7 Dec 2021

Disease-Causing Plant Pathogens and Their Symptoms (Part-1)

Anjali Singh

Table of Contents

Introduction

Plants are the primary source of nourishment for animals and source approximately 80% of the food consumed by humans. Thus, maintaining plant health is a serious concern for food security and safety, availability of pharmaceuticals and livelihoods, and a healthy environment. However, the yield and availability of crops are often threatened by pests ad pathogens.

Plant diseases caused by viruses, bacteria, or fungi have been a central concern of agriculturists. The loss of crops caused by these pathogens affects crop yield and economic efficiency. This reduces food security at national and global levels, and may also lead to starving conditions.

However, over the past century, modern techniques have been developed to protect plants from these deadly pathogens. The study of these pathogens, their symptoms, and the development of solutions for plant diseases is a central concern to enhance crop efficiency and availability.

This article brings you to the six most common plant pathogens, their symptoms for recognition, and possible solution for their prevention to increase the crop yield.

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Six Most Common Disease-Causing Pathogens and Their Symptoms

The plant diseases can be classified either based on the symptoms of the affected plant parts, such as leaf spots, fruit rots, petal blights, and other diseases or based on the disease-causing pathogens. Some pathogens affect an extensive range of hosts, while some others are only restricted to one genus.

Fungi

Fungi is a versatile class of fungal organisms, ranging from mushrooms to molds, and consist of around 10,000 species. Where some of the fungi cause diseases in plants and humans, some others are beneficial for their health and are consumed as food.

Their body is consists of fine microscopic threads, called hyphae, which grow and branch out inside their species, whether it’s plant or animal. A group of hyphae is called mycelium. And, when a huge mass of hyphae is developed inside hosts, they become visible to the eyes in the form of symptoms.

For example, black spots on rose, powdery mildew, brown patch, down mildew, conks, and oak leaf blisters. These are also one of the most common symptoms, observed in fungal-affected plants.

Figure: An image showing verticillium wilt caused due to a fungal species.

Identification: Fungal infection in plants is generally of two types: saprophytic and pathogenic. And, their identification can be performed by simple microscopic examination.

In a microscope, you need to look for any fruiting bodies or mycelial structures, and then its morphological features can be compared with the reference to decide whether it’s a saprophytic or pathogenic fungus.

However, sometimes simply microscopic examinations are not enough, especially when the mycelial structures are absent or not visible. In such cases, either a selective media is used for the isolation, identification, or promotion of sporulation of fungus, or fungi are incubated under certain temperature, aeration, or light conditions to produce spores.

Bacteria

Bacteria are microscopic single-celled organisms. They are ubiquitous in nature and invade plant tissues through wounded tissues or stomata, lenticels, hydathodes, or other flower nectaries—unlike fungi that invade plants through cuticles. Moreover, it can spread from plant to plant through insects, infected seeds, splashing water, soil, or/and pruning tools.

A total of 80-100 species of bacteria have been found to cause diseases in plants. The diseases are categorized into three groups:

Wilting: It’s due to vascular system invasion or intrusion in water-conducting vessels. For example: Cucumber wilt.
Necrotic Blights, Rots, and Leaf Spots: Here the parenchymal tissue is killed, as in the case of fire blight, soft rot of iris and other plants with rhizomes or fleshy roots, and delphinium black spot.
Overgrowth of tissues or hyperplasia: Uncontrollable growth of tissues as in case of crown gall or hairy root.

Another class of bacteria that harm crops is phytoplasma. They are bacteria without a cell wall, having a single unit membrane. They inhabit plant phloem and mainly cause aster yellows, which can rapidly spread to other plants as well by leafhoppers.

Their symptoms include shoot proliferation, yellowing of foliage, and abnormal green color of flower petals.

Figure: Leaf spot diseases caused by a bacterium.

Identification: Being single-celled organisms, bacteria can only be observed using a microscope and by physiological parameter determination. But for some genus and species of bacteria, the study is performed by using selective media.

But the confirmatory test involves the isolation of bacteria and inoculation of a whole colony in plants to observe if they produce the same symptoms as observed in plants. Some immunodiagnostic assays or serodiagnostic assays such as ELISA (enzyme-linked immunosorbent assay), agglutination and precipitation, and fluorescent antibody staining can be performed for the identification of bacteria.

Phytoplasmas can be identified by following several parameters including microscopical examination, grafting, symptoms determination, transformation, and susceptibility to tetracyclines.

Viruses and Viroids

Viruses are particles of DNA and RNA wrapped in a protein coat. They enter plants cells and use the plant machinery to multiply.

They show a spectrum of symptoms ranging from green and yellow mottling or mosaic, color break on flowers, ring spots, vein clearing, stunting of leaves, flowers, or whole plant, leaf malformation, short proliferation, and necrotic spots.

Viral infection is systemic and spreads to whole plants once infected in a region. Thus, growing plants using infected mother plants through vegetative means, such as grafting, cutting, or division, may cause infection to all plants that are produced.

Viroids are virus-like particles that cause the same symptoms as viruses and spread through mechanical or vegetative propagation means.

Figure: A bright yellow mosaic cause due to the Rose rosette virus.

Identification: The techniques used to diagnose diseases caused by viruses or viroids are:

Virus transmission is tested on specific host plants through inoculation, grafting, or certain insect, nematode, fungus, and mite vectors.
Electron microscopy techniques, as negative staining of virus particles in leaf dip or purified preparations, and immune-specific electron microscope techniques are also used.
Immunoblot technique, such as electrophoretic tests and hybridization of commercially available radioactive DNA complementary to a certain virus DNA or RNA, or viroid RNA, with the DNA or RNA present in plant sap and attached to a membrane filter.

In the second part of the subject, you will learn about nematodes, parasitic plants, and protozoans affecting plants and their symptoms.

Happy Culturing!!