THREE PHASES OF PENETRATION OF PATHOGENESIS IN PLANTS

Read this article to learn about the Phases of Penetration of Pathogenesis in Plants !

INTRODUCTION
Pathogenesis is the process of infection or the actual way in which the disease develops in plant body. Infection is the establishment of a pathogenic microorganism within the host, following entrance.
It signifies the sum of biological processes which takes place in the host body after penetration of the pathogen, independent of the fact whether the pathogen causes a disease or not. As the result of infection visible or latent diseases are produced in the host plants. The potential capacity of infection of any pathogen is called its pathogenicity.

The pathogenicity of every pathogen is its specific feature. This characteristic depends upon the capacity of parasitic adaptation and struggle for existence of the pathogen. The phenomenon of pathogenesis can be understood easily by studying the three phases of penetration of the pathogen, viz., pre-penetration, during penetration and post-penetration phases. These three phases of penetration are briefly discussed below.

Images showing Plant pathology; by Author:

The phenomenon of pathogenesis can be understood easily by studying the three phases of penetration of the pathogen, viz., pre-penetration, during penetration and post-penetration phases.
(I) Pre-penetration Changes:
The pre-penetration phase includes the growth of the pathogen before actual entry or penetration into the host. Spores of various pathogenic fungi germinate on the surface of the host. During germination, metabolic activities of spores increase considerably.
Spore germination, besides various physical factors (moisture, temperature, light, pH, oxygen, carbon-dioxide, etc.), is mainly influenced by non-parasitic micro-organisms present in the rhizosphere and phyllosphere.

Certain chemicals secreted in the rhizosphere by roots are known to accelerate or inhibit germination of spores of pathogens. During germination, the spore produces filamentous germ tube from one or more germ pores. The formation of germ tube is affected by environmental factors and susceptibility of the plant.

Direct penetration diagram

(II) Penetration Process:
Spores show various penetration mechanisms. The infection threads of pathogenic fungi may enter the host through natural openings (stomata, lenticels or hydathodes), through wounds or by direct penetration. Although most of the pathogens enter the host only by one method, some pathogens adopt more than one ways.

Most of the rusts and downy mildews enter through stomata. When germ tube reaches the stoma, its terminal part swells to form a vesicle, known as appressorium. Most of the protoplast of the germ tube accumulates in the appressorium and the appressorium is separated from the germ tube by a septum.
A blade-like wedge grows from the appressorium through the stomatal slits and it swells to form a substomatal vesicle. The contents of the appressorium pass in the vesicle. One or more penetration hyphae grow from this vesicle and they form infer or intracellular mycelium.

Stomata, hydathodes and nectaries are the common pathways for the entry of many parasitic bacteria. All powdery mildews and a few downy mildews usually penetrate host tissue through epidermis by mechanical process.

(III) Post-penetration changes:
The post-penetration phase includes the development and growth of the pathogen after penetration. In this phase, colonisation of the pathogen takes place. After successful penetration inside the host, the pathogens secrete several types of substances, such as enzymes, toxins or growth regulators. These substances bring out physiological, anatomical and morphological disturbances in the host plant.

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Plant pathology?