Fundamentals of Plant Pathology and Entomology
Lecture 1: Introduction
Why did the Romans sacrifice red animals on April 25th
- Against yield losses through cereal rusts
How come that witches were burned at the stake?
- Hallucinating compounds in moldy rye
Why are there so many Irish descendants in the USA?
- The Irish potato famine
Why do the English drink tea?
- Destruction of coffee cultivation by coffee rust
History of plant diseases
Theophrastus (370 - ~285 BC), successor of Aristotle (Athens, Greece).
Two large botanical treatises; “The nature of Plants” and “Reasons of Vegetable Growth”
Theophrastus was the first do describe disease symptoms of trees, cereals and legumes that
we now call leaf scorch, rots, scab and rust.
He believed that Gods controlled the weather that ‘brought about’ diseases (spontaneous
generation).
Biblical times
- Locusts comprise one of the seven plaques in the Bible
- Locust swarms can destroy entire crops
- These warms occur cyclically, especially in Africa and Central Asia
- Population explosions depend on “gregarization” (transformation of solitary insects
into a swarm due to rapid growth in population), which in turn depends on the
interplay of density, plant nutritional quality and abiotic factors
Roman times: robigalia
- “Robigalia”, a public festival on April 25 in honor of Robigus, the god of wheat rust
and grain blight, to preserve the fields from those diseases.
- The sacrifices offered were the entrails of a red dog (referring to the rust color) and a
sheep, together with aromatic resin and wine.
Middle ages: ergotism
- Rye was important food crop, and produced “ergots” during cool and wet weather
(Claviceps purpurea infection)
- Ergots were inadvertently ground with grains to make bread
- Constrictions in blood vessels, gangrene, loss of extremities: holy fire or St. Anthony’s
fire
- Hallucination (witchcraft accusations)
- Ergot decreased when potato was increasingly used a staple food
, Mycotoxin (toxic substance produced by a fungus):
ergoline
Ergot of cereals. Fungus develops overwintering
structures (sclerotia) where kennels should be.
Toxicity depends on concentration
Ergoline and it derivatives
The Irish famine
- Half 19th century (1846-1850)
- Caused by the oomycete Phytophtora infestans
- Potato crop was lost for several years in Ireland
- 1 million deaths; 1.5 million emigrants
Why the English drink tea
- The main crop on the British crown colony Ceylon (now Sri Lanka) was coffee
- In 1869, the coffee-rust fungus, Hemileia vastatrix, killed the majority of the coffee
plants and estate owners had to diversify into other crops in order to avoid total ruin
Attackers of plants
- Insects - Oomycetes: fungal-like non-photosynthetic brown algae
- Bacteria - Viroids: small (a few hundred nucleotides), complementary,
- Fungi circular, single-stranded RNA
- Viruses - Phytoplasmas: small “bacteria” without cell wall
- Nematodes - Parasitic plants
,Interaction between micro-organisms, insects and plants
Multiple attackers may co-operate
- Tomato spotted wilt virus destroys many crops, transmitted by thrips, which itself also
damages the plant
Dutch elm disease
- Causal agent identified in the Netherlands
- Fungus Oophiostoma (novo-)ulmi
- Carried with bark beetles
What is a disease?
- A disease is the abnormal functioning of an organism
• Internal dysfunction
• External factors
▪ Biotic agents
▪ Other causes
- Infectious disease is the abnormal functioning of an organism caused by disease-
causing agents, their multiplication and the reaction of host tissues to these
organisms or the toxins they produce
Diagnosis is required to reveal the cause of a disease
Various symptoms on various tissues:
- Leaves: discoloration (mosaic, mottle, streak, stripe yellowing)
- Leaves: necrosis (spots (small), blotches (large), blights (whole leaves), scorches
(edge of leaves))
- Fruits: soft rots, dry rots
- Wood: bleeding, gummosis, canker
- Whole plant: stunting, wilting (xylem)
- Seedlings: damping off
Leaf spots: Cercospora infection in peanut.
Brown lesions: early stage leaf spot infection
Black lesions: late stage leaf spot infection
Plants with leaf spots show some level of resistance.
The disease is contained; even in the later stage, the
disease does not spread anymore.
Pathogens reside in the xylem when they cause wilt; responsible for water transport.
Parasites
- Endoparasite = parasite living within the body of the host.
- Ectoparasite = parasite living outside the body of the host.
• E.g. powdery mildew
Nematodes affect water transport; may cause wilting
Nematode symptoms in a field shows a typical patchy appearance, because it shows the
typical ploughing direction of the famer.
Disease symptoms describe malfunction of plant tissues that are affected, but disease
symptoms are not always sufficient for accurate diagnosis of many plant diseases.
, How to identify a disease
- Is the causal agent a biotic agent?
- Find and identify the pathogen
• Signs of biotic pathogens or its parts or products on a host (e,g, spores,
fruiting bodies, eggs)
- Signs of abiotic agents
• More plants show them
• Similar age of development
- Primary versus secondary infection
Disease diagnosis: Koch’s postulates
Four criteria to establish a causal relationship between causative microbe and disease:
1. The microorganism must be found in abundance in all organisms suffering from the
disease, but should not be found in healthy organisms.
2. The microorganisms must be isolated from a diseased organism and grown in pure
culture.
3. The cultured microorganism should cause disease when introduced into a healthy
organism.
4. The microorganism must ben reisolated from the inoculated, diseased experimental
host and identified as being identical to the original specific causative agent.
Koch’s postulates cannot be applied for
a) Vascular wilt pathogens
b) Bacterial plant pathogens
c) Obligate biotrophic pathogens
d) Nematodes
e) Viruses
f) Fungi and oomycetes in general
Parasite feeding styles
- Biotroph: parasite that lives and multiples on another living organism, but can also be
cultured on synthetic media
• Obligate biotroph: parasite that lives and multiplies only on another living
organism, and cannot be cultured on synthetic media
- Necrotroph: parasite that kills host cells and obtains nutrients from dead cells
- Saprotroph: organism that feeds on dead organic matter; not parasitic
• Can colonize material killed by another organism → competition between
different microorganisms on plant tissue. The organism responsible for killing
has to be identified
