart rot disease.