An assignment on Mode of entry of pathogen; Plant resistance and Control Measure

     
                                                    Mukesh Paneru, IAAS                               
  The process of germination or multiplication of infective propagules in host through the establishment of a parasitic relationship between pathogen and host is called infection. It is affected by nature of host, pathogen and external environment. If any of the phase of infection inhibited by any of these factors then infection process is incomplete as a result no disease occurs in host. There are air borne infection, seed borne infection, soil borne infection, insects infection and parasite borne infection. Infection process can be described in following steps.
1)Pre-entry
During the penetration of pathogen in a host tissue the spore must germinate on the surface of plant. Some pathogens develop appressoria (specialized penetration structure) and infect the tissue. Some pathogens utilizes openings of plant surface. It this step there is contact between infective propagules and host tissue which is called inoculation. Pathogen formation is affected by temperature, moisture, light, aeration, nutrient availability and pH.
2)Entry
Pathogens attach to the plant surface with the help of pressure and then enzymatic degradation of the cuticle and cell wall takes place. With application of turgor pressure in host tissue, a penetration peg is formed which apply high pressure to the host cuticle and cell wall. Some pathogen enter via pre-existing openings like hydathode, stomata and wounds.

3)Colonization
In this stage there is establishment of parasitic relationship between host tissue and pathogen. Pathogens can be categorized in two steps.
• Biotrops:- that attack and cause infection in living cell.
• Necrotrops:- that kill cell before colonization process by secreting toxins
Pathogen colonize in the plant surface and sucking of nutrient through haustoria in epidermal and mesophyll cells. As a result sub-cuticular infection, mesophyll infection, parenchyma tissue infection, vascular infection and systemic infection takes place.

                  Fig:- Generalized diagram of infection and colonization patterns of a) biotrophic
                             pathogens; b) necrotrophic pathogens; and c) biotrophic mutualists.   
         
          Fungi, bacteria and viruses have different mode of entries. The infection mechanism of fungi through,
- Deposition of spores on host body surface
- Germination
- Establishment of appressoria in the germ tubes
- Formation of penetration tubes from appressoria.
For germination of fungus spores should favourable to external factors ( i.e. atmospheric humidity, light and temperature).
In terms of bacteria and viruses infection is through,
- Mechanical penetration of host tissue
- Enter through openings

      The entry of pathogen to host from different ways which are as given below.
1. By penetration through uninjured surface like epidermis
2. By means of penetration tube
3. Through natural openings of stomata, lenticels etc.
4. Through thin and weak walled organs like root hair, plant tip
5. Through incidental wounds
6. Through injured and openings caused by other pathogens
7. Through wounds caused by other agents like insects.

                     Fig:-showing bacterial pathogen in different plant parts

           To effectively cause a disease, the pathogens should enter to a host tissue in large population. But the population or number of pathogens may vary to cell to cell or plant to plant. When pathogens enter inside the host tissues they obtain nutrients from host and multiplies rapidly. On the other side, host tissue have own immunity natural defence power to prevent the infection caused by pathogens. The symptoms of disease is only seen when pathogens inhibit the defence power of host tissue. For efficient spread of the pathogen host parasite reaction is important. When the pathogens enters into the intercellular space, it may builds up different types of relationships with the host which are given below.
i. Pathogens send haustoria and obtain nutrient and relationship formed
ii. By producing pectolytic or celluloytic enzymes which dissolve the cell wall and pathogen spreads.
iii. It may produce specific toxic substance and killing of host cell.
iv. Pathogen causes mechanical blockage of the tissue.


Plant disease resistance:-
                        The capacity of host to prevent the entry, growth and development and establishment of pathogen is called resistance. Resistance to disease in plants varies from plant to plant. It is due to agronomical factors, climatic factors, nutritional factors, resistance qualities, availability of NPK fertilizer and unbalanced fertilization. There are various factors for resistance which are discussed as below.
A. Morphological resistance
           In 1892, Cobb discussed about resistance to rust in wheat varieties was morphological characters. He told that due to morphological behaviour of plants pathogens are restricted to do infection. The thickness of leaf epidermis in wheat, thickness of skin of in potato tuber, higher content of silica in epidermal layer of rice are give resistance to different pathogens. Also he reported that the thicker sclerenchyma around vascular bundle of sugarcane is resistance against red rot disease

B. Functional resistance
      Some physiological functions of host plant prevent the infection of pathogen. Many air-borne pathogens attacks the host through stomata openings but in some cases the stomata remain closed at adverse condition (i.e. condition of pathogens to enter). No. of stomata per unit area in rice leaves responsible with a degree of resistance of the varieties to Pyricularia infections. In wheat varieties which are resistance to rust, the stomata remain closed in the early morning which prevent fungal infection.

C. Protoplasmic resistance
         The supply of food materials in host tissue may affect the pathogens. In the highly susceptible variety, although there is presence of required quantity and quality of nutrient the pathogen spreads as fast as it can and results in epiphytic disease condition. Many workers believed that the physiological and biochemical interaction occurring in host tissue inhibit the infection. Several biochemical factors which are responsible for disease resistance are as follows.
 Nutrients lack quality and quantity, as in wheat for rust infection.
 Presence of tannin which is toxic, as in beans.
 Acidity of sap, as tartaric acid in grapes against black rot.
 Protocatechuic acid and catechol in red onion.
 Phenol in wheat against Puccinia triticina.
 Alkaloids or organic acid in monocots against root rot fungus.
 Riboflavin in groundnut or peanut provide resistance.

Control measures
         The aim of plant disease management is to reduce the economic and aesthetic damage caused by plant disease. Many strategies, tactics and techniques used in disease control.  There are two principles of disease management or control, they are:-
I. Prevention:- Includes disease management practices applied before infections as a result plant is protected from disease.
II. Therapy or curative action:- Applied after the plant is infected and treatment of disease.

But H.H. Whetzel described four general disease control principles i.e. exclusion, eradication, protection and immunization. There are various control methods of plant disease which are described as below.
1) Exclusion
In this method the entry of pathogen in a country, locality or field should restricted by various precautions. Governmental action and quarantine regulations are strictly followed. There are some quarantine (legal forbidden on the transportation of agricultural commodities). Exclusion can be effective by seed certification programmes, by selecting the effective propagative organs, by selecting the disease free material and establishment of natural barrier(such as wind breaks and cleaning farming equipment).

            Fig:- Cleaning of equipment before applying in agricultural field
2) Eradication
It is the process, which involves eliminating of pathogens or destroying the infested area. The main goal of disinfestation is to kill the spores and pathogenic organisms contamination the host surface . Roughing of infested plants, destruction of susceptible alternative host, killing the diseased plant parts, seed treatment, drenching the infested soil with chemicals are some control measures.



3) Protection
It is the method of control of pathogens by creating a barrier between the susceptible host and pathogens. This may be done by several ways. Modification of the environment such as changing soil reaction, cold storage for vegetables and fruits, wind breaks, chemical protection etc. are the important method. The chemical protectants  both external and internal can be used.

              Fig:- Protection of banana from adverse environmental condition

 

    Some of the common fungicides which are used for plant disease control are:-
• Copper fungicides
• Sulphur fungicides
• Organo-mercurials
• Organic fungicides





4) Immunization
Use of disease resistant plants is the common method to manage plant diseases. Use of those resistant plants eliminates the need for additional efforts to reduce disease losses. By hybridization and genetics such type if plant can be produced.


                                             Fig:- Introduction of antigenic genes into plants


5) Integrated disease management
Integrated disease management is a concept derived from successful integrated pest management. Integrated disease management consists the timely application of all agricultural practices. These may include site selection and preparation, utilizing resistant cultivars, alternation of planting method, drainage, irrigation, training,   pruning, shedding and applying pesticides if necessary. IDM balances the fertilizer application.


6) Cultural methods and field sanitation
Cultural methods for disease control refer to those methods that reduce pathogen level or reduce the rate of disease development. These includes sanitation, crop rotation, host eradication.
References
1. http://bugs.bio.usyd.edu.au/learning/resources/PlantPathology/infection/infection_process.html
2. https://www.apsnet.org/edcenter/intropp/topics/Pages/PlantDiseaseManagement.aspx
3. http://erec.ifas.ufl.edu/plant_pathology_guidelines/module_07.shtml
4. Arneson, P. A. 2001. Plant Disease Epidemiology .
5. https://www.shutterstock.com/image-photo/vascular-bundle-stem-sugar-cane-magnification-28610887
6. http://bugs.bio.usyd.edu.au/learning/resources/PlantPathology/infection/image_pages/hydathode.htm
7. https://www.intechopen.com/books/fungal-pathogenicity/infection-mechanisms-and-colonization-patterns-of-fungi-associated-with-soybean
8. http://agriinfo.in/default.aspx?page=topic&superid=5&topicid=1843
9. Rangaswami G., Bagyaraj D.J.,Agricultural Microbiology, September 2013
10. Singh R.S.,Introduction to Principles of Plant Pathology,2010