Plant Disease General Characteristics and Core Concepts

A plant disease is a disease of any part of a plant and may have various forms. Usually plant diseases affect plants in ways that reduce the survival, growth, and yield of the plants. Plant diseases are either caused by a microbe (bacteria, virus, or fungus) or by an organism of any size, including microscopic organisms (protozoa, algae, parasites).

Microbes are organisms less than 1 micron in size. Microbes that live in or on plants are referred to as plant microorganisms. Microbes that live in the environment around plants and cause diseases are called environmental microbes. The diseases caused by environmental microbes are called pests. Some plant diseases are diseases that affect all parts of a plant. Other plant diseases are diseases caused only by one or two different organisms. 

A disease caused by multiple microorganisms or an organism may have a variety of forms. Plant diseases can be caused by biological, physical, chemical or mechanical damage.

Biological Damage: Any agent that affects plants using biochemical, microbiological, genetic, chemical, mechanical, thermal, or other processes. Biological damage can be caused by bacteria, viruses, or fungus. Biological damage is usually a result of physical damage, such as mechanical damage, or exposure to cold, hot, wet, or dry conditions.

Physical Damage: Any agent that affects plants by causing physical changes to plants or their parts, such as a disease caused by mechanical injury to plant parts. Examples include diseases caused by an insect vector, such as by a louse insect.

Chemical Damage: Any agent that affects plants by causing chemical changes to plants or their parts. Chemicals may enter plants directly, be sprayed on plants to control plant disease or come from the environment around plants, such as soil.

A disease is caused by certain microbes, fungi, viruses, nematodes, insects, and herbivores that cause serious damage to plants. Most plant diseases do not kill the plant, but instead, cause the plant to be weakened, making it more vulnerable to attack by disease-causing organisms.

Some diseases are not visible to the human eye, so it is important to develop a method of inspection to accurately identify the disease before it is too late. A good method of inspection is to collect a sample from the affected plant and send it to a diagnostic laboratory for examination.

The following are some of the most common agricultural diseases:

Soybean rust

This is an Asian fungal disease that is damaging and reducing the value of the crop. The infected plants are chlorotic and lose their leaves as the disease progresses. Once the leaf blades are removed, the stems are also infected and die. Because this disease is found mainly in Asia, some areas of the United States such as South Carolina and the Florida area will be considered as a refuge.

Soybean rust is spread by contact with infected debris from the infected soybean plants, insects, wind, or other means. It thrives in humid, warm, and dry conditions during the spring and fall. It thrives on dead, rotting plant material. The fungus that causes the rust is in the genus Phakopsora. The pathogen is spread by the fungus-producing spores called pycnidia (infected puffs on the leaves) that break off and spread the disease by wind and rain.

Soybean rust attacks the vascular system of the plant resulting in the reduction of the plant's water-conducting abilities and ultimately, the death of the plant. In infected areas, this disease can be very detrimental to soybean yields.

There are several races of soybean rust. Each year there are new strains of the fungus that are resistant to specific fungicides that are used to control the disease. If there is a disease-resistant variety of soybean available that can be grown in the area, it is worthwhile to plant it and hope for the best. If a good yield is expected, the best fungicide to use is a product that contains a mixture of fungicides. Pyramid products that have two, three, or four fungicides on one product label offer maximum efficacy for soybean rust control.

Soybean rust symptoms vary depending on the strain of the fungus that is present. When the disease is at the lower stage of development, it is possible to be infected by a rust strain, but it will not have the same effect as a higher stage of development. It is common to see many small, round pustules on the upper portion of the leaf (upper portion of the leaf blade). These pycnidia are usually the first indication of soybean rust.

Pseudothecia

When pustules turn into pycnidia, pycnidia will produce more spores, resulting in more rust on the plant and less disease resistance. Pycnidia may produce one, two, or more spores in a cluster on the underside of the leaf blade. At any given time, there is a variable amount of spores present on a leaf. The rust may appear on both sides of the leaf. One side of the leaf blade may be covered with rust while the other is not. When leaves turn yellow or pale, this is a good indication of rust on the leaf.

What is Plant Disease?

In general, a plant is called to be a diseased plant when it is continuously disturbed by the causal agent resulting in an abnormal physiological process that eventually disturbs the plant’s normal growth, structure and function. Plant diseases are classified according to the nature of their causal agent - infectious or non - infectious. The infectious plant disease general characteristics are caused by the pathogens such as bacteria, fungus, virus, mycoplasma, nematodes or parasites. Non - infectious plant diseases are caused by unfavourable growing conditions such as toxic substances in the soil and atmosphere, extreme temperature, disadvantageous relationships between oxygen and moisture and deficiency or excess of minerals.

Characteristics of Healthy Plants

A plant is healthy when it is completely disease-free. They grow stronger and their leaves are always firm. The parts of the plant - shoot, leaves, flowers, fruits all are well grown.

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Classification of Plant Diseases by the Causal Agent

Plant diseases are classified as per their symptoms. However, many diseases produce similar symptoms and signs but are caused by different pathogens. They also require different control methods. Hence, the classification of plant diseases as per their symptoms is inadequate. Another reason for this is that the causal agents may induce several symptoms which often intergrade. Instead, classification can be done based on the species of the plant affected. The best and most used classification method of plant diseases is based on the causal agent i.e, infectious or non - infectious. 

Non - Infectious Diseases

The non - infectious diseases of plants are generally caused by unfavourable growth conditions. They are not caused by any living organisms and can not be transmitted from one plant to another. Non - infectious diseases are very common but should be assessed carefully and should be removed immediately for the healthy growth of the plants. Examples of non - infectious diseases include nutritional deficiencies, sun scorch, salt injury, soil compaction and moisture extremes.

Infectious Diseases

Plants get infected by thousands of species from a very diverse group of organisms. They can be microscopic or macroscopic. These infectious agents or pathogens are grouped as bacteria, viruses and viroids, fungi, nematodes and parasites. We will learn about the general characteristics of the diseases caused by the fungi here. Infectious diseases are spread through water, wind, insects, animals and even humans. 

Fungal Diseases in Plants

Almost two-thirds of the infections of the plants are caused by fungi. These infections include leaf curls, needle casts, white & true rusts, smuts, sooty moulds, mildew; root, stem, fruit, bud, leaf rots; leaf, flower and fruit spots; wilts; shoot, leaf and bud galls and many more. In its life cycle, one single plant can be attacked by many fungi.

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General Characteristics of Fungal Infection in Plants

A fungus is a member of a large and diverse group of eukaryotic microorganisms. They have chlorophyll-less cells which have membrane-bound nuclei, mitochondria and rigid cell walls. Fungi have a plantlike vegetative body that consists of microscopic branching. They have thread-like filaments of different lengths known as hyphae, some of which extend in the air and others penetrate the substrate on which these fungi grow. These hyphae are arranged in a network which is called mycelium . It is because of this mass of mycelium that the fungal growth looks cottony. They reproduce by both sexual and asexual methods. They produce spores of various types in large quantities.

Signs and Symptoms of Fungal Diseases in Plants

The fungal infection can either reduce the growth of plants due to hypoplasia and atrophy or can induce excessive growth in plants due to hyperplasia and hypertrophy. The symptoms of fungal infection may include leaf spots, scabs, rots,blights, anthracnose, damping-off, canker, dieback, clubroot, warts, galls and leaf curls. Sometimes, the fungi that infect the plant produce growth on the plant parts that appears as a mass of mycelium and looks like cotton. This is a sign of infection.