Plant Disease Triangle, Koch's Postulates, and Diagnosis PDF

Disease triangle, Koch postulate, and Disease development CONDITIONS NECESSARY FOR DISEASE CAUSED BY PATHOGENS In order for disease to occur, three conditions must be met: 1. Susceptible Host 2. Virulent Pathogen 3. Suitable Environment Disease Requirement 1. Host It is necessary to have a s...

Disease triangle, Koch postulate, and Disease development CONDITIONS NECESSARY FOR DISEASE CAUSED BY PATHOGENS In order for disease to occur, three conditions must be met: 1. Susceptible Host 2. Virulent Pathogen 3. Suitable Environment Disease Requirement 1. Host It is necessary to have a susceptible host plant. Each species of plant is capable of being infected by only certain organisms (pathogens). The plant must be in a stage of development susceptible to infection by the disease agent. Disease Requirement 1. Host A host or host plant is the plant infected by the pathogen. There are different levels of susceptibility, which include: ◦ Immune ◦ Susceptible ◦ Resistant Host – Levels of Susceptibility Immune means that the host plant cannot be infected. Susceptible means that the host plant can be infected. Resistant means that the host may or may not be infected and that the host plant is able to prevent the pathogen from completely killing the plant i.e., the plant may be infected, but at the time of infection the host plant produces inhibitory or defense compounds that kill or prevent the spread of the pathogen. Disease Requirement 2. Pathogen The second requirement is the presence of an active pathogen in a stage of development conducive to infecting the host plant. If there is no pathogen present, there can be no disease. Disease Requirement 2. Pathogen A pathogen is a living organism, which infects a susceptible host. The severity of disease is regulated by the virulence of the pathogen The level of virulence determines the aggressiveness of the pathogen. Disease Requirement 3. Environment The third condition is an environment suitable for the pathogen to infect the plant. Temperature and moisture are important. ◦ Humidity is the critical factor limiting the spread of many foliage diseases The diseases of major importance are those which attack below the soil line where moisture and temperature conditions are favorable. Exception - viruses are plant pathogens that are little affected by climate. Disease Requirement 3. Environment Environmental conditions that favor the development of disease are those conditions that: ◦ encourage the growth and virulence of the pathogen ◦ discourage the growth and defense reactions by the host. Disease Triangle  Since all plants can be considered hosts and a number of pathogens survive in the landscape, it is usually the environment that regulates the severity and incidence of disease.  However, the severity and incidence of disease is also determined by the susceptibility or resistance of the host and the virulence of the pathogen.  The two most important environmental factors regulating plant disease are temperature and moisture, with moisture levels being critical.  In general, cool moist conditions are going to favor the development of disease, while hot, dry conditions will deter it. Disease Triangle Robert Koch  Koch’s Postulates A pathogen must be found associated with all the diseased plants examined (constant association with symptomatic host). The microbe must be isolated from the host with the disease and grown in pure culture on a nutrient media and its characteristics described. The specific disease must be reproduced when a pure culture of the microbe is inoculated into a healthy susceptible host. The microbe must be recoverable from the experimentally infected host. PLANT DISEASE DIAGNOSIS Diagnosis is the act or process of deciding the nature of a disease, problem, etc. by examination and analysis. Proper diagnosis involves the following: 1) Critical observation of the symptoms with the naked eye first, then with hand lens and finally with the dissecting microscope. All information is noted down and should include; extent of the lesion, shape, colour and any signs of the fruiting structures. 2) microscopy; Involves more thorough examination of signs and symptoms. Fruiting structures are examined at a field of x10 and x40. Staining with cotton blue may be necessary. Heating helps to clear the specimen and enhances visibility of the structures. 3) Incubation; If distinct reproductive structures are not visible, specimen is placed in a moist sandwich box lined with whatman paper or cellulose wadding and just enough water to keep it moist is added. Once closed, the relative humidity close to 100% is obtained. Examination is done again after one day. Isolation If incubation does not give clear results, isolation is done. Small sections cut from the diseased specimen,sterilised with 5% sodium or calcium hypochlorite for 2 minutes and rinsed with 2-3 changes of sterile distilled water, blotted dry and placed in sterile cooled solid medium. Isolation cont’d Petridishes are incubated at preferably 25°C or room temperature for 2-3 days and examined for fungal growth. Subcultures are made from advancing colonies in order to obtain pure cultures of the pathogenic fungi. Isolation media; many types like potato dextrose agar, sucrose agar malt extract agar are general for most fungi. Isolation cont’d Other media like oatmeal agar is good for Aspergillus, Czapek autolysate agar(CYA) good for Pencillium and Spezieller nahrstorffarmer agar(SNA)is good for Fusarium There are many other media specific for different isolations. DISEASE DEVELOPMENT  It is important to understand how plant diseases develop in order to control them.  By the time it becomes obvious that a plant has a disease, it is generally too late to do anything about it in that growing season.  Plants cannot be cured in the way people expect their own ills to be cured.  The process by which diseases develop can be broken into five distinct phases: 1. Inoculation 4. Infection 2. Incubation 5. Disease 3. Penetration Disease Development 1. Inoculation Introduction of the pathogen to the host plant tissue. Wind, rain, or running water can move pathogens and introduce them to a host. Also birds, insects, people, or equipment. Some pathogens move themselves short distances, but most rely on other means. Sources of inoculum include plant debris, seed, perennial plants, and soil. Disease Development 2. Incubation Period of development during which the pathogen undergoes changes to develop into a form which can penetrate or infect the new host plant. Some fungi, for instance, grow a structure called a penetration peg that can grow through the cell walls of the plant. Disease Development 3. Penetration Process of getting inside the plant. It may be an active or passive process. Some pathogens produce enzymes to dissolve the cutin and cellulose layers of plant material between them and the cell contents. Some pathogens can swim through water on a plant's surface and into the plant through natural openings (such as stomata, lenticels, or hydathodes) or through wounds. Some pathogens are put inside the plant by insects, pruning tools, or driving rain. Disease Development 3. Penetration Disease Development 4. Infection Pathogen invades the plant tissue Establishes a parasitic relationship between itself and the host. Disease Development 5. Disease The host plant responds to the presence of the pathogen, a disease exists. The host's response results in symptoms of the disease, such as blight or necrosis. As the pathogen matures, it produces inoculum in the form of spores, virus particles, and bacterial cells that can be spread or disseminated to other adjacent, healthy plants. SYMPTOMS, SIGNS, & SYNDROMES Symptom the physical expression of a change in the appearance and function of the plant. Examples Blights of Symptoms Rots sudden death of twigs, general decomposition and foliage, flowers. destruction of tissue. Cankers Necrosis dead places on bark and death of tissue. cortex of stems; often discolored and raised or Spots sunken. circular or irregular lesions on above-ground tissue. Galls abnormal, localized swellings on leaf, stem, or root tissue. SYMPTOMS, SIGNS, & SYNDROMES Sign the visible presence of the pathogen, such as a fruiting body or discharge associated with the disease Examples  Conks of Signs  Pycnidia fungal fruiting structures minute, fungal, asexual formed on rotting woody fruiting structures, usually plants (shelf or bracket fungi). globose and black, formed on plant surfaces.  Mycelia masses of fungal threads  Rhizomorphs (hyphae) which compose the string-like strands of fungal vegetative body of the mycelia sometimes found fungus. under bark of trees.  Ooze (flux) viscid mass of juices composed of host and parasite substances found exuding from some diseased plants. SYMPTOMS, SIGNS, & SYNDROMES Syndrome  the group of signs and symptoms which collectively characterize a disease.  Familiarity with a disease's signs or symptoms is not enough to diagnose a disease; it is necessary to know the syndrome and case history.  Seeing a spot on a leaf doesn't tell you much, but finding pycnidia in that spot and knowing the plant species and recent weather conditions might aid in disease diagnosis.  Other times, laboratory work is necessary. Disease IPM Familiarity with crops and the diseases and insects that affect them is useful in planning control programs. Some diseases occur every season; others occur sporadically. Some can be controlled easily by using proper methods; others must be tolerated. Knowing which problem falls into which category comes with experience. Knowing the proper method to use at the proper time is a part of integrated pest management. Spores of Magnaporthe grisea germinating and forming appressoria appressorium Stanley, M. S., Callow, M. E., Perry, R., Alberte, R. S., Smith, R., and Callow, J. A. 2002. Inhibition of fungal spore adhesion by zosteric acid as the basis for a novel, nontoxic crop protection technology. Phytopathology 92:378-383.