Understanding Plant Diseases

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Questions and Answers

What type of organism is classified as a biotic causal agent of plant diseases?

Soil acidity

Fungi (correct)

Pesticides

Minerals

Which of the following is an abiotic factor that can cause plant diseases?

Too low or too high temperature (correct)

Viruses

Fungi

Bacteria

What historical factor contributed to the control efforts of plant diseases?

Cultivation of diverse crops

Existence of plant pathogens

Advanced agricultural techniques

Lack of information on causes of disease (correct)

Which of the following is NOT considered a noninfectious cause of plant disease?

Fungi Signup and view all the answers

Homer mentioned the therapeutic properties of which substance for controlling plant diseases?

Sulfur Signup and view all the answers

What was a common consequence of plant diseases in historical agrarian societies?

Destruction of crops and famines Signup and view all the answers

Which of the following is a way by which pathogens can be classified?

By their causal agents Signup and view all the answers

Democritus recommended controlling plant blights using which material?

Olive grounds Signup and view all the answers

What is the initial manifestation of a plant disease?

Localized affliction to one or a few cells Signup and view all the answers

Which type of infection primarily affects the ability to absorb water and minerals?

Root infection Signup and view all the answers

What type of interference is caused by foliage infections?

Interruption of photosynthesis Signup and view all the answers

In which type of disease are infected cells induced to divide faster than normal?

Crown gall disease Signup and view all the answers

Which of the following classification methods is considered the most useful for plant diseases?

The causal agent that causes the disease Signup and view all the answers

What is a symptom of phloem infection?

Cankers that interfere with downward translocation Signup and view all the answers

What can multiple host pathogens affect?

Several dozen or even hundreds of species of plants Signup and view all the answers

Which term describes the visible changes resulting from plant disease?

Disease symptoms Signup and view all the answers

What did Beijerinck conclude caused the tobacco mosaic disease?

A virus Signup and view all the answers

Which organisms were transferred to the kingdom protozoa?

Myxomycota and Plasmodiophoromycota Signup and view all the answers

What term describes the wall-less bodies observed by Doi in 1967 that infect plants?

Mycoplasma-like organisms Signup and view all the answers

What is the smallest infectious nucleic acid molecule identified in plant diseases?

Viroid Signup and view all the answers

Which disease was linked to the potato spindle tuber viroid?

Spindle tuber disease Signup and view all the answers

What was observed by Millardet in 1885 regarding chemical control of plant diseases?

Copper sulfate and lime prevented disease Signup and view all the answers

What type of symptoms were previously thought to be caused by viruses but later lacked evidence of viral presence?

Witches’ broom and yellowing Signup and view all the answers

What is a characteristic of phytoplasmas and spiroplasmas?

They are classified as Mollicutes Signup and view all the answers

What are mycotoxins?

Toxic compounds produced by certain fungi Signup and view all the answers

How can plant diseases directly impact farmers' earnings?

By requiring the use of less productive resistant varieties Signup and view all the answers

What is a potential financial impact of controlling a plant disease?

Higher control costs that may exceed crop returns Signup and view all the answers

Why might shippers incur higher expenses in dealing with plant diseases?

Due to the need for refrigerated storage and transportation Signup and view all the answers

Which of the following strategies may be insufficient for controlling certain plant diseases?

Implementing cultural practices and resistant varieties Signup and view all the answers

What additional cost may arise from the need to separate healthy and diseased products?

Increased handling costs Signup and view all the answers

What is a potential indirect financial loss due to plant disease?

Expenses for chemicals and control measures Signup and view all the answers

How can plant diseases limit a grower's market opportunities?

By forcing early sales during periods of low prices Signup and view all the answers

What did Kühn identify as a cause of plant diseases?

Unfavorable environment and parasitic organisms Signup and view all the answers

What are Koch’s postulates used for?

To prove that a disease is caused by a pathogen Signup and view all the answers

Who first reported nematodes associated with plant disease?

Needham Signup and view all the answers

What disease did Burrill link to bacteria in 1878?

Fire blight Signup and view all the answers

What discovery was made regarding Agrobacterium tumefaciens?

It acts as a natural genetic engineer of plants Signup and view all the answers

What did Adolph Mayer inject into healthy tobacco plants?

Juice from infected tobacco leaves Signup and view all the answers

What did Mayer conclude about the cause of the mosaic patterns in tobacco plants?

It was likely caused by bacteria Signup and view all the answers

What is the basis of Plant Biotechnology derived from Agrobacterium tumefaciens?

Loading DNA segments for desirable traits Signup and view all the answers

Study Notes

Plant Disease

Plant disease is a response of plant cells and tissues to pathogens or environmental factors that lead to changes in form, function, or integrity.
These changes are visible to the naked eye as the plant becomes affected.
Different types of cells and tissues are affected by different diseases, and the affected area determines the physiological function that is disrupted.
Root infection causes root rots and disruption of water and mineral absorption.
Xylem infection causes vascular wilts, which affect the transport of water to the crown of the plant.
Phloem infection causes cankers, impacting the translocation of photosynthetic products.
Foliage infection causes symptoms like leaf spots, blights, rusts, and mildews, impacting photosynthesis.
Flowers and fruits are also vulnerable to infection which affects their reproductive functionality.
While most infected cells weaken or die, some cause abnormal growth such as tumors in crown gall disease due to hyperplastic and hypertrophic cells.
Each plant has a considerable number of diseases it can be affected with.
Pathogens vary in their host specificity. Some may affect only one variety (single-host), while others can infect dozens or even hundreds of plant species.

Plant Disease Classification

Classification can be done based on symptoms (e.g., root rots, wilts, leaf spots), affected plant part (e.g., root, stem, or foliage), type of plant affected (e.g., field crops, vegetables), or the causal agent (most useful method).
Classification based on the causal agent indicates the cause of the disease, suggesting potential development, spread, and control measures.

Pathogens

Living organisms that cause diseases are called pathogens.
These include fungi, bacteria, viruses, viroids, nematodes, protozoa, parasitic plants, and green algae.

Environmental Factors

Non-infectious diseases are caused by abiotic factors.
These include extreme temperatures, inadequate or excessive soil moisture, light exposure, oxygen, air pollution, mineral deficiencies, toxicities, soil pH, pesticides, and improper cultivation practices.

History of Plant Pathology

Plant diseases have been a major factor in crop losses, leading to famines and hardships throughout history.
Evidence of plant diseases is found in ancient texts such as Homer (c. 1000 B.C.) and the Old Testament (c. 750 B.C.).
Ancient writers like Homer mentioned the therapeutic properties of sulfur to control plant diseases.
The first book on plant pathology by Kühn (“Diseases of Cultivated Crops, Their Causes and Their Control”) recognized the role of both unfavorable environments and parasitic organisms in plant diseases.
Koch’s postulates (1887) standardized procedures for proving that a disease is caused by a specific pathogen, mainly bacteria.

Discovery of Other Disease-Causing Agents

Nematodes: Needham (1743) observed nematodes in wheat galls but didn't associate them as the causal agent. Later, in 1855, root knot nematodes were discovered in cucumber galls.
Bacteria: Burrill (1878) proved that bacteria cause fire blight disease in pear and apple trees.
Viruses: Mayer (1886) injected juice from mosaic-diseased tobacco plants into healthy plants, proving the disease's transmission. Beijerinck (1898) discovered that the disease was caused by a "contagious living fluid" he named a virus.
Protozoa: Stahel (1931) discovered flagellates infecting coffee trees, causing abnormal phloem formation and wilting.
Mollicutes: After the discovery of viruses, numerous plant diseases exhibiting yellowing, reddening, or shoot proliferation remained unexplained. These diseases were initially attributed to viruses.
Doi and his colleagues (1967) identified wall-less bodies in the phloem of plants exhibiting yellows and witches' broom symptoms, naming them Mycoplasma-like (prokaryotic cells lacking cell walls).
Later, these MLOs were reclassified as Mollicutes, including phytoplasmas (circular) and spiroplasmas (helical).
Viroids: The potato spindle tuber disease was found to be caused by a small, single-stranded, circular RNA molecule called a viroid (1971).
Viroids are the smallest known infectious nucleic acid molecules, and they are only found in plants.

Plant Pathology in the 20th Century

The focus shifted towards chemical control of plant diseases.
Millardet (1885) discovered that a copper sulfate and lime mixture stopped grapevine diseases.
The discovery of antibiotics and fungicides led to the development of numerous chemical control methods.
However, the overuse of chemicals resulted in environmental damage and the emergence of resistant pathogens.

Economic Consequences

Plant diseases lead to direct yield and quality losses and have many indirect financial impacts.
Farmers may have to plant less productive or expensive disease-resistant varieties.
Disease management costs include chemicals, machinery, labor, and storage space, all of which lead to financial losses.
Plant diseases can limit product freshness and require extra handling costs, further impacting profitability.
Although disease control methods are available, it’s important to consider the costs associated with management and the potential financial losses.

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Description

This quiz explores the various plant diseases caused by pathogens and environmental factors. It covers the impact on different plant tissues and the physiological disruptions resulting from infections. Test your knowledge on symptoms, types of infections, and their effects on plants.